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Title: Elementary Zoology, Second Edition
Author: Kellogg, Vernon L. (Vernon Lyman), 1867-1937
Language: English
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ELEMENTARY ZOOLOGY
BY
VERNON L. KELLOGG, M.S.
_Professor of Entomology, Leland Stanford Junior University_
_SECOND EDITION, REVISED_
[Illustration]
NEW YORK
HENRY HOLT AND COMPANY
1902
Copyright, 1901,
BY
HENRY HOLT & CO.
ROBERT DRUMMOND, PRINTER, NEW YORK
PREFACE
It seems to the author that three kinds of work should be included in
the elementary study of zoology. These three kinds are: (_a_)
observations in the field covering the habits and behavior of animals
and their relations to their physical surroundings, to plants, and to
each other; (_b_) work in the laboratory, consisting of the study of
animal structure by dissection and the observation of live specimens in
cages and aquaria; and (_c_) work in the recitation- or lecture-room,
where the significance and general application of the observed facts are
considered and some of the elementary facts relating to the
classification and distribution of animals are learned.
These three kinds of work are represented in the course of study
outlined in this book. The sequence and extent of the study in
laboratory and recitation-room are definitely set forth, but the
references to field-work consist chiefly of suggestions to teacher and
student regarding the character of the work and the opportunities for
it. Not because the author would give to the field-work the least
important place,--he would not,--but because of the utter
impracticability of attempting to direct the field-work of students
scattered widely over the United States. The differences in season and
natural conditions in various parts of the country with the
corresponding differences in the "seasons" and course of the
life-history of the animals of the various regions make it impossible
to include in a book intended for general use specific directions for
field-work. Further, the amount of time for field-work at the disposal
of teacher and class and the opportunities afforded by the topographic
character of the region in which the schools are located vary much. The
initiation and direction of this must therefore always depend on the
teacher. On the other hand, the work of the other two phases of study
can to a large extent be made pretty uniform throughout the country. For
dissection, specimens properly killed and preserved are about as good as
fresh material, and by modifying the suggested sequence of work a little
to suit special conditions or conveniences, the examination of live
specimens in the laboratory can in most cases be accomplished.
The author believes that elementary zoological study should not be
limited to the examination of the structure of several types. The
student should learn by observation something of the functions of
animals and something of their life-history and habits, and should be
given a glimpse of the significance of his particular observations and
of their general relation to animal life as a whole. The drill of the
laboratory is perhaps the most valuable part of the work, but as a
matter of fact the high school is trying to teach elementary zoology,
an elementary knowledge of animals and their life, and dissection
alone cannot give the pupil this knowledge. On the other hand, without
a personal acquaintance with animals, based on careful actual
observations of their life-history and habits and on the study of the
structural characters of the animal body by personally made
dissections, the pupil can never really appreciate and understand the
life of animals. Reading and recitation alone can never give the
student any real knowledge of it.
The book is divided into three parts, of which Part I should be[1]
first undertaken. This is an introduction to an elementary knowledge of
animal structure, function, and development. It consists of practical
exercises in the laboratory, each followed by a recitation in which the
significance of the facts already observed is pointed out. The general
principles of zoology are thus defined on a basis of observed facts.
Part II is devoted to a consideration of the principal branches of the
animal kingdom; it deals with[2] systematic zoology. In each branch
one or more examples are chosen to serve as types. The most important
structural features of these examples are studied, by dissection, in
the laboratory. The directions for these dissections consist of
technical instructions for dissecting, the calling attention to and
naming of principal parts, together with questions and demands
intended to call for independent work on the part of the student. The
directions follow the actual course of the dissection instead of being
arranged according to systems of organs, and are intended for the
orientation of the student and not to be in themselves expositions of
the anatomy of the types. The condensation of these directions is made
more feasible by the presence of anatomical plates (drawn directly
from dissections). Following the account of the dissection of the type
are brief notes on its life-history and habits. Then follows a
general account of the branch to which the example dissected belongs
and brief accounts of some of the more interesting members of the
branch. In these accounts technical directions are given for brief
comparative examinations and for the study of the life-history and
habits of some of the more accessible of these forms.
It will not be possible, of course, to undertake with any thoroughness
the consideration of all of the branches of animals in a single year.
But all are treated in the book, so that the choice of those to be
studied may rest with the teacher. This choice will of necessity depend
largely on the opportunities afforded by the situation of the school,
as, for example, whether on the seashore or in the interior near a lake
or river, or on the dry plains, and on the relation of the school-terms
to the seasons of the year. The branches are arranged in the book so
that the simplest animals are first considered, the slightly complex
ones next, and lastly the most highly organized forms. But if in order
to obtain examples for study it is necessary to take up branches
irregularly, that need not prove confusing. The author would suggest
that whatever other branches are studied, the insects and birds, which
are readily available in all parts of the country, be certainly
selected, and with this selection in view has given them special
attention. Indeed some teachers may find these two branches to offer
quite sufficient work in classificatory and ecological lines.
Part III is devoted to a necessarily brief consideration of certain of
the more conspicuous and interesting features of animal ecology. It
has in it the suggestion for much interesting field-work. The work of
this part should be taken up in connection with that of Part II, as,
for example, the consideration of social and communal life in
connection with the insects, parasitism in connection with the worms,
and also with the insects, distribution in connection with the birds,
perhaps, and so on.
In appendices there are added some suggestions for the outfitting of
the laboratory, and a list of the equipment each student should have.
Here, also, is appended a list of a few good authoritative reference
books which should be accessible to students and to which specific
references are made in the course of this book. Some practical
directions for the collecting and preserving of specimens are also
given. (Suggestions for the obtaining of material for the various
laboratory exercises outlined in the book are to be found in
"technical notes" included in the directions for each exercise.) The
author believes that the building up of a single school-collection in
which all the pupils have a common interest and to which all
contribute is to be encouraged rather than the making of separate
collections by the pupils. Waste of life is checked by this, and in
time, with the contributions of succeeding classes, a really good and
effective collection may be built up. The "collecting interest" can be
taken advantage of just as well in connection with a school-collection
as with individual collections.
The plates illustrating the dissections have all been drawn originally
for the book from actual dissections. Most of the other figures are
original, either drawn or photographed directly from nature, or from
preserved specimens. Credit is given in each case for figures not
original. The drawings for all of the figures of dissections and for
all original figures not otherwise accredited were made by Miss Mary
H. Wellman, to whom the author expresses his obligations. The thanks
of the author are due to Mr. George Otis Mitchell, San Francisco, who
kindly made the photo-micrographs of insect structure from the
author's slides; to Professor Mark V. Slingerland, Cornell University,
for electros of his photographs of insects; to Dr. L. O. Howard, U.
S. Entomologist, for electros of figs. 45, 52, 56, 68, 81, 82, 83, 84,
87, 90, and 92; to Professor L. L. Dyche, University of Kansas, for
photographs of his mounted groups of mammals; to Mrs. Elizabeth
Grinnell, Pasadena, Calif., for photographs of birds; to Mr. J. O.
Snyder, Stanford University, for photographs of snakes; to Mr. Frank
Chapman, editor of "Bird-lore," for electros of photographs of birds;
to Mr. G. O. Shields, editor of "Recreation," for an electro of the
photograph of a bird; to the American Society of Civil Engineers for
electros of photographs of boring marine worms; to Cassell & Co., for
electros of three photographs from nature; to Geo. A. Clark, secretary
Fur Seal Commission for photographs of seals; and to the Whitaker and
Ray Co., San Francisco, for electros of figs. 46, 59, 60, 61, 64, 65,
93, 94, 97, 98, 99, 100, 102, 119, and 166 to 172, published
originally in Jenkins & Kellogg's "Lessons in Nature Study." The
origin of each of these pictures is specifically indicated in
connection with its use in the book.
The author's sincere thanks are also due to Mrs. David Starr Jordan
and to Mr. J. C. Brown, graduate student in zoology in Stanford
University, for their assistance in the correction of the MS., and in
the preparation of the laboratory exercises respectively. The chapters
of Part II relating to the vertebrates were read in MS. by President
David Starr Jordan, whose aid and courtesy are gratefully
acknowledged. Similar acknowledgments are due Professors Harold Heath
and R. E. Snodgrass for reading the proofs of the directions for the
laboratory exercises.
VERNON LYMAN KELLOGG.
STANFORD UNIVERSITY, May, 1901.
FOOTNOTES:
[1] This is true if a strictly logical treatment of the subject is
held to. As a matter of fact, it is often of advantage to begin with,
or at least to take up from the beginning in connection with the
indoor work, some field-work, such as the collecting and classifying
of insects and the observation of their metamorphosis. As most schools
begin work in the fall, advantage must be taken of the favorable
opportunities for field-work at the beginning of the year. These
opportunities are of course much less favorable in the winter.
[2] The classification of animals used in this book is that adopted in
Parker and Haswell's "Text-book of Zoology" (2 vols., 1897, Macmillan
Co.). Exception is made in the case of the worms, which are considered
as a single branch, Vermes, instead of as several distinct branches.
CONTENTS
PART I
STRUCTURE, FUNCTIONS, AND DEVELOPMENT OF
ANIMALS
I.--THE STUDY OF ANIMALS AND THEIR LIFE.
Our familiar knowledge of animals and their life, 1.--Zoology and its
divisions, 2.--A first course in Zoology, 3.
II.--THE GARDEN TOAD (BUFO LENTIGINOSUS).
[Laboratory exercise], 5.--External structure, 5.--Internal structure,
7.
III.--THE STRUCTURE AND FUNCTIONS OF THE ANIMAL BODY.
Organs and functions, 14.--The animal body a machine, 14.--The
essential functions or life-processes, 15.
IV.--THE CRAYFISH (CAMBARUS SP.).
[Laboratory exercise], 17.--External structure, 17.--Internal
structure, 21.
V.--THE MODIFICATION OF ORGANS AND FUNCTIONS.
Difference between crayfish and toad, 26.--Resemblances between
crayfish and toad, 27.--Modification of functions and structure to fit
the animal to the special conditions of its life, 29.--Vertebrate and
invertebrate, 30.
VI.--AMŒBA AND PARAMŒCIUM.
[Laboratory exercise], 31.--Amœba, 31.--The slipper-animalcule
(PARAMŒCIUM SP.), 34.
VII.--THE SINGLE-CELLED ANIMAL BODY; PROTOPLASM
AND THE CELL.
The single-celled animal body, 36.--The cell, 37.--Protoplasm, 39.
VIII.-CELLULAR STRUCTURE OF THE TOAD (OR FROG).
[Laboratory exercise], 40.--The blood, 40.--The skin, 40.--The liver,
41.--The muscles, 41.
IX.--THE MANY-CELLED ANIMAL BODY; DIFFERENTIATION
OF THE CELL.
The many-celled animal body, 43.--Differentiation of the cell, 43.
X.--HYDRA.
[Laboratory exercise], 46.
XI.--THE SIMPLEST MANY-CELLED ANIMALS.
Cell-differentiation and body-organization in Hydra, 52.--Degrees in
cell-differentiation and body-organization, 54.
XII.--DEVELOPMENT OF THE TOAD.
[Field and laboratory exercise], 55.
XIII.--MULTIPLICATION AND DEVELOPMENT.
Multiplication, 57.--Spontaneous generation, 58.--Simplest
multiplication and development, 59.--Birth and hatching,
61.--Life-history, 62.
PART II
SYSTEMATIC ZOOLOGY
XIV.--THE CLASSIFICATION OF ANIMALS.
[Laboratory exercise and recitation], 65.--Basis and significance of
classification, 65.--Importance of development in determining
classification, 67.--Scientific names, 68.--An example of
classification, 68.--Species, 69.--Genus, 70.--Family, 72.--Order,
72.--Class and branch, 73.
XV.--BRANCH PROTOZOA: THE ONE-CELLED ANIMALS.
EXAMPLE: THE BELL ANIMALCULE (VORTICELLA SP.) [Laboratory exercise], 75.
OTHER PROTOZOA.
Form of body, 78.--Marine Protozoa, 80.
XVI.--BRANCH PORIFERA: THE SPONGES.
EXAMPLE: THE FRESH-WATER SPONGE (SPONGILLA SP.) [Laboratory exercise],
84.
EXAMPLE: A CALCAREOUS OCEAN-SPONGE (GRANTIA SP.) [Laboratory
exercise], 85.
EXAMPLE: A COMMERCIAL SPONGE [Laboratory exercise], 86.
OTHER SPONGES.
Form and size, 87.--Skeleton, 88.--Structure of body, 88.--Feeding
habits, 88.--Development and life-history, 89.--The sponges of
commerce, 90.--Classification, 91.
XVII.--BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES,
CORALS, AND JELLYFISHES.
POLYPS, SEA-ANEMONES, CORALS, AND JELLYFISHES.
General form and organization of body, 93.--Structure, 94.--Skeleton,
95.--Development and life-history, 95.--Classification, 96.--The
polyps, colonial jellyfishes, etc. (Hydrozoa), 97.--The large
jellyfishes, etc. (Scyphozoa), 101.--The sea-anemones and corals
(Actinozoa), 102.--The Ctenophora, 107.
XVIII.--BRANCH ECHINODERMATA: THE STARFISHES, SEA-URCHINS,
SEA CUCUMBERS, ETC.
EXAMPLE: STARFISH (ASTERIAS SP.) [Laboratory exercise].--External
structure, 108.--Internal structure, 110.--Life-history and habits, 113.
EXAMPLE: SEA-URCHIN (STRONGYLOCENTRUS SP.) [Laboratory
exercise].--External structure, 113.
OTHER STARFISHES, SEA-URCHINS, SEA CUCUMBERS, ETC.
Shape and organization of body, 116.--Structure and organs,
117.--Development and life-history, 119.--Classification,
120.--Starfishes (Asteroidea), 121.--Brittle stars (Ophiuroidea),
122.--Sea-urchins (Echinoidea), 123.--Sea-cucumbers (Holothuroidea),
124.--Feather-stars (Crinoidea), 125.
XIX.--BRANCH VERMES: THE WORMS.
EXAMPLE: THE EARTHWORM (LUMBRICUS SP.) [Laboratory
exercise].--External structure, 127.--Internal structure,
129.--Life-history and habits, 133.
OTHER WORMS.
Classification, 135.--Earthworms and leeches (Oligochætæ), 136.--Flat
worms (Platyhelminthes). 137.--Round worms (Nemathelminthes),
140.--Wheel-animalcules (Rotifera), 142.
XX.--BRANCH ARTHROPODA: THE CRUSTACEANS, CENTIPEDS,
INSECTS, AND SPIDERS.
CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS, ETC.
EXAMPLE: THE CRAYFISH (CAMBARUS SP.). Structure, 146.--Life-history
and habits, 146.
OTHER CRUSTACEANS.
Body form and structure, 147.--Water-fleas (_Cyclops_),
148.--Wood-lice (_Isopoda_), 150.--Lobsters, shrimps, and crabs
(Decapoda), 151.--Barnacles, 155.
XXI.--BRANCH ARTHROPODA (CONTINUED).
CLASS INSECTA: THE INSECTS.
EXAMPLE: THE RED-LEGGED LOCUST (MELANOPLUS FEMUR-RUBRUM). [Laboratory
exercise]. External structure, 157.--Life-history and habits, 161.
EXAMPLE: THE WATER-SCAVENGER BEETLE (HYDROPHILUS SP.) [Laboratory
exercise]. External structure, 163.--Internal structure,
166.--Life-history and habits, 169.
EXAMPLE: THE MONARCH BUTTERFLY (ANOSIA PLEXIPPUS) [Laboratory
exercise]. External structure, 171.--Life-history and habits, 175.
EXAMPLE: LARVA OF MONARCH BUTTERFLY [Laboratory exercise]. Structure,
177.
OTHER INSECTS.
Body form and structure, 181.--Development and life-history,
188.--Classification, 191.--Locusts, cockroaches, crickets, etc.
(Orthoptera), 192.--The dragon-flies and May-flies (Odonata and
Ephemerida), 194.--The sucking-bugs (Hemiptera), 197.--The flies
(Diptera), 201.--The butterflies and moths (Lepidoptera), 205.--The
beetles (Coleoptera), 206.--The ichneumon flies, ants, wasps, and bees
(Hymenoptera), 212.
CLASS MYRIAPODA: THE CENTIPEDS AND MILLIPEDS.
CLASS ARACHNIDA: THE SCORPIONS, SPIDERS, MITES, AND TICS.
XXII.--BRANCH MOLLUSCA: THE MOLLUSCS.
EXAMPLE: THE FRESH-WATER MUSSEL. (UNIO SP.) [Laboratory exercise].
Structure, 239.--Life-history and habits, 243.
OTHER MOLLUSCS.
Body form and structure, 245.--Development, 246.--Classification,
246.--Clams, scallops, and oysters (Pelecypoda), 246.--Snails, slugs,
nudibranchs, and "sea-shells" (Gastropoda), 252.--Squids,
cuttlefishes, and octopi (Cephalopoda), 255.
XXIII.--BRANCH CHORDATA: THE ASCIDIANS, VERTEBRATES,
ETC.
Structure of the vertebrates, 259.--Classification of the Chordata,
260.--The ascidians, 261.
XXIV.--BRANCH CHORDATA (CONTINUED).
CLASS PISCES: THE FISHES.
EXAMPLE: THE GOLDEN SUNFISH (EUPOMOTIS GIBBOSUS) [Laboratory
exercise]. External structure, 263.--Internal structure,
265.--Life-history and habits, 270.
OTHER FISHES.
Body form and structure, 271.--Development and life-history,
276.--Classification, 277.--The lancelets (Leptocardii), 277.--The
lampreys and hag-fishes (Cyclostomata), 278.--The true fishes
(Pisces), 279.--The sharks, skates, etc. (Elasmobranchii), 279.--The
bony fishes (Teleostomi), 281.--Habits and adaptations,
285.--Food-fishes and fish-hatcheries, 288.
XXV.--BRANCH CHORDATA (CONTINUED).
CLASS BATRACHIA: THE BATRACHIANS.
Body form and organization, 292.--Structure, 293.--Life-history and
habits, 295.--Classification, 297.--Mud-puppies, salamanders, etc.
(Urodela), 297.--Frogs and toads (Anura), 299.--Cœcilians
(Gymnophiona), 302.
XXVI.--BRANCH CHORDATA (CONTINUED).
CLASS REPTILIA: THE SNAKES, LIZARDS, TURTLES, CROCODILES, ETC.
EXAMPLE: THE GARTER SNAKE (THAMNOPHIS SP.) [Laboratory exercise].
Structure, 303.--Life-history and habits, 308.
OTHER REPTILES.
Body form and organization, 310.--Structure, 311.--Life-history and
habits, 312.--Classification, 313.--Tortoises and turtles (Chelonia),
314.--Snakes and lizards (Squamata), 317.--Crocodiles and alligators
(Crocodilia), 325.
XXVII.--BRANCH CHORDATA (CONTINUED).
CLASS AVES: THE BIRDS.
EXAMPLE: THE ENGLISH SPARROW (PASSER DOMESTICUS) [Laboratory
exercise]. External structure, 327.--Internal structure [Laboratory
exercise], 329.--Life history and habits, 335.
OTHER BIRDS.
Body form and structure, 336.--Development and life-history,
339.--Classification, 340.--The ostriches, cassowaries, etc. (Ratitæ),
341.--The loons, grebes, auks, etc. (Pygopodes), 343.--The gulls,
terns, petrels, and albatrosses (Longipennes), 345.--The cormorants,
pelicans, etc. (Steganopodes), 346.--The ducks, geese, and swans
(Anseres), 347.--The ibises, herons, and bitterns (Herodiones),
347.--The cranes, rails, and coots (Paludicolæ), 348.--The snipes,
sand-pipers, plovers, etc. (Limicolæ), 349.--The grouse, quail,
pheasants, turkeys, etc. (Gallinæ), 358.--The doves and pigeons
(Columbæ), 351.--The eagles, hawks, owls, and vultures (Raptores),
351.--The parrots (Psittaci), 353.--The cookoos and kingfishers
(Coccyges), 354.--The woodpeckers (Pici), 354.--The whippoorwills,
chimney-swifts, and humming-birds (Macrochires), 356.--The perchers
(Passeres), 357.--Determining and studying the birds of a locality,
359.--Bills and feet, 362.--Flight and songs, 364.--Nestling and care
of the young, 366.--Local distribution and migration, 367.--Feeding
habits, economics, and protection of birds, 370.
XXVIII.--BRANCH CHORDATA (CONTINUED).
CLASS MAMMALIA: THE MAMMALS.
EXAMPLE: THE MOUSE (MUS MUSCULUS) [Laboratory exercise]. Structure,
373.--Life-history and habits, 379.
OTHER MAMMALS.
Body form and structure, 381.--Development and life-history,
387.--Habits, instincts, and reason, 387.--Classification, 388.--The
opossums (Marsupialia), 389.--The rodents or gnawers (Glires),
390.--The shrews and moles (Insectivora), 391.--The bats (Chiroptera),
391.--The dolphins, porpoises, and whales (Cete), 393.--The hoofed
mammals (Ungulata), 394.--The carnivores (Feræ), 396.--The man-like
mammals (Primates), 398.
PART III
ANIMAL ECOLOGY
XXIX.--THE STRUGGLE FOR EXISTENCE, ADAPTATION,
AND SPECIES-FORMING.
The multiplication and crowding of animals, 404.--The struggle for
existence, 406.--Variation and natural selection, 406.--Adaptation and
adjustment to surroundings, 407.--Species forming, 408.--Artificial
selection, 409.
XXX.--SOCIAL AND COMMUNAL LIFE, COMMENSALISM, AND
PARASITISM.
Social life and gregariousness, 410.--Communal life,
411.--Commensalism, 413.--Parasitism, 415.
XXXI.--COLOR AND PROTECTIVE RESEMBLANCES.
Use of color, 424.--General, variable, and special protective
resemblance, 426.--Warning colors, terrifying appearances, and
mimicry, 430.--Alluring coloration, 433.
XXXII.--THE DISTRIBUTION OF ANIMALS.
Geographical distribution, 435.--Laws of distribution, 437.--Modes of
migration and distribution, 437.--Barriers to distribution,
438.--Faunæ and zoogeographic areas, 440.--Habitat and species,
441.--Species-extinguishing and species-forming, 442.
APPENDICES
EQUIPMENT AND METHODS
APPENDIX I.--EQUIPMENT AND NOTES OF PUPILS.
Equipment of pupils, 447.--Laboratory drawings and notes, 447.--Field
observations and notes, 448.
APPENDIX II.--LABORATORY EQUIPMENT AND METHODS.
Equipment of laboratory, 450.--Collecting and preparing material for
use in the laboratory, 451.--Obtaining marine animals, microscopic
preparations, etc., 453.--Reference-books, 454.
APPENDIX III.--REARING ANIMALS AND MAKING COLLECTIONS.
Live cages and aquaria, 457.--Making collections, 461.--Collecting and
preserving insects, 463.--Collecting and preserving birds,
466.--Collecting and preserving mammals, 470.--Collecting and
preserving other animals, 472.
PART I
STRUCTURE, FUNCTIONS, AND DEVELOPMENT
OF ANIMALS
CHAPTER I
THE STUDY OF ANIMALS AND THEIR LIFE
=Our familiar knowledge of animals and their life.=--We are familiarly
acquainted with dogs and cats; less familiarly probably with toads and
crayfishes, and we have little more than a bare knowledge of the
existence of such animals as seals and starfishes and reindeer. But
what real knowledge of dogs and toads does our familiar
acquaintanceship with them give? Certain habits of the dog are known
to us: it eats, and eats certain kinds of food; it runs about; it
responds to our calls or even to the mere sight of us; it evidently
feels pain when struck, and shows fear when threatened. Another class
of attributes of the dog includes those things that we know of its
bodily make-up: its possession of a head with eyes and ears, nose and
mouth; its four legs with toes and claws; its covering of hair. We
know, too, that it was born alive as a very small helpless puppy which
lived for a while on food furnished by the mother, and that it has
grown and developed from this young state to a fully grown, fully
developed dog. We know also that our dog is a certain kind of dog, a
spaniel, perhaps, while our neighbor's dog is of another kind, a
greyhound, it may be. We know accordingly that there are different
kinds of tame dogs, and we may know that wolves are so much like dogs
that they might indeed be called wild dogs, or dogs called a kind of
tame wolf. But how little we really know about the dog's body and its
life is apparent at a moment's thought. We see only the outside of the
dog, but what an intricate complex of parts really composes this
animal! We see it eat and breathe and run; of what is done with the
food and air inside its body, and of the series of muscle contractions
and mechanical processes which cause its running, we have but the
slightest conception. We see that the pup gets larger, that is, grows;
that it changes gradually in appearance, that is, develops; but of the
real processes and changes that take place in growth and development
how little we know! We know that there are other kinds of dogs; that
wolves and foxes are relatives of the dog; and we have heard that cats
and tigers are relatives also, although more distant ones. We know,
too, that all the backboned animals, some of them very unlike dogs,
are believed to be related to each other, but of the thousands of
these animals and of their relationships our knowledge is scanty.
Finally, of the relations of the dog, and of other animals, to the
outside world, and of the wonderful manner in which the dog's make-up
and behavior fit it to live in its place in the world under the
conditions that surround it, we have probably least knowledge of all.
=Zoology and its divisions.=--What things we do know about the dog,
however, and about its relatives, and what things others know, can be
classified into several groups, namely, things or facts about what the
dog does, or its behavior, things about the make-up of its body,
things about its growth and development, things about the kind of dog
it is and the kinds of relatives it has, and things about its
relations to the outer world, and its special fitness for life.
All that is known of these different kinds of facts about the dog
constitutes our knowledge of the dog and its life. All that is known
by scientific men and others of these different kinds of facts about
all the 500,000 or more kinds of living animals, constitutes our
knowledge of animals and is the science _zoology_.[3] Names have been
given to these different groups of facts about animals. The facts
about the bodily make-up or structure of animals constitute that part
of zoology called animal _anatomy_ or _morphology_; the facts about
the things animals do, or the functions of animals, compose animal
_physiology_; the facts about the development of animals from young to
adult condition are the facts of animal _development_; the knowledge
of the different kinds of animals and their relationships to each
other is called _systematic_ zoology or animal _classification_; and
finally the knowledge of the relations of animals to their external
surroundings, including the inorganic world, plants and other animals,
is called animal _ecology_.
Any study of animals and their life, that is, of zoology, may include
all or any of these parts of zoology. Most zoologists do, indeed,
devote their principal attention to some one group of facts about
animals and are accordingly spoken of as anatomists, or physiologists,
systematists, and so on. But such a specialization of study should be
made only after the zoologist has acquired a knowledge of the
principal or fundamental facts in all the other branches of zoology.
=A first course in zoology.=--The first "course," then, in the study
of animals should include the fundamental facts in all these branches
or parts of zoology. That is what the course outlined in this book
tries to cover. But no text-book of zoology can really give the
student the knowledge he seeks. He must find out most of it for
himself; a text-book, based on the experiences of others, is chiefly
valuable for telling him how to work most effectively to get this
knowledge for himself. And the best students always find out things
which are not in books. Especially can the beginning student find out
things not known before, "new to science," as we say, about the
behavior and habits of animals, and their relations to their
surroundings. The life-history of comparatively few kinds of animals
is exactly known; the instincts and habits of comparatively few have
been studied in any detail. The kinds of food demanded, the feeding
habits, nest-building, care of the young, cunning concealment of nest
and self, time of egg-laying or of producing young, duration of the
immature stages and the habits and behavior of the young animals--a
host, indeed, of observations on the actual life of animals, remain to
be made by the "field naturalist." Any beginning student can be a
"field naturalist" and can find out new things about animals, that is,
can add to the science of zoology.
[Illustration: FIG. 1.--Dissection of the Garden Toad (_Bufo
lentiginosus_).]
FOOTNOTE:
[3] Zoology is formed from two Greek words: _zoon_, meaning animal,
and _logos_, meaning discourse.
CHAPTER II
THE GARDEN TOAD (_Bufo lentiginosus_)
LABORATORY EXERCISE
TECHNICAL NOTE.--Although this description is written for the toad
it will fit for the dissection of the frog. It will be found,
after casting aside a few ungrounded prejudices, that the toad is
the better for class dissection. Toads are best collected about
dusk, when they can be picked up in almost any garden in town or
in the country. During the spring many can be found in the ponds
where they are breeding. To kill the toad place it in an air-tight
vessel with a piece of cotton or cloth saturated in chloroform or
ether. When the toad is dead, wash off the specimen and put in a
dissecting pan for study. Several specimens should be placed in a
nitric acid solution for a day or so (for directions for
preparing, see p. 12) to be used later for the study of the
nervous system. Also several specimens should be injected for the
better study of the circulatory system. With an injecting mass
made as directed on p. 451 introduce through a small canula into
the ventricle of the heart. This will inject the arterial system,
and with increased pressure the injecting mass may be forced
through the valves of the heart, thus passing into the auricles
and throughout the venous system. After injecting use the specimen
fresh or after it has been preserved in 4% formalin.
=External structure.=--Note that the body of the toad is divided into
several principal regions or parts, as is the human body, namely, a
_head_, _upper limbs_, _trunk_, and _lower limbs_. As you look at the
toad note the similarity of the parts on one side to those of the other,
as right leg corresponding to left leg, right eye to left eye, etc. This
arrangement of the body in similar halves among animals is known as
_bilateral symmetry_. As a rule animals which show bilateral symmetry
move in a definite direction. The part that moves forward is the
_anterior end_, while the opposite extremity is the _posterior end_. In
most animals we note two other views or aspects; that which is called
the "back" and with most animals is, under ordinary conditions,
uppermost is the _dorsum_ or _dorsal aspect_, while that which lies
below is the _venter_ or _ventral aspect_. When referring to a view from
one side we speak of it as a right or left _lateral aspect_. These terms
hold good for most of the animals that we shall study.
Note at the anterior end of the toad a wide transverse slit, the
_mouth_. What other openings are on the anterior end? Note the two large
_eyes_, the organs of sight. Just back of each eye note an elliptical,
smooth membrane. This is the tympanum of the outer _ear_, and through
this membrane the vibrations produced by sound-waves are transferred to
the inner ear, which receives sensations and transmits them to the
brain. Open the mouth by drawing down the lower jaw. Note just within
the angle of the lower jaw the _tongue_. How is it attached to the wall
of the mouth? On the tongue are a great many fine _papillæ_ in which is
located the sense of taste. It has now been seen that most of the
special senses of the toad have their seat in the head. Pass a straw or
bristle into one of the nostrils. Where does it come out? These internal
openings to the nose are the _inner nares_. Note in the roof of the
mouth just posterior to each of the eyeballs an opening. These are the
internal openings to the wide _Eustachian tubes_, which lead to the
mouth from the chamber of the ear behind the tympanum.
Note far back in the mouth an opening through which food passes. This
is the _œsophagus_ or _gullet_. Note just below this gullet an
elevation in which is a perpendicular slit, the _glottis_. This is the
upper end of the _laryngo-tracheal chamber_, and the flaps within on
either side of the slit are the _vocal cords_.
Note at the posterior end of the body in the median line an opening.
This is the _anal opening_ or _anus_. Note the general make-up of the
toad. How do its arms compare with our own? How do its fore feet
(hands) differ from its hind feet? Note that the body is covered by a
tough enveloping membrane, the _skin_. In the skin are many glands
which by their excretion keep it soft and moist.
=Internal structure.=--TECHNICAL NOTE.--With a fine pair of
scissors make a longitudinal median cut through the skin of the
venter from the anal opening to the angle of the lower jaw. Spread
the cut edges apart and pin back in the dissecting-pan.
Note the complex system of _muscles_ which govern the movements of the
tongue. Observe a number of pairs of muscles overlying the bones which
support the arms. These are attached to the _pectoral_ or
_shoulder-girdle_. Note the large sheet of muscles covering the
ventral aspect of the toad. These are the _abdominal muscles_, which
consist of two sets, an outer and an inner layer. Note that
posteriorly the abdominal muscles are attached to a bone. This is the
_pubic bone_ of the _pelvic girdle_ which supports the hind legs.
TECHNICAL NOTE.--With the scissors cut through the muscles of the
body wall at the pubic bone and pass the points forward to the
shoulder-girdle. Separate the bones of the shoulder-girdle and pin
out the flaps of skin and muscle to right and left in the
dissecting-pan (see fig. 1). Cover the dissection with clear water
or weak alcohol.
Note two large conspicuous soft brown lobes of tissue. These form the
_liver_, an organ which produces a secretion that assists in the process
of digestion. Note just anterior to the liver and extending between its
two lobes a pear-shaped organ, the _heart_, which may yet be pulsating.
Are these pulsations regular? How many occur in a minute? The lower end
or apex of the heart, _ventricle_, undergoes a contraction, forcing
blood out into the _blood-vessels_. This is followed by a relaxation of
the apex and a contraction of the basal portion, the _auricle_. The
heart is surrounded by a delicate semi-transparent sac, the
_pericardium_. The pericardium is filled with a watery fluid,
_body-lymph_, which bathes the heart. Note between the lobes of the
liver a small bladder-shaped transparent organ of a pinkish color. This
is the _gall-cyst_, or _gall-bladder_, a reservoir for the _bile_, the
secretion from the liver. Separate the lobes of the liver and note,
beneath, the long convoluted tube which fills most of the body-cavity.
This is part of the _alimentary canal_. Is the alimentary canal of
uniform character? The most anterior portion of the canal, the _gullet_
or _œsophagus_, leads to a large U-shaped enlargement, the _stomach_.
From the lower end of the stomach there extends a long, slender, very
much convoluted tube, the _small intestine_, which is followed by a much
larger one, the _large intestine_. This large intestine after one or two
turns passes directly back into the _rectum_, which opens at last to the
exterior through the anus. Note just ventral to the rectum a large
thin-walled membranous sac. This is the _urinary bladder_ which acts as
a reservoir for the secretion from the _kidneys_. Notice a many-branched
yellow structure with a glistening appearance, the _fat-body_ (_corpus
adiposum_). Now push liver and intestine to one side and note the
pinkish sac-like bodies (perhaps filled with air), the _lungs_. The
lungs are paired bodies which open into the laryngo-tracheal chamber.
The toad takes air into its mouth through its nostrils, and then forces
it, by a kind of swallowing action, through the laryngo-tracheal chamber
into the lungs.
Now lift the stomach and note in the loop between its lower end and the
small intestine a thin transparent tissue. This is a part of the
_mesentery_, which will be found to suspend the whole alimentary canal
and its attached organs to the dorsal wall of the body. Note in the
loop of the stomach in the mesentery an irregular pinkish glandular
structure which leads by a small duct into the intestine. This gland is
the _pancreas_, and the duct is the _pancreatic duct_. From it comes a
secretion which aids in the digestion of food. Near the upper end of the
pancreas note a round nodular structure, generally dark red. This is the
_spleen_, a ductless gland, the use of which is not altogether known.
Make a drawing which will show as many of the organs noted as possible.
TECHNICAL NOTE.--Pass two pieces of thread under the rectum near
the pubic bone. Tie these threads tightly a short distance apart
and then cut the rectum in two between the threads. Now carefully
lift up the alimentary canal with attached organs (liver, etc.),
and cut it off near the region of the heart.
How is the heart situated with regard to the lungs? The _heart_
consists of a lower chamber with thick muscular walls, the tip, called
the _ventricle_, and two upper thin-walled chambers, the _right_ and
_left auricles_. Can you make out these three chambers? The purified
blood from the lungs flows into the left auricle, while the venous
blood from all over the body laden with its carbon dioxide enters the
right auricle. From these two chambers the blood enters the ventricle.
Here the pure and impure blood are mixed. From the ventricle the blood
enters a large muscular tube on the ventral side of the heart. This is
the _conus arteriosus_, which gives off three branches on each side;
the anterior ones, the _carotid arteries_, supply the head, the next
ones, the _systemic arteries_, or _aortæ_, carry blood to the rest of
the body, while the posterior vessels, the _pulmonary arteries_, go
directly to the lungs and there break up into fine vessels
(_capillaries_) where the carbon dioxide is given off and oxygen is
taken from the air. From the lungs the blood returns through the
_pulmonary vein_ to the left auricle. Meanwhile the blood which has
passed through the systemic arteries and body capillaries is collected
again into other vessels going back to the heart; these are the
_veins_, which empty into a large thin-walled reservoir, the _sinus
venosus_, which in turn connects with the right auricle of the heart.
Three large veins enter the sinus venosus, namely, two _pre-caval
veins_ at the anterior end, and a single _post-caval vein_ at the
posterior end. Trace out the larger arteries and veins from the heart
to their division into or origin from the smaller vessels.
TECHNICAL NOTE.--Carefully remove the heart together with the lungs.
The lungs may be inflated by blowing into them through the
laryngo-tracheal chamber with a quill and tying them tightly, after
which they should be left for several days to dry. When perfectly
dry, sections may be cut through them in various places with a sharp
knife, and by this means a very good idea of the simple lung
structure of the lower backboned animals can be obtained. With a
sharp knife cut the heart open, beginning at the tip (ventricle) and
cutting up through the conus arteriosus and the two auricles. Note
the valves in the heart which separate the different compartments.
Note on either side of the median line in the dorsal region a pair of
reddish glandular bodies (_the kidneys_). From each kidney trace a
tube (_ureter_) posteriorly toward the region of the anus. The kidneys
are the principal excretory organs of the body. The blood which flows
through the delicate blood-vessels in the kidney gives up there much
of its waste products. These pass out through small tubules of the
kidneys into the ureters, which carry the wastes toward the anus.
Along one side of each kidney may be seen a yellowish glistening mass,
the _adrenal body_.
In some of the specimens studied, the body cavity may be filled with
thousands of little black spherical bodies. These are undeveloped
_eggs_. They are deposited by the mother toad in the water in long
strings of transparent jelly, which are usually wound around sticks or
plant-stems at the bottom of the pond near the shore. From these eggs
the young toads hatch as tadpoles and in their life-history pass
through an interesting metamorphosis. (See Chapter XII.)
TECHNICAL NOTE.--The teacher should be provided with several
well-cleaned skeletons of the toad in order that the bones may be
carefully studied. Boil in a soap solution a toad from which most
of the muscles and skin have been removed (see p. 452). Leave in
this solution until the muscles are quite soft and then pick off
all bits of muscles and tissue from the bones. If this is
carefully done, the ligaments which bind the bones will be left
intact and the skeleton will hold together.
Note that the _skeleton_ (fig. 2) consists of a head portion which is
composed of many bones joined together to form a bony box, the
_skull_; of a series of small segments, the _vertebræ_, forming the
_vertebral column_, which with the skull forms the _axial skeleton_;
and of the _appendicular skeleton_, consisting of the bones of the
fore and hind limbs. Note that the skull is composed of many bones
joined together, some by _sutures_, while others are fused. Do the
limbs attach directly to the axial skeleton? The anterior limbs (arms)
articulate with the _pectoral_ or _shoulder-girdle_. The arms will be
seen to be made up of a number of bones placed end to end. Note that
the uppermost, the _humerus_, is attached to the pectoral girdle,
while at its lower end it articulates with the _radio-ulna_. At the
lower end of the radio-ulna is a small series of _carpal_ bones which
afford attachments for the slender finger-bones, the _phalanges_ or
_digital_ bones. The bones of the leg are articulated with a closely
fused set of bones, the _pelvic girdle_. The leg-bones, proceeding
from the pelvic girdle, are named _femur_, _tibio-fibula_, _tarsal_
bones, and _phalanges_ or _digits_. To what bones of the arm do these
correspond? Determine the other principal bones of the skeleton by
reference to figure 2.
[Illustration: FIG. 2.--Skeleton of the garden toad.]
TECHNICAL NOTE.--In a specimen which has been macerated for some
time in 20% nitric acid dissect out the nervous system. Place the
specimen in a pan ventral side uppermost and pin out. Carefully pick
away the vertebræ and the roof of the mouth-cavity, thereby exposing
the central nervous system, which will appear light yellow.
Examine the _brain_. In front of the true brain are the _olfactory
lobes_, the nervous centre for the sense of smell. The brain itself is
composed of several parts. The anterior portion consists of two
elongated parts, the _cerebral hemispheres_; just back of these are the
optic lobes or _midbrain_, consisting of two short lobes, which are
followed by the small _cerebellum_, which in turn is followed by a long
part, the _medulla oblongata_, which runs imperceptibly into the long
dorsal nerve, the _spinal cord_. Note the large _optic nerves_ running
out to each eye. How far backward does the spinal cord extend? Note the
many pairs of nerves given off from the brain and spinal cord. These
nerves branch and subdivide until they end in very fine fibres. Some end
in the muscle-fibres, and through them the central nervous system
innervates the muscles. These are _motor endings_. Still others pass to
the surface and receive impressions from the outside. These last are
_sensory endings_. Note that the _spinal nerves_ arise from the spinal
cord by two roots, an _anterior_ or _ventral_, and a _posterior_ or
_dorsal root_. Trace the principal spinal nerves to the body-parts
innervated by them. These nerves are numbered as first, second, etc.,
according to the number of the vertebræ (counting from the head
backward) from behind which they arise.
CHAPTER III
THE STRUCTURE AND FUNCTIONS OF THE
ANIMAL BODY
=Organs and functions.=--The body of the toad is composed of various
parts, such as the lungs, the heart, the muscles, the eyes, the
stomach, and others. The life of the toad consists of the performance
by it of various processes, such as breathing, digesting food,
circulating blood, moving, seeing, and others. These various processes
are performed by the various parts of the body. The parts of the body
are called _organs_, and the processes (or work) they perform are
called their _functions_. The lungs are the principal organs for the
function of breathing; the heart, arteries and veins are the organs
which have for their function the circulation of the blood; the
principal organ concerned in the digestion of food is the alimentary
canal, the function of seeing is performed by the organs of sight, the
eyes, and so one might continue the catalogue of all the organs of the
body and of all the functions performed by the animal.
=The animal body a machine.=--The whole body of the toad is a machine
composed of various parts, each part with its special work or business
to do, but all depending on one another and all co-operating to
accomplish the total work of living. The locomotive engine is a
machine similarly composed of various parts, each part with its
special work or function, and all the parts depending on one another
and so working together as to perform satisfactorily the work for
which the locomotive engine is intended. An important difference
between the locomotive engine and the toad's body is that one is a
lifeless machine and the other a living machine. But there is a real
similarity between the two in that both are composed of special parts,
each part performing a special kind of work or function, and all the
parts and functions so fitted together as to form a complex machine
which successfully accomplishes the work for which it is intended. And
this similarity is one which should help make plain the fundamental
fact of animal structure and physiology, namely, the division of the
body into numerous parts or organs, and the division of the total work
of living into various processes which are the special work or
functions of the various organs.
=The essential functions or life-processes.=--The toad has a great
many different special parts in its body. Its body is very complex. It
performs a great many different functions, that is, does a great many
different things in its living. And the structure and life of most of
the other animals with which we are familiar are similarly complex: a
fish, or a rabbit, or a bird has a body composed of many different
parts, and is capable of doing many different things. Are all animals
similarly complex in structure, and capable of doing such a great
variety of things? We shall find that the answer to this question is
No. There are many animals in which the body is composed of but a few
parts, and whose life includes the performance of fewer functions or
processes than in the case of the toad. There are many animals which
have no eyes nor ears nor other organs of special sense. There are
animals without legs or other special organs of locomotion; some
animals have no blood and hence no heart nor arteries and veins. But
in the life of every animal there are certain processes which must be
performed, and the body must be so arranged or composed as to be
capable of performing these necessary life-processes. All animals take
food, digest it, and assimilate it, that is, convert it into new body
substance; all animals take in oxygen and give off carbonic acid gas;
all animals have the power of movement or motion (not necessarily
locomotion); all animals have the power of sensation, that is, can
feel; all animals can reproduce themselves, that is, produce young.
These are the necessary life-processes. It is evident that the toad
could still live if it had no eyes. Seeing is not one of the necessary
functions or processes of life. Nor is hearing, nor is leaping, nor
are many of the things which the toad can do; and animals can exist,
and do exist, without any of those organs which enable the toad to see
and hear and leap. But the body of any animal must be capable of
performing the few essential processes which are necessary to animal
life. How surprisingly simple such a body can be will be later
discovered. But in most animals the body is a complicated object, and
is able to do many things which are accessory to the really essential
life-processes, and which make its life complex and elaborate.
CHAPTER IV
THE CRAYFISH (_Cambarus_ sp.)
LABORATORY EXERCISE
TECHNICAL NOTE.--The crayfish, or crawfish, is found in most of
the fresh-water ponds and streams of the United States. (It is not
found east of the Hoosatonic River, Mass. In this region the
lobster may be used. On the Pacific coast the crayfishes belong to
the genus _Astacus_.) Crayfishes may be taken by a net baited with
dead fish, or they may be caught in a trap made from a box with
ends which open in, and baited with dead fish or animal refuse of
any sort. This box should be placed in a pond or stream frequented
by crayfish. If possible the student should study the living
animal and observe its habits. Crayfish which are to be kept alive
should be placed in a moist chamber in a cool place. They will
keep for a longer time in a moist chamber than in water. Some
fresh specimens should be injected by the teacher for the study of
the circulatory system. A watery solution of coloring matter or,
better, of an injecting mass of gelatine (see p. 451) is injected
into the heart through the needle of a hypodermic syringe. For the
purpose of injecting, a small bit of the shell may be removed from
the cephalothorax above the heart. Specimens which are to be kept
for some time should be placed in alcohol or 4% formalin.
=External structure= (fig. 3).--Place a specimen in a pan for study.
Note that the body, which of course differs much in shape from that of
the toad, is also unlike that of the toad in being covered by a hard
calcareous _exoskeleton_, which acts as a covering for the soft parts
and also as a place of attachment for the muscles, just as the
internal skeleton does in the case of the toad. The body is composed
of an anterior part, the _cephalothorax_, and a posterior part, the
_abdomen_. The cephalothorax is covered above and on the sides by the
_carapace_, which is divided into parts corresponding to the head and
thorax of the toad by the transverse _cervical suture_. The abdomen
is composed of segments. How many? The flattened terminal segment is
called the _telson_. Is the cephalothorax composed of segments? Where
is the mouth of the crayfish? Where is the anal opening?
[Illustration: FIG. 3.--Ventral aspect of crayfish (_Cambarus_ sp.),
with the appendages of one side disarticulated.]
At the anterior end of the cephalothorax note a sharp projection, the
_rostrum_. Where are the eyes? Remove one of them and examine its
outer surface with a microscope. A bit of the outer wall should be
torn off and mounted on a glass slide. Note that it is made up of a
great many little facets placed side by side. Each of these facets is
the external window of an eye element or _ommatidium_. An eye composed
in this way is called a _compound eye_. In front of the eyes note two
pairs of slender many-segmented appendages. The shorter pair, the
_antennules_, are two-branched. Remove one of them and note at its
base a small slit along the upper surface. This slit opens into a
small bag-like structure which contains fine sand-grains. The bag is
protected by a series of fine bristles along the edge of the slit.
This bag-like structure is believed to be an auditory organ. The
longer pair of appendages are the _antennæ_, and in the fine hair-like
projections upon the joints is believed to be located the sense of
smell. Thus it will be seen that the sense-organs of the crayfish,
like those of the toad, are located on the head. Beneath the basal
portion of each antenna there is a flat plate-like projection, at the
base of which on the upper edge will be noted a small opening, the
exit of the kidney, or _green gland_.
Make a drawing of the surface of part of an eye; also of an antennule;
and of an antenna.
TECHNICAL NOTE.--Stick one point of the scissors under the
posterior end of the carapace on the right side, and cut forward,
thus exposing a large cavity, the gill-chamber. Remove all of the
mouth-parts, legs and abdominal appendages from the right side,
being careful to leave the fringe-like parts, the gills, attached
to their respective legs. Place all of the appendages in order on
a piece of cardboard.
Examine the abdominal appendages, called _pleopods_, or swimming feet.
How many pairs are there? Each is composed of a basal part, the
_protopodite_, and two terminal segments, an inner one, the
_endopodite_, and an outer, the _exopodite_. In the males the first
and second pleopods of the abdomen are larger and less flexible than
the others. In the female the pleopods serve to carry the eggs and the
first two pairs are very small or absent. Note the last set of
abdominal appendages. These are the _uropods_, which together with the
telson form the tail.
Make a drawing of the pleopods of one side.
Examine the appendages of the cephalothorax. Like the appendages of
the abdomen the typical composition of each includes a protopodite, an
exopodite and an endopodite, but some of these appendages are much
modified, and show a loss of one of these parts, or the addition of an
extra part. The cephalothoracic appendages may be divided into three
groups, an anterior group of three pairs of mouth-parts (belonging to
the head) of which the first pair is the _mandibles_ and the others
are the _maxillæ_; a second group of three pairs of foot-jaws or
_maxillipeds_, belonging to the thorax, and a third group of five
pairs of _walking-legs_. The mandibles, lying next to the
mouth-opening, are hard and jaw-like and lack the exopodite; the first
maxillæ are small and also lack the exopodite; the second maxillæ have
a large paddle-like structure which extends back over the gills on
each side within the space, the _branchial chamber_, above the gills.
It is by means of this paddle-like structure (the _scaphognathite_)
that currents of water are kept up through the gill-chambers. The
maxillipeds increase in size from first to third pair. Each pair of
walking-legs except the last bears _gills_. These gills are the organs
by which the blood is purified. The blood of the crayfish flows into
the large vessels on the outer sides of the gill and thence into the
fine vessels in the little leaf-like lamellæ. At the same time the air
which is mixed with the water bathing the gills passes freely through
the thin membranous walls of these lamellæ and blood-vessels, and the
blood gives off its carbonic acid gas to the water and takes up oxygen
from the air in the water. Thus it will be seen that the office of the
gill is like that of the lung in the toad, namely, to act as an organ
for the elimination of carbonic acid gas and the taking up of oxygen.
Note the pincer-like appendages of the first pair of legs. These pincers
are the _chelæ_, with which food is torn into bits and placed in the
mouth. In the basal segment of each of the last pair of legs of the male
note the _genital pore_. In the female the _genital pores_ are in the
basal segments of the next to last pair of legs. Is the crayfish
bilaterally symmetrical? Note the repetition of parts in the crayfish,
that is, the recurrence of similar parts in successive segments. This
serial repetition of parts among animals is called _metemerism_.
=Internal structure= (fig. 4).--TECHNICAL NOTE.--With a pair of
scissors cut through the dorsal wall of the cephalothorax into the
body-cavity. Cut the body-wall away from both sides and remove the
middle portion.
[Illustration: FIG. 4.--Diagrammatic median longitudinal section of
crayfish (_Cambarus_ sp.).]
At the anterior end of the cephalothorax note the large membranous
sac, the _stomach_. Attached to each end of this are sets of muscles
which control its movements. To the right and left of the stomach
notice attached to the shell large muscles which connect by stout
ligaments at their lower ends with the mandibles. Note a yellow
fringe-like structure, the _digestive gland_, which fills most of the
region about the stomach. It connects by a pair of small tubes, the
_bile-ducts_, with the alimentary canal. Within the posterior portion
of the cephalothorax note a pentagonal sac, the _heart_, contained
within a delicate membrane, the _pericardium_. Remove the pericardium
and note a pair of dorsal openings into the heart, called _ostia_.
(There are also two lateral pairs and a ventral pair of ostia.) Note
passing anteriorly from the heart along the median line to the eyes a
blood-vessel, the _ophthalmic artery_. Arising from the anterior
portion of the heart are the _antennary arteries_, running to the
antennæ. Yet another pair running anteriorly from the heart to the
stomach and digestive glands are called the _hepatic arteries_. From
the posterior end of the heart arises the _dorsal abdominal artery_,
running back to the telson. Below this arises the _sternal_ artery,
which will be seen later.
In the region below the heart are located the reproductive organs.
They are whitish glandular masses from each of which runs a tube which
opens at the base of the last pair of walking-legs in the male, and at
the base of the third pair of walking-legs in the female.
TECHNICAL NOTE.--Cut longitudinally through the dorsal wall of the
abdomen on either side of the median line and remove the piece of
shell.
Note the powerful _muscles_ within which flex and extend the abdomen. By
a rapid contraction of these muscles the tail is brought beneath the
body, propelling the animal strongly backwards. When the crayfish crawls
it generally goes forward, but in swimming it reverses this direction.
Make a drawing showing, in their natural position, the internal organs
which have been studied.
Examine the alimentary canal for its whole length. Note that the large
bladder-shaped stomach is attached to the mouth-opening by a short
tube. What part of the canal is this? From the posterior end of the
stomach is a short thick-walled part, the _small intestine_, followed
by a long straight tube, the _large intestine_, which opens to the
exterior through the _anus_.
TECHNICAL NOTE.--Remove the alimentary canal, detaching it from
the anal end first, and working forward.
Cut the stomach open. Note an anterior portion, the _cardiac chamber_,
and a smaller posterior portion, the _pyloric chamber_. Examine its
inner surface. What do you find here? This structure is called the
_gastric mill_. Food, which for the most part consists of any dead
organic matter, is chewed by the "stomach-teeth" into fine bits, and
is then passed into the pyloric chamber. It is here that the digestive
glands empty their secretion into the food. These glands have the same
office as have the liver and pancreas combined in the toad, and so
they are often called the _hepato-pancreas_. When the stomach has been
removed there will be noted in the anterior portion of the body
paired, flattened bodies, already mentioned, which connect with
openings at the base of each of the antennæ by means of wide
thin-walled sacs, the _ureters_. These organs are the _kidneys_, or
_green glands_. Their office is similar to that of the kidneys in the
toad, namely, the elimination of waste from the body.
TECHNICAL NOTE.--Carefully remove all of the alimentary canal,
digestive glands, and reproductive organs. This process will
expose the floor of the cephalothorax. Now cut away from either
side the horny floor or bridge at the bottom of the cephalothorax.
If the specimen has not already been immersed, place it in clear
water for further dissection.
The foregoing dissection will expose the _central nervous system_. It
extends as a series of paired _ganglia_ connected by a double nerve-cord
along the ventral median line from the œsophagus to the last segment of
the abdomen. From what points do the lateral nerves arise? Anteriorly
the double nerve-cord divides, the two parts passing upward on each
side of the œsophagus, where they again meet to form the
_supra-œsophageal ganglion_ or _brain_. Where do the nerves run which
rise from the brain? What is the difference between the position of the
central nervous system in the crayfish and in the toad?
Make a drawing of the nervous system.
Just beneath the nerve-cord note a blood-vessel extending the length
of the body. This is the _sternal artery_, which arises from the
posterior end of the heart and passes ventrally at one side of the
alimentary canal and between the nerve-cords. Here the sternal artery
divides into an anterior and a posterior branch, from which lesser
branches are given off to each one of the appendages. The various
arteries running to all parts of the body finally pour out the blood
into the body-cavity, where it flows freely in the spaces among the
various tissues and organs. After the blood has bathed the body
tissues it flows to the gills on either side, passing up the outer
side of the gill through delicate thin-walled vessels, where it is
oxygenated as has already been described. From the gills the purified
blood flows back on the inner side through a large chamber, _sinus_,
into the pericardium, through the ostia of the heart, whence it is
driven into the arteries once more. This sort of a circulatory system
in which the blood in places is not enclosed in a definite vessel is
known as an _open system_. In the toad we find the blood in a _closed
system_, i.e., arteries leading into capillaries which in turn lead
into veins, in no case allowing the blood to pass freely through the
spaces of the body.
CHAPTER V
THE MODIFICATION OF ORGANS AND FUNCTIONS
=Differences between crayfish and toad.=--In the dissection of the
crayfish one of the most important things in the study of zoology has
been learned. It is plain that the crayfish has a body composed, like
the toad's, of parts or organs, and that most of these organs,
although differing much in appearance and actual structure from those
of the toad, correspond to similarly named organs of the toad, and
perform the same functions or processes, although with many striking
differences, essentially in the same way as in the toad. But the
structure of the body is very different in the two animals. The toad
has an internal body skeleton to which the muscles are attached, and a
soft, yielding, outer body-covering or skin; the crayfish has no
internal skeleton, but has its body covered by a horny, firm body-wall
to which the muscles are attached. The toad has its main nervous chain
lying just beneath the dorsal wall of the body; the crayfish has its
main nervous chain lying just above the ventral wall of the body. The
toad has lungs and takes up oxygen from the air of the atmosphere; the
crayfish has gills and takes up oxygen from the air which is mixed
with the water. The toad has a single pair of jaws; the crayfish has
several pairs of mouth-parts. The toad has four legs fitted for
leaping; the crayfish has numerous legs fitted for crawling or
swimming. The crayfish's body is composed of a series of body-rings
or segments; the toad's body is a compact apparently unsegmented mass.
The toad has eyes each with a single large lens and capable of moving
in the head and of changing their shape and hence their focus; the
crayfish's eyes are immovable and have a fixed focus, and are composed
of hundreds of tiny eyes each with lens and special retina of its own.
And so a long list of differences might be gone through with.
=Resemblances between toad and crayfish.=--But on the other hand there
are many resemblances--resemblances both in structure and life-processes
or physiology. Both toad and crayfish have organs for the prehension of
food, its digestion and its assimilation. And these organs, the organs
of the digestive system, while differing in details are alike in being
composed principally of a long tube, the alimentary canal, running
through the body, open anteriorly for the taking in of food, and open
posteriorly for the discharge of indigestible useless matter. Both
alimentary canals are divided into various special regions for the
performance of the various special processes connected with the
digestion and assimilation of food. Each is adapted for the special kind
of food which it is the habit of the particular animal to take. The two
sets of organs are essentially alike and have the same essential
function or life-process to perform. But this process differs in the
details of its performance, and the organs which perform this function
and which constitute the digestive system of each are modified to suit
the special habits or kind of life of the animal.
Both toad and crayfish have a heart with blood-vessels leading from
it. In the case of the toad the heart is more complex than in the
crayfish, and the system of blood-vessels is far more extensive and
elaborate. But the heart and blood-vessels in both animals subserve
the same purpose; their function is the circulation of the blood,
this being the means by which oxygen and food are carried to all
growing or working parts of the body, and by which carbonic acid gas
and other poisonous waste products are brought away from these parts.
But this function differs somewhat in its performance in the two
animals, and the organs which perform the function are correspondingly
modified in structural condition.
Both toad and crayfish have organs for respiration, that is, for
breathing in oxygen and breathing out carbonic acid gas. But the toad
takes its oxygen from the atmosphere about it; its respiratory organs
are the lungs, the sac-like tube leading to the mouth, and the
external openings for the ingress and exit of the gases. The crayfish,
living mostly in the water, takes its oxygen from the air which is
mechanically mixed with the water. Its respiratory organs are its
gills. There is a great difference, apparently, in the structural
conditions of the organs of respiration in the two animals. As a
matter of fact the difference is less great than, at first sight,
appears to be the case. The lungs of the toad are composed primarily
of a thin membrane, in the form of a sac, richly supplied with
blood-vessels. Air is brought to this thin respiratory membrane and by
osmosis the oxygen passes through the membrane and through the thin
walls of the fine blood-vessels, and is taken up by the blood. At the
same time the carbonic acid gas brought by the blood to the lungs from
all parts of the body is given up by it and passes through the
membranes in order to leave the body. The air comes in contact with
the respiratory membrane (which is situated inside the body) by means
of a system of external openings and a conducting chamber, and by
these same openings and chamber the carbonic acid gas leaves the body.
In the crayfish the gills are nothing else than a large number of
small flattened sacs each composed of a thin membrane richly supplied
with blood-vessels. This respiratory membrane is not, in the crayfish,
situated inside the body, but on the outer surface, although protected
by being in a sort of pocket with a covering flap, and it comes into
immediate contact with the air held in the water which freely bathes
the gills. By osmosis the oxygen of this air passes in through the
gill-membranes, while the carbonic acid gas brought by the blood
passes out through them. Exactly the same exchange of gases is
accomplished as in the toad. But because of the great difference in
the conditions of life of the toad and crayfish, one living in water,
the other living out of water, the character of the performance of the
function of respiration, and correspondingly the structural condition
of the organs performing this function, are strikingly different.
=Modification of functions and structure to fit the animal to special
conditions of its life.=--As has been done with the organs of
digestion, circulation, and respiration, so we might compare the other
organs of the crayfish and the toad. There would be found not only
many very marked differences between organs which have the same
general function in the two animals, but we should find also numerous
organs in the toad which are not present at all in the crayfish, and
conversely; and this means, of course, that the toad can do numerous
things, perform numerous functions, which the crayfish cannot, and,
conversely, that the crayfish does some things which the toad cannot.
But both of these animals agree in possessing in common the capability
of performing those processes such as taking food, breathing,
reproducing, etc., to which attention has been called as being
indispensable to all animal life. These processes, however, are
performed by the two animals in different ways and the organs for the
performance of these processes, although at very bottom essentially
alike, are in outer and superficial details of position, appearance
and general structure markedly different. Animals are fitted to live
in different places amid different surroundings by having their bodies
modified and the performance of their life-processes modified to suit
the special conditions of their life.
=Vertebrate and invertebrate.=--In selecting the toad and the crayfish
as the first animals to study and to compare with each other, we have
chosen representatives of the two great groups into which the complexly
organized animals are divided, viz., the group of backboned or
vertebrate animals, and the group of backboneless or invertebrate
animals. To the vertebrates belong all those which have an internal bony
skeleton (and a few without such a skeleton) and which have also an
arrangement of body-organs on the general plan of the toad's body. A
conspicuous feature of this arrangement is the situation of the spinal
cord or main great nerve-trunk along the back or dorsal wall of the
animal, and inside of a backbone. All the fishes, batrachians (frogs,
toads, salamanders, etc.), reptiles (snakes, lizards, alligators, etc.),
birds, and mammals (quadrupeds, whales, seals, etc.) belong to the
vertebrates. The backboneless or invertebrate animals have no internal
bony skeleton and have their main nerve-trunk usually along the ventral
wall of the body, sometimes in a circle around the mouth, but never in a
backbone. To the invertebrates belong all insects, lobsters, crabs,
clams, squids, snails, worms, starfishes and sea-urchins, corals and
sponges, altogether a great host of animals, mostly small.
CHAPTER VI
AMŒBA AND PARAMŒCIUM
LABORATORY EXERCISE
=Amœba.=--TECHNICAL NOTE.--_Amœbæ_ are found in stagnant pools of
water on the dead leaves, sticks and slime at the bottom. To
obtain them, collect slime and water from various puddles in
separate bottles and take them to the laboratory. Place a small
drop of slime on a slide under a cover-glass. Examine under the
low power first and note any small transparent or opalescent
objects in the field. Examine these objects with the higher power
and note that some are mere granular jelly-like specks, which
slowly (but constantly) change their form. These are _Amœbæ_.
A teacher of zoology recommends the following method of obtaining
a large supply of _Amœbæ_: "For rearing _Amœbæ_ place two or
three inches of sand in a common tub, which is then filled with
water and placed some feet from a north window; three or four
opened mussels, with merest trace of the mud from the stream in
which they are taken, are partially buried in the sand and a
handful of _Nitella_ and a couple of crayfish cut in two are
added; as decomposition goes on a very gentle stream is allowed
to flow into the tub, and after from two to four weeks abundant
_Amœbæ_ are to be found on the surface of the sand and in the
scum on the sides of the tub; small _Amœbæ_ appear at first, and
later the large ones."
Having found an _Amœba_ (fig. 5) note its irregular shape, and if it
moves actively observe its method of moving. How is this accomplished?
The viscous, jelly-like substance which composes the whole body of an
_Amœba_ is called _protoplasm_. The little processes which stick out
in various directions are the "false feet" (_pseudopodia_). Note that
the outer portion, the _ectosarc_, of the protoplasmic body is clear,
while the inner, the _endosarc_, is more or less granular in
structure. Has _Amœba_ a definite body-wall? Do the pseudopodia
protrude only from certain parts of the body? Within the endosarc
note a clear globular spot which contracts and expands, or pulsates,
more or less regularly. This is the _contractile vacuole_. Note the
small granules which move about within the endosarc. These are
food-particles which have been taken in through the body-wall. Note
how pseudopodia flow about food-particles in the water and how these
are digested by the protoplasm. If an _Amœba_ comes into contact with
a particle of sand, note how it at once retreats. Note within the
endosarc an oval transparent body which shows no pulsations. This is
the _nucleus_, a very complex little structure of great importance in
the make-up of _Amœba_.
[Illustration: FIG. 5.--_Amœba_ sp.; showing the forms assumed by a
single individual in four successive changes. (From life.)]
Note that _Amœba_ has no mouth or alimentary canal; no nostrils or
lungs, no heart or blood-vessels, no muscles, no glands. It is an
animal body not made up of distinct organs and diverse tissues. Its
whole body is a simple minute speck of protoplasm, a single animal
cell. But it takes in food, it moves, it excretes waste matter from
the body, is sensitive to the touch of surrounding objects, and, as we
may be able to see, it can reproduce itself, i.e., produce new
_Amœbæ_. _Amœba_ is the simplest living animal.
It is only rarely that we can find an _Amœba_ actually reproducing.
The process, in its gross features, is very simple. First the _Amœba_
draws in all of its pseudopodia and remains dormant for a time. Next,
certain changes take place in the nucleus, which divides into equal
portions, one part withdrawing to one end of the protoplasmic body,
the other to the opposite end. Soon the body protoplasm itself begins
to divide into two parts, each part collecting about its own half of
the nucleus. Finally the two halves pull entirely away from each other
and form two new _Amœbæ_, each like the original, but only half as
large. This is the simplest kind of reproduction found among animals.
_Amœbæ_ continue to live and multiply as long as the conditions
surrounding them are favorable. But when the pond dries up the _Amœbæ_
in it would be exterminated were it not for a careful provision of
nature. When the pond begins to dry up each _Amœba_ contracts its
pseudopodia and the protoplasm secretes a horny capsule about itself.
It is now protected from dry weather and can be blown by the winds
from place to place until the rains begin, when it expands, throws off
the capsule and commences active life again in some new pond.
=The Slipper Animalcule= (_Paramœcium_ sp.)--_Technical
Note_.--_Paramœcia_ can be secured in most pond water where leaves
or other vegetation are decaying. However, if specimens are not
readily secured place some hay or finely cut dry clover in a glass
dish, cover with water and leave in the sun for several days. In
this mixture specimens will develop by thousands. Place a drop of
water containing _Paramœcia_ on a slide with cover-glass over it.
Using a low power, note the many small animals darting hither and
thither in the field. Run a thin mixture of cherry gum in water
under the cover-glass. In this mixture they can be kept more quiet
and be better studied.
How does _Paramœcium_ (fig. 6) differ from _Amœba_ in form and
movement? Has the body an anterior and a posterior end? The delicate,
short, thread-like processes, on the surface of the body, which beat
about very rapidly in the water are called _cilia_, and they are
simply fine prolongations of the body protoplasm. What is their
function? Note a fine _cuticle_ covering the body. Note also many
minute oval sacs lying side by side in the ectosarc. These are called
_trichocysts_ and from each a fine thread can be thrust out.
Note on one side, beginning at the anterior end, the _buccal groove_
leading into the interior through the _gullet_. Observe also that by the
action of the cilia in the buccal groove food-particles are swept into
the gullet. Rejected or waste particles are ejected from the body
occasionally. Where? Note about midway of the _Paramœcium_ an ovoid body
with a smaller oval one attached to its side, the former being the
_macronucleus_, the latter the _micronucleus_. Note that there are two
contractile vacuoles in the _Paramœcium;_ also that the food-vacuoles
have a definite course in their movement inside the endosarc.
Make a drawing of a _Paramœcium_.
In comparing _Paramœcium_ with _Amœba_ it is apparent that the body of
the first is less simple than that of the second. The definite opening
for the ingress of food, the two nuclei, the fixed cilia, and the
definite cell-wall giving a fixed shape to the body, are all
specializations which make _Paramœcium_ more complex than _Amœba_. But
the whole body is still composed of a single cell, and there is, as in
_Amœba_, no differentiation of the body-substance into different
tissues, and no arrangement of body-parts as systems of organs.
_Paramœcium_ may occasionally be found reproducing. This process takes
place very much as in _Amœba_. The animal remains dormant for a while,
the micronucleus then divides, the macronucleus elongates and finally
divides in two, the protoplasm of the body becomes constricted into two
parts, each part massing itself about the withdrawn halves of the macro-
and micro-nuclei, and lastly the whole breaks into two smaller organisms
which grow to be like the original. After multiplication or reproduction
has gone on in this way for numerous generations (about one hundred), a
fusion of two _Paramœcia_ seems necessary before further divisions take
place. This process of fusion, called _conjugation_, may be noted at
some seasons. Two _Paramœcia_ unite with their buccal grooves together,
part of the macronucleus and micronucleus of each passes over to the
other, and the mixed elements fuse together to form a new macro- and
micronucleus in each half. The conjugating _Paramœcia_ now separate, and
each divides to form two new individuals.
[Illustration: FIG. 6.--_Paramœcium_ sp.; buccal groove at right.
(From life.)]
CHAPTER VII
THE SINGLE-CELLED ANIMAL BODY.--PROTOPLASM
AND THE CELL
=The single-celled body.=--The study of _Amœba_ and _Paramœcium_ has
made us acquainted with an animal body very different from that of the
toad or the crayfish. These extraordinarily minute animals have a body
so simple in its composition, compared with the toad's, that if the
toad's body be taken for the type of the animal body, _Amœba_ might
readily be thought not to be an animal at all. The body of _Amœba_ is
not composed of organs, each with a particular function or work to
perform. Whatever an _Amœba_ does is done, we may say, with its whole
body. But as we learn the things that this formless viscid speck of
matter does, we see that it is truly an animal; that it really does
those things which we have learned are the necessary life-processes of
an animal. _Amœba_ takes up and digests food composed of organic
particles; it has the power of motion; it knows when its body comes in
contact with some external object, that is, it can feel or has the
power of sensation. _Amœba_ takes in oxygen and gives out carbonic
acid gas, and it can produce new individuals like itself, that is, it
has the power of reproduction. But for the performance of these
various life-processes or functions it has no special parts or organs,
no mouth or alimentary canal, no lungs or gills, no legs, no special
reproductive organs. We have here to do with one of the "simplest
animals." With a minute, organless, soft speck of viscous matter
called protoplasm for a body, the simplest structural condition to be
found among living beings, _Amœba_ nevertheless is capable of
performing, in the simplest way in which they may be performed, those
processes which are essential to animal life.
_Paramœcium_ has a body a little less simple than _Amœba_. The
food-particles are taken into the body always at a certain spot; this
might be spoken of as a mouth. And the body has some special
locomotory organs, if they may be so called, in the presence of the
cilia. The body, too, has a definite shape or form. But, as in _Amœba_
there is no alimentary canal, nor nervous system, nor respiratory
system, nor reproductive system. The whole body feels and breathes and
takes part in reproduction.
A long jump has been made from the toad and crayfish to _Amœba_ and
_Paramœcium_; from the complex to the simplest animals. But, as will
later be seen, the great difference between the bodies of these
simplest animals and those of the highly complex ones is only a
difference of degree; there are animals of all grades and stages of
structural condition connecting the simplest with the most complex.
When animals are studied systematically, as it is called, we begin
with the simplest and proceed from them to the slightly complex, from
these to the more complex, and finally to the most complex. There are
hundreds of thousands of different kinds of animals, and they
represent all the degrees of complexity which lie between the extremes
we have so far studied.
=The cell.=--The characteristic thing about the body of _Amœba_ and
_Paramœcium_ and the other "simplest animals"--for there are many
members of the group of "simplest animals," or Protozoa--is that it is
composed, for the animal's whole lifetime, of a single cell. A cell is
the structural unit of the animal body. As will be learned in the
next exercise, the bodies of all other animals except the Protozoa,
the simplest animals, are composed of many cells. These cells are of
many kinds, but the simplest kind of animal cell is that shown by the
body of an _Amœba_, a tiny speck of viscous, nearly colorless
protoplasm without fixed form. The protoplasm composing the cell is
differentiated to form two parts or regions of the cell, an inner
denser part, called the nucleus, and an outer clearer part, called the
cytoplasm. Sometimes, as in the _Paramœcium_, the cell is enclosed by
a cell-wall which may be simply a denser outer layer of the cytoplasm,
or may be a thin membrane secreted by the protoplasm. Thus the cell is
not what its name might lead us to expect, typically cellular in
character; that is, it is not (or only rarely is) a tiny sac or box of
symmetrical shape. While the cell is composed essentially of
protoplasm, yet it may contain certain so-called cell-products, small
quantities of various substances produced by the life-processes of the
protoplasm. These cell-products are held in the protoplasmic body-mass
of the cell, and may consist of droplets of water or oil or resin, or
tiny particles of starch or pigment, etc. The cell cannot be said to
be composed of organs, because the word organ, as it is commonly used
in the study of an animal, is understood to mean a part of the animal
body which is composed of many cells. But the single cell can be
somewhat differentiated into parts or special regions, each part or
special region being especially associated with some one of the
life-processes. In _Paramœcium_, for example, the food is always taken
in through the so-called mouth-opening; the fine protoplasmic cilia
enable the cell to swim freely in the water, the waste products of the
body are always cast out through a certain part, and so on. But this
is a very simple sort of differentiation, and the whole body is only
one of those structural units, the cells, of which so many are
included in the body of any one of the complex animals.
=Protoplasm.=--The protoplasm, which is the essential substance of the
typical animal cell and hence of the whole animal body, is a substance
of very complex chemical and physical make-up. No chemist has yet been
able to determine its exact chemical constitution, and the microscope
has so far been unable to reveal certainly its physical characters.
The most important thing known about the chemical constitution of
protoplasm is that there are always present in it certain complex
albuminous substances which are never found in inorganic bodies. And
it is certain that it is on the presence of these substances that the
power possessed by protoplasm of performing the fundamental
life-processes depends. Protoplasm is the primitive physical basis of
life, but it is the presence of the complex albuminous substances in
it that makes it so.
The physical constitution of protoplasm seems to be that of a viscous
liquid containing many fine globules of a liquid of different density
and numerous larger globules of a liquid of still other density. Some
naturalists believe the fine globules to be solid grains, while still
others believe that numerous fine threads of dense protoplasm lie
coiled and tangled in the clearer, viscous protoplasm. But the little
we know of the physical structure of protoplasm throws almost no light
on the remarkable properties of this fundamental life-substance.
CHAPTER VIII
CELLULAR STRUCTURE OF THE TOAD (OR
FROG)
LABORATORY EXERCISE
=The blood.=--TECHNICAL NOTE.--The blood of a frog can be studied
as it flows through the small vessels in the membranes between the
toes while the animal is alive. Place a frog on a small flat board
which has had a hole cut near one end, and with a piece of cloth
bind it to the board. Spread the web between two toes over the
hole in the board and keep it in place with pins. This done,
examine the distended web under the compound microscope first with
low then with higher power, and observe the blood-vessels and the
blood circulating in them. For a further study of the blood kill a
toad or frog and place a drop of the blood on a slide with a
cover-glass over it.
Put the prepared slide under the microscope and note that the blood,
which as seen with the unaided eye appears to be a red fluid, is made
up of a great many yellowish elliptical disks or _cells_, the
_blood-corpuscles_, floating in a liquid, the _blood-plasma_. Here and
there you may notice _amœboid blood-corpuscles_. These are
irregular-shaped cells which move about by thrusting out pseudopodia.
They look like some of the unicellular animals, as the _Amœba_. Can
you distinguish a nucleus and cell-wall in the blood-cells?
Make drawings of these blood-cells.
=The skin.=--TECHNICAL NOTE.--Keep a live toad or frog in water
for some time and note if its skin becomes loose or begins to slip
away. If the outer skin, epidermis, comes off, take some of the
shed skin and wash it in water, then stain for three or four
minutes in a solution of methyl-green and acetic acid (see p.
451). Cut the pieces of stained skin into small bits and examine
one of these under the microscope.
With the low power of the microscope you will note that the skin is
made up of a great many flat _cells_ placed edge to edge. Each one has
its cell-wall and a central darkly stained nucleus.
Make a drawing of a portion of the toad's skin.
=The liver.=--TECHNICAL NOTE.--Cut through the fresh liver of a
toad, and with a knife-blade scrape from the cut surface some of
the liver-cells and place them on a slide with cover-glass.
Examine under the microscope and observe many polygonal _cells_. Place
some of the methyl-green acetic stain under the cover-glass and note,
after the cells are stained, that they have definite boundaries and a
central nucleus.
Draw some of these scattered liver-cells.
=The muscles.=--TECHNICAL NOTE.--Take a piece of intestine from a
freshly killed toad, wash it thoroughly and place it in a
concentrated solution of salicylic acid in 70% alcohol for 24 hours,
then gradually heat until about the boiling-point, when the muscles
will fall to pieces. Transfer the preparation to a watch-crystal and
tease small bits of isolated muscle with dissecting-needles. Place
some of the teased muscle-fibres on a slide, cover with cover-glass,
and add a drop of the methyl-green acetic acid.
Note the small spindle-shaped _muscle-fibres_. Each one of these
fibres is a _cell_ possessing all of the structures common to cells,
namely, cell-wall, nucleus, etc.
Make a drawing of a few isolated fibres of muscle.
From this study of some of the tissues in a toad it will be noted that
in the first case we had in the blood separate cells which moved about
freely in the plasma. In the second case, that of the epidermis, the
cells are fixed edge to edge, thus forming a thin tissue; while in the
third and fourth cases, that of the liver and muscle, the cells are not
only placed edge to edge, but aggregated into vast masses or bundles,
in one case to form the liver and in the other case a muscle. The entire
body of the toad is built up of a colony of simple units (cells)
combined in various forms to make all the various tissues and organs.
CHAPTER IX
THE MANY-CELLED ANIMAL BODY.--DIFFERENTIATION
OF THE CELL
=The many-celled animal body.=--In the study of certain of the tissues
and organs of the toad we have learned that the body of this animal is
composed of many cells, thousands and thousands of these microscopic
structural units being combined to form the whole toad. This
many-celled or multicellular condition of the body is true of all the
animals except the simplest, the unicellular Protozoa. Corals,
starfishes, worms, clams, crabs, insects, fishes, frogs, reptiles,
birds, and mammals, all the various kinds of animals in which the body
is composed of organs and tissues, agree in the multicellular
character of the body, and may be grouped together and called the
many-celled animals in contrast to the one-celled animals. This
division is one which is recognized by many systematic zoologists as
being more truly primary or fundamental than the division of animals
into Vertebrates and Invertebrates. The one-celled animals are called
Protozoa, and the many-celled animals Metazoa.
=Differentiation of the cell.=--It is apparent at first glance that
the cells which compose the body of a many-celled animal are not like
the simple primitive cell which makes up the body of the _Amœba_, nor
are they like the more complexly arranged cell of the _Paramœcium_.
Nor are they all like each other. The cells in the toad's blood are of
two kinds, the white blood-cells, which are very like the body of
_Amœba_, and the elliptical disk-like red blood-cells. The cells
composing the muscles are, moreover, like neither kind of blood-cells,
and the cells of which the liver is composed are not like the cells of
the muscles. That is, there are many different kinds of cells in the
body of a many-celled animal. While the single cell which composes the
whole body of the _Amœba_ is able to do all the things necessary to
maintain life, the various cells in the body of a complex animal are
differentiated or specialized, certain cells devoting themselves to a
certain function or special work, and others to other special
functions. For example, the cells which compose the organs of the
nervous system, the brain, ganglia, and nerves, devote themselves
almost exclusively to the function of sensation, and they are
especially modified for this purpose. The highly specialized
nerve-cells resemble very little the primitive generalized body-cell
of _Amœba_. The muscle-cells of the complex animal body have developed
to a high degree that power of contraction which is possessed, though
in but slight degree, by _Amœba_. These muscle-cells have for their
special function this one of contraction, and massed together in great
numbers they form the strongly contractile muscular tissue and muscles
of the body on which the animal's power of motion depends. The cells
which line certain parts of the alimentary canal are the ones on which
the function of digestion chiefly rests. And so we might continue our
survey of the whole complex body. The point of it all is that the
thousands of cells which compose the many-celled animal body are
differentiated and specialized; that is, have become changed or
modified from the generalized primitive amœboid condition, so that
each kind of cell is devoted to some special work or function and has
a special structural character fitting it for its special function. In
the Protozoan body the single cell can perform and does perform all
the functions or processes necessary to the life of the animal. In the
Metazoan body each cell performs, in co-operation with many other
similar cells, some one special function or process. The total work of
all the cells is the living of the animal.
CHAPTER X
HYDRA
LABORATORY EXERCISE
TECHNICAL NOTE.--_Hydra_ lives in fresh water, attached to stones,
sticks, or decayed leaves. It can be found in most open
fresh-water ponds not too stagnant, often attached to _Chara_.
There are two species occurring commonly, _H. viridis_, the green
_Hydra_, and _H. fuscus_, the brown or flesh-colored _Hydra_. Both
are very small forms and have to be looked for carefully.
Specimens should be brought to the laboratory, put into a large
dish of water and left in the light. _Hydra_ is best studied
alive. Place a living specimen attached to a bit of weed in a
watch-crystal filled with water or on a slide with plenty of water
and examine with the low power of the microscope.
Note the cylindrical body (fig. 7, _A_, _B_) with its flat basal
attachment and _radial tentacles_ (varying in number) which crown the
upper end and surround the centrally located _mouth_. Note the movements
of _Hydra_, its powers of contraction, and method of taking in food.
TECHNICAL NOTE.--To feed _Hydra_, place very small "water-fleas"
(_Daphnia_ sp.) in the water with it.
Observe the method by which "water-fleas" are taken into the mouth.
Food is caught on stinging cells (to be studied later) and conveyed to
the mouth by the tentacles. Note that the cylindrical body encloses a
cavity, the _digestive cavity_. How is this connected with the
exterior? If _Hydra_ captures prey too large or is no longer hungry,
the prey is released.
[Illustration: FIG. 7.--A, _Hydra fusca_, with expanded body and a
budding individual; B, _H. fusca_, contracted; C, _H. fusca_, part of
outer surface of a tentacle, greatly magnified. (A and B drawn from live
specimens, C, from a preparation) D, _Grantia_ sp. (a sponge), three
individuals; E, _Grantia_ sp., longitudinal section; F, _Grantia_ sp.,
spicules. (D, E, and F drawn from preserved specimens.)]
TECHNICAL NOTE.--Place small slips of paper on the slide near the
_Hydra_, put cover-glass over the whole, and examine with the low
power of the microscope.
Note that the whole animal is made up of cells closely joined. Are the
cells in the tentacles all alike? Note nodule-like projections above
some of the cells; these are _stinging cells_, or _cnidoblasts_. In
some cases a small hair-like process, the trigger hair or _cnidocil_,
may be seen projecting above the surface of the cell. Note in some of
the tentacles dark-colored particles. These are food-particles which
have been taken through the mouth into the digestive cavity and have
passed thence into the tentacles. The central digestive cavity
communicates freely with the cavities in the tentacles, for the
tentacles are merely evaginations of the body-wall.
Make drawings of the _Hydra_ expanded and of the same individual
contracted.
TECHNICAL NOTE.--From the preparation which you have under the
microscope pull out the slips of paper, thus letting the cover-glass
drop down on the specimen. With a small pipette put a drop of
anilin-acetic stain (see p. 451) on the slide at one side of the
cover-glass and with a piece of filter-paper draw the water through
from the other side of the cover-glass. When the stain is diffused
press down the cover-glass gently and examine the tentacles first
under a low power of the microscope, then under a high one.
Note the distortion that the animal has undergone through the action
of the reagent. Observe the cnidoblasts of the tentacles and note that
many of them have thrown out long whip-like processes (fig. 7, _C_).
On what parts of the body do the cnidoblasts occur? Carefully examine
one of the cnidoblasts which has been discharged and note a clear
transparent bag-like structure within, the _nematocyst_, to which is
attached the long whip-like process. In another cnidoblast cell which
has not been discharged note that the whip-like process is coiled
about inside of the bag-like structure. The whole apparatus is like
the inturned finger of a glove which can be blown out by pressure from
the inside. The mechanism is simple. The cnidocil or trigger-hair is
touched by some animal, an impulse is conveyed to the delicate fibres
interspersed among the cells (nerve-cells) which stimulate the
cnidoblast cell, whereupon there is a contraction of the contents and,
the cnidoblast being compressed, the inverted whip-like process turns
wrong side out and impales the animal on its points or barbs.
TECHNICAL NOTE.--The teacher should be provided with microscopical
sections, both transverse and longitudinal, of the _Hydra_ stained
in some good general stain (hæmatoxylin or borax carmine). If the
teacher has no means of making such preparations, they may be
procured from dispensers of microscopical supplies.
From the cross-section of the _Hydra_ make out the general structure
of the body. Note that it is a hollow cylinder consisting of two
well-defined layers of cells, an outside _ectoderm_ layer and an inner
_endoderm_ layer. Between these two is yet another thin non-cellular
layer called the _mesoglœa_.
Thus it will be seen that _Hydra_ is made up of two layers of cells,
the outer ectoderm or skin, which is specialized to perform the office
of capturing prey as well as that of protection, and the inner
endoderm, which surrounds the digestive cavity and performs the
function of digestion. The endoderm lines the body-cavity, particles
taken in as food being digested by certain digestive cells which
thrust out amœboid processes and ingest particles of food. Other cells
in the endoderm have long flagellate processes which vibrate back and
forth in the digestive cavity, thereby creating currents in the water
containing food-particles.
Note, in a cross-section, that there are small ovoid or cuboid cells
at the bases of the large ectoderm cells. These are the _interstitial
cells_. Some of the interstitial cells become modified and pushed up
between the ectoderm cells to form cnidoblast cells. Many of the
endoderm as well as ectoderm cells have muscle-processes which spread
out from the base of the cell and which serve to contract and expand
the body.
TECHNICAL NOTE.--In the specimens which have been collected
perhaps two methods of reproduction will be observed. Place
healthy _Hydræ_ in a wide-mouthed jar in the sunlight with plenty
of water and food. In a few days active budding will take place.
Observe the method of reproduction in _Hydra_. Commonly the parent
produces small buds, which at first are only evaginations of the
body-wall, but which later develop tentacles and a mouth of their own.
Subsequently the bud becomes constricted at the base, separates from
the parent, and the young _Hydra_ begins a distinct existence.
Another mode of reproduction takes place which, in distinction from the
asexual method just mentioned, is called sexual reproduction. This last
is the method common to most of the higher organisms. You may note that
in some _Hydræ_ there is a swelling or bulging of the ectoderm of the
body-wall in the region just below the tentacles. These are the
_sperm-glands_. Within these are produced sperm-cells which break away
in great clusters to fertilize the ova, or eggs. Note a larger bulging
of the body-wall nearer the lower end of the body which, under high
power, has a granular appearance. This is the _egg-gland_, in which
develops a single _ovum_ or _egg_. The ovum breaks from its covering and
is fertilized by sperm-cells from another individual. In forms like
_Hydra_, where both sexes are represented in a single individual, the
organism is termed _monœcious_ or _hermaphroditic_. In connection with
reproduction Chapter XIII should be studied.
An instructive experiment can be performed by cutting a _Hydra_ into
two or more parts, when (usually) each of the various parts will
develop into a complete _Hydræ_. This process may be called
reproduction by fission, but it rarely occurs naturally.
CHAPTER XI
THE SIMPLEST MANY-CELLED ANIMALS
=Cell differentiation and body organization in Hydra.=--From the
examination of _Hydra_ we have learned that there are true many-celled
animals which are much less complex in structure than the toad and
crayfish. The body of _Hydra_, like the body of the toad, is composed of
many cells, but these cells are of only a few different kinds; that is,
show but little differentiation. There is relatively little division of
the body into distinct organs. Still, certain parts of the body devote
themselves principally to certain particular functions. Thus all the
food is taken in through the single "mouth-opening" at the apical free
end of the cylindrical body, and there are certain organs, the
tentacles, whose special business or function it is to find and seize
food and to convey it to the mouth. After the food is taken into the
cylindrical body-cavity it is digested by special cells which line the
cavity. Some of these cells are unusually large, and each contains one
or more contractile vacuoles. From the free ends of these cells, the
ends which are next to the body-cavity, project pseudopods or flagella.
These protoplasmic processes are constantly changing their form and
number. In addition to these large sub-amœboid cells there are, in this
inner layer of cells lining the body-cavity, and especially abundant
near the base or bottom of the cavity, many long, narrow, granular
cells. These are gland-cells which secrete a digestive fluid. The food
captured by the tentacles and taken in through the mouth-opening
disintegrates in the body-cavity, or digestive cavity as it may be
called. The digestive fluid secreted by the gland-cells acts upon it so
that it becomes broken into small parts. These particles are seized by
the projecting pseudopods of the sub-amœboid cells and taken into the
body-protoplasm of these cells. The cells of the outer layer of the body
do not take food directly, but receive nourishment only by means of and
through the cells of the inner layer. The body-cavity of _Hydra_ is a
very simple special organ of digestion.
In the outer layer of cells there are some specially large cells whose
inner ends are extended as narrow pointed prolongations directed at
right angles with the rest of the cell. These processes are very
contractile and are called muscle-processes. Each one is simply a
specially contractile continuation of the protoplasm of the cell-body.
There are also in this layer some small cells very irregular in shape
and provided with unusually large nuclei. These cells are more
irritable or sensitive than the others and are called nerve-cells. We
have thus in _Hydra_ the beginnings of muscular organs and of
nerve-organs. But how simple and unformed compared with the muscular
and nervous systems of the toad and crayfish! There is no circulatory
system, nor are there any special organs of respiration.
But _Hydra_ is far in advance of _Amœba_ or _Paramœcium_. Its body is
composed of thousands of distinct cells. Some of these cells devote
themselves especially to food-taking, some especially to the digestion
of food; some are specially contractile, and on them the movements of
the body depend, while others are specially irritable or sensitive,
and on them the body depends for knowledge of the contact of prey or
enemies. In the cnidoblast cells, those with the stinging threads,
there is a very wide departure from the simple primitive type of
cells. There is in _Hydra_ a manifest differentiation of the cells
into various kinds of cells. The beginnings of distinct tissues and
organs are indicated.
=Degrees in cell differentiation and body organization.=--In the study
of the cellular constitution of the tissues and organs of the toad, we
found to what a high degree the differentiation of the cells may
attain, and in the study of the anatomy of the toad we found how
thoroughly these differentiated cells may be combined and organized
into body-parts or organs. The body of the toad is made up of distinct
organs, each composed of highly differentiated or specialized cells.
The body of _Hydra_ is composed of cells for the most part only
slightly differentiated and hardly recognizably grouped or combined
into organs. These two conditions are the extremes in the
body-structure of the many-celled animals. Between them is a host of
intermediate conditions of cell differentiation and body organization.
When we come to the study of other members of the great branch of
simple many-celled animals to which _Hydra_ belongs (see Chapter
XVII), it will be found that some of them show a slight advance in
complexity beyond _Hydra_. Higher in the scale of animal life the
forms will be found still more and more complex, with ever-increasing
differentiation of the cells, with the combination of the
differentiated cells into distinct organs, and the co-ordination of
organs into systems of organs up to the extreme shown by the birds and
mammals. And hand in hand with this increasing complexity of structure
goes ever-increasing complexity or specialization of function.
Breathing is a simple function or process with _Hydra_, where each
body-cell takes up oxygen for itself, but it is a complex business
with the toad, or with a bird or mammal, where certain complex
structures, the lungs and accessory parts, and the heart,
blood-vessels and blood all work together to distribute oxygen to all
parts of the body.
CHAPTER XII
DEVELOPMENT OF THE TOAD
FIELD AND LABORATORY EXERCISE
TECHNICAL NOTE.--As the work of this chapter, or some similar work
in getting acquainted with the postembryonic development of a
many-celled animal, should be done early in the course, and as
most schools open in the fall, it will perhaps be impossible to
make this first study of development from live specimens in the
field. In such case the examination of a series of prepared
specimens, previously obtained by the teacher, must be resorted
to. In the spring the development of several kinds of animals,
including the toad, can be studied from live specimens in the
field or in breeding-cages and aquaria in the laboratory. The eggs
of the toad may be found in April and May (the toads are heard
trilling at egg-laying time) in ponds. The eggs look like the
heads of black pins, and are in single rows in long strings of
transparent jelly, which are usually wound around sticks or
plant-stems at the bottom of the pond near the shore. Bring some
of these strings into the schoolroom and keep them in water in
shallow dishes. Keep them in the light, but not in direct
sunlight. In the dishes put some small stones and mud from the
pond, arranging them in a slope, thus making different depths of
water. Stones with green algæ on should be selected, for algæ are
the food of the tadpoles. The eggs will hatch in two or three
days, and if too many tadpoles are not kept in the dish, and the
little aquarium be well cared for, the whole postembryonic
development of the toad can be well observed. For the study of the
development from prepared specimens the teacher should have a
complete series of stages from egg to adult toad in alcohol. The
specimens may be examined by the students in connection with a
talk from the teacher on the life-history of the toad.
If the study is made from prepared specimens, make drawings of
egg-strings, and of a single egg magnified and shaded to indicate its
color. Draw each specimen of the series of tadpoles, noting in the
youngest the presence of gills and tail and absence of legs and eyes;
in the older the appearance of eyes, the shrivelling of the gills,
shrinking of the tail and development of legs; in the still older the
characteristic shape, in miniature, of the adult toad.
In observing the course of development of the living specimens there
should be made, in addition to the drawings, notes showing the
duration of the egg stage, and the time elapsing between all important
changes (as seen externally) in the body of the young. Observations
and notes on the general behavior of tadpoles should also be made;
note the swimming, the feeding, the gradual leaving of the water, etc.
In addition to the easily seen external changes in the body, very
important ones in the internal organs take place during development.
Perhaps the most important of these concerns the lungs. The young
gilled toad breathes as a fish does, but gradually its gills are lost,
while at the same time lungs develop and the tadpole comes to the
surface to breathe air like any lunged aquatic animal. The toad on
leaving the water changes its diet from vegetable to animal food; a
tadpole feeds on aquatic algæ; a toad preys on insects. Correlated
with the change in habit, the intestine during development undergoes
some marked changes, becoming relatively diminished in length.
For an account of the development of the toad see Gage's "Life-history
of a Toad" or Hodge's "The Common Toad."
CHAPTER XIII
MULTIPLICATION AND DEVELOPMENT.--MULTIPLICATION
OF ONE-CELLED ANIMALS
=Multiplication.=--We know that any living animal has parents; that
is, has been produced by other animals which may still be living or be
now dead or, as with _Amœba_, may have changed, by division, into new
individuals. Individuals die, but before death, they produce other
individuals like themselves. If they did not, their kind or species
would die with them. This production of new animals constantly going
on is called the reproduction or multiplication of animals. The
process is well called multiplication, because each female animal
normally produces more than one new individual. She may produce only
one at a time, one a year, as many of the sea-birds do or as the
elephant does, but she lives many years. Or she may produce hundreds,
or thousands, or even millions of young in a very short time. A
lobster lays 10,000 eggs at a time. Nearly nine millions of eggs have
been taken from the body of a thirty-pound female codfish. As a matter
of fact but very, very few of these eggs produce new animals which
reach maturity. From the 10,000 eggs produced by the lobster each year
an average of but two new mature lobsters is produced. There is always
a struggle for food and for place going on among animals, for many
more are produced than there are food and room for, and so of all the
new or young animals which are born the great majority are killed
before they reach maturity. In a later chapter more attention will be
given to this great struggle for life.
In the preceding paragraph it has been stated that "we know that any
living animal has parents; that is, has been produced by other animals
which may still be living or be now dead." This is a statement,
however, which has found complete acceptance only in modern times. It
is a familiar fact that a new kitten comes into the world only through
being born; that it is the offspring of parents of its kind. But we
may not be personally familiar with the fact that a new starfish comes
into the world only as the production of parent starfish, or that a
new earthworm can be produced only by other earthworms. But
naturalists have proved these statements. All life comes from life;
all organisms are produced by other organisms. And new individuals are
produced by other individuals of the same kind. That these statements
are true all modern observations and investigations of the origin of
new individuals prove. But in the days of the earlier naturalists the
life of the microscopic organisms like _Amœba_ and _Paramœcium_, and
even that of many of the larger but unfamiliar animals, was shrouded
in mystery. And various and strange beliefs were held regarding the
origin of new individuals.
=Spontaneous generation.=--The ancients believed that many animals
were spontaneously generated. The early naturalists thought that flies
arose by spontaneous generation from the decaying matter of dead
animals. Frogs and many insects were thought to be generated
spontaneously from mud, and horse-hairs in water were thought to
change into water-snakes. But such beliefs were easily shown to be
based on error, and have been long discarded by zoologists. But the
belief that the microscopic organisms, such as bacteria and infusoria,
were spontaneously generated in stagnant water or decaying organic
liquids was held by some naturalists until very recent times. And it
was not so easy to disprove the assertions of such believers. If some
water in which there are apparently no living organisms, however
minute, be allowed to stand for a few days, it will come to swarm with
microscopic plants and animals. Any organic liquid, as a broth or a
vegetable infusion, exposed to the air for a short time becomes foul
through the presence of innumerable microscopic organisms. But it has
been certainly proved that these organisms are not spontaneously
produced in the water or organic fluid. A few of them enter the water
from the air, in which there are always greater or less numbers of
spores of microscopic organisms. These spores germinate quickly when
they fall into water or some organic liquid, and the rapid succession
of generations soon gives rise to the hosts of bacteria and one-celled
animals which infest all standing water. If all the active organisms
and inactive spores in a glass of water are killed by boiling the
water, and this sterilized water be put into a sterilized glass, and
this glass be so well closed that germs or spores cannot pass from the
air without into the sterilized liquid, no living animals will ever
appear in it. We know of no instance of the spontaneous generation of
animals, and all the animals whose life-history we know are produced
by other animals of the same kind.
=Simplest multiplication and development.=--The simplest method of
multiplication and the simplest kind of development shown among animals
are exhibited by such simple animals as _Amœba_ and _Paramœcium_. The
production of new individuals is accomplished in _Amœba_ by a simple
division or fission of its body (a single cell) into two practically
equivalent parts. An _Amœba_ which has grown for some time contracts all
of its finger-like processes, the pseudopodia, and its body becomes
constricted. This constriction or fissure increases inwards so that the
body is soon divided fairly in two. There are now two _Amœbæ_, each half
the size of the original one; each, indeed, actually one-half of the
original one. The original _Amœba_ was the parent; the two halves of it
are the young. Each of the young possesses all of the characteristics
and powers of the parent; each can move, eat, feel, grow, and reproduce
by fission. The only change necessary for the young or new _Amœba_ to
become like its parent, is that of simple growth to a size about twice
its present size. The development here is reduced to a minimum. Just as
the simplest animals perform the other life-processes, such as taking
and digesting food, breathing and feeling, in an extremely primitive
simple way, so do they perform the necessary life-process of
reproduction or multiplication in the simplest way shown among animals.
In the case of _Paramœcium_ the process of multiplication is slightly
more complex than that of _Amœba_ in the fact that sometimes before
the simple fission of the body takes place the interesting phenomenon
of conjugation occurs. _Paramœcium_ may reproduce itself for many
generations by simple fission, but a generation finally appears in
which conjugation takes place. Two individuals come together and each
exchanges with the other a part of its nucleus. Then the two
individuals separate and each divides into two. The result of the
conjugation, or the coming together, of two individuals with mutual
interchange of nuclear substance is to give to the new _Paramœcia_
produced by the conjugating individuals a body which contains part of
the body-substance of two distinct individuals. If the two conjugating
individuals differ at all--and they always do differ, because no two
individual animals, although belonging to the same species, are
exactly alike--the new individual, made up of parts of each of them,
will differ slightly from both. Nature seems intent on making every
new individual differ slightly from the individual which precedes it.
And the method of multiplication which Nature has adopted to produce
the result is the method which we have seen exhibited in its simplest
form in the case of _Paramœcium_--the method of having two individuals
take part in the production of a new one.
The development of the new _Paramœcia_ is a little more complex than
that of _Amœba_. Not only must the new _Paramœcium_ grow to the size
of the original one, but it must develop those slight, but apparent,
modifications of the parts of its body which we can recognize in the
full-grown, fully developed _Paramœcium_ individual. A new
mouth-opening must develop on the new individual formed of the hinder
half of the original _Paramœcium_ and new cilia must be developed.
Thus there is a slight advance in complexity of development, just as
there is in complexity of structure in _Paramœcium_ as compared with
_Amœba_. In the many-celled animals this complexity of development is
carried to an extreme.
=Birth and hatching.=--When a young animal is born alive, it usually
resembles in appearance and structure the parent, although of course
it is much smaller, and requires always a certain time to complete its
development and become mature. A young kangaroo or opossum is carried
for some time after its birth in an external pouch on the mother's
body and is a very helpless animal. A young kitten is born with eyes
not yet opened and must be fed by the mother for several weeks. On the
other hand young Rocky Mountain sheep are able to run about swiftly
within a few hours after birth.
Most animals appear first as eggs laid by the mother. This is true of
the birds, the reptiles, the fishes, the insects, and most of the
hosts of invertebrate animals. This egg may be cared for by the parent
as with the birds, or simply deposited in a safe place as with most
insects, or perhaps dropped without care into the water as with most
marine invertebrates. The young animal which issues from the egg may
at the time of its hatching resemble the parent in appearance and
structural character (although always much smaller) as with the birds,
some of the insects, and many of the other animals. Or it may issue in
a so-called _larval_ condition, in which it resembles the parent but
slightly or not at all, as is the case with the gill-bearing, legless,
tailed tadpole of the frog or the crawling, wingless, wormlike
caterpillar of the butterfly, or the maggot of the house-fly.
=Life-history.=--Any animal which hatches from an egg has undergone a
longer or shorter period of development within the egg-shell before
hatching. The development of an animal from first germ-cell to the time
it leaves the egg, for example, the development of the embryo chick from
the first cell to time of hatching, is called its _embryonic_
development; and the development from then on, for example, that of the
chick to adult hen or rooster, or that of tadpole to frog, is called the
_post-embryonic_ development. Beginning students of animals cannot study
the embryonic development (_embryology_) of animals readily, but they
can in many cases easily follow the course of the post-embryonic
development, and this study will always be interesting and valuable.
When the "life-history" of an animal is spoken of in this book, or other
elementary text-book of zoology, it is the history of the life of the
animal from the time of its birth or hatching to and through adult
condition that is meant, not the complete life-history from beginning
single egg-cell to the end. In all of the study of the different kinds
of animals to which the rest of this book is devoted, attention will be
paid to their life-history.
PART II
SYSTEMATIC ZOOLOGY
CHAPTER XIV
THE CLASSIFICATION OF ANIMALS
=Basis and significance of classification.=--It is the common
knowledge of all of us that animals are classified: that is, that the
different kinds are arranged in the mind of the zoologist and in the
books of natural history, in various groups, and that these various
groups are of different rank or degree of comprehensiveness. A group
of high rank or great comprehensiveness includes groups of lower rank,
and each of these includes groups of still lower rank, and so on, for
several degrees. For example, we have already learned that the toad
belongs to the great group of back-boned animals, the Vertebrates, as
the group is called. So do the fishes and the birds, the reptiles and
the mammals or quadrupeds. But each of these constitutes a lesser
group, and each may in turn be subdivided into still lesser groups.
In the early days of the study of animals and plants their
classification or division into groups was based on the resemblances
and the differences which the early naturalists found among the
organisms they knew. At first all of the classifying was done by
paying attention to external resemblances and differences, but later
when naturalists began to dissect animals and to get acquainted with
the structure of the whole body, the differences and likenesses of
inner parts, such as the skeleton and the organs of circulation and
respiration, were taken into account. At the present time and ever
since the theory of descent began to be accepted by naturalists (and
there is practically no one who does not now accept it), the
classification of animals, while still largely based on resemblances
and differences among them, tells more than the simple fact that
animals of the same group resemble each other in certain structural
characters. It means that the members of a group are related to each
other by descent, that is, genealogically. They are all the
descendants of a common ancestor; they are all sprung from a common
stock. And this added meaning of classification explains the older
meaning; it explains why the animals are alike. The members of a group
resemble each other in structure because they are actually blood
relations. But as their common ancestor lived ages ago, we can learn
the history of this descent, and find out these blood relationships
among animals only by the study of forms existing now, or through the
fragmentary remains of extinct animals preserved in the rocks as
fossils. As a matter of fact we usually learn of the existence of this
actual blood relationship, or the fact of common ancestry among
animals, by studying their structure and finding out the resemblances
and differences among them. If much alike we believe them closely
related; if less alike we believe them less closely related, and so
on. So after all, though the present-day classification means
something more, means a great deal more, in fact, than the
classification of the earlier naturalists means, it is largely based
on and determined by resemblances and differences just as was the old
classification. Sometimes the fossil remains of ancient animals tell
us much about the ancestry and descent of existing forms. For example,
the present-day one-toed horse has been clearly shown by series of
fossils to be descended from a small five-toed horse-like animal which
lived in the Tertiary age.
=Importance of development in determining classification.=--A very
important means of determining the relationships among animals is by
studying their development. If two kinds of animals undergo very
similar development, that is, if in their development and growth from
egg-cell to adult they pass through similar stages, they are nearly
related. And by the correspondence or lack of correspondence, by the
similarity or dissimilarity of the course of development of different
animals much regarding their relationship to each other is revealed.
Sometimes two kinds of animals which are really nearly related come to
differ very much in appearance in their fully developed adult
condition because of the widely different life-habits the two may
have. But if they are nearly related their developmental stages will
be closely similar until the animals are almost fully developed. For
example, certain animals belonging to the group which includes the
crabs, lobsters, and crayfishes, have adopted a parasitic habit of
life, and in their adult condition live attached to the bodies of
certain kinds of true crabs. As these parasites have no need of moving
about, being carried by their hosts, they have lost their legs by
degeneration, and the body has come to be a mere sac-like pulsating
mass, attached to the host by slender root-like processes, and not
resembling at all the bodies of their relatives the crabs and
crayfishes. If we had to trust, in making out our classification,
solely to structural resemblances and differences, we should never
classify the _Sacculina_ (the parasite) in the group Crustacea, which
is the group including the crabs and lobsters and crayfishes. But the
young _Sacculina_ is an active free-swimming creature resembling the
young crabs and young shrimps. By a study of the development of
_Sacculina_ we find that it is more closely related to the crabs and
crayfishes and the other Crustaceans than to any other animals,
although in adult condition it does not at all, at least in external
appearance, resemble a crab or lobster.
=Scientific names.=--To classify animals then, is to determine their
true relationships and to express these relationships by a scheme of
groups. To these groups proper names are given for convenience in
referring to them. These proper names are all Latin or Greek, simply
because these classic languages are taught in the schools and colleges
of almost all the countries in the world, and are thus intelligible to
naturalists of all nationalities. In the older days, indeed, all the
scientific books, the descriptions and accounts of animals and plants,
were written in Latin, and now most of the technical words used in
naming the parts of animals and plants are Latin. So that Latin may be
called the language of science. For most of the groups of animals we
have English names as well as Greek or Latin ones and when talking
with an English-speaking person we can use these names. But when
scientific men write of animals they use the names which have been
agreed on by naturalists of all nationalities and which are understood
by all of these naturalists. These Latin and Greek names of animals
laughed at by non-scientific persons as "jaw-breakers," are really a
great convenience, and save much circumlocution and misunderstanding.
AN EXAMPLE OF CLASSIFICATION.
TECHNICAL NOTE.--There should be provided a small set of bird-skins
which will serve just as well as freshly killed birds, and which may
be used for successive classes, thus doing away with the necessity
of shooting birds. The birds suggested for use are among the
commonest and most easily recognizable and obtainable. They may be
found in any locality at any time of the year. The skins can be
made by some boy interested in birds and acquainted with making
skins, or by the teacher, or can be purchased from a naturalists'
supply store, or dealer in bird skins. The skins will cost about 25
cents each. This example or lesson in classification can be given
just as well of course with other species of birds, or with a set of
some other kinds of animals, if the teacher prefers. Insects are
especially available, butterflies perhaps offering the most readily
appreciated resemblances and differences.
=Species.=--Examine specimens of two male downy woodpeckers (the males
have a scarlet band on the back of the head). (In the western States
use Gardiner's downy woodpecker.) Note that the two birds are of the
same size, have the same colors and markings, and are in all respects
alike. They are of the same kind; simply two individuals of the same
kind of animal. There are hosts of other individuals of this kind of
bird, all alike. This one kind of animal is called a _species_. The
species is the smallest[4] group recognized among animals. No attempt
is made to distinguish among the different individuals of one kind or
species of animal as we do in our own case.
Examine a specimen of the female downy woodpecker. It is like the male
except that it does not have the scarlet neck-band. But despite this
difference we know that it belongs to the same species as the male
downy because they mate together and produce young woodpeckers, male
and female, like themselves. There are thus two sorts of
individuals,[5] male and female, comprised in each species of animal.
A _species_ is a group of animals comprising similar individuals which
produce new individuals of the same kind usually after the mating
together of individuals of two sexes which may differ somewhat in
appearance and structure.
Examine a male hairy woodpecker and a female; (in western States
substitute a Harris's hairy woodpecker). Note the similarity in markings
and structure to the downy. Note the marked difference in size. Make
notes of measurements, colors and markings, and drawings of bill and
feet, showing the resemblances and the differences between the downy
woodpecker and the hairy woodpecker. These two kinds of woodpeckers are
very much alike, but the hairy woodpeckers are always much larger
(nearly a half) than the downy woodpeckers and the two kinds never mate
together. The hairy woodpeckers constitute another species of bird.
=Genus.=--Examine now a flicker (the yellow-shafted or golden-winged
flicker in the East, the red-shafted flicker in the West). Compare it
with the downy woodpecker and the hairy woodpecker. Make notes
referring to the differences, also the resemblances. The flicker is
very differently marked and colored and is also much larger than the
downy woodpecker, but its bill and feet and general make-up are
similar and it is obviously a "woodpecker." It is, however, evidently
another species of woodpecker, and a species which differs from either
the downy or the hairy woodpecker much more than these two species
differ from each other. There are two other species of flickers in
North America which, although different from the yellow-shafted
flicker, yet resemble it much more than they do the downy and hairy
woodpeckers or any other woodpeckers. We can obviously make two groups
of our woodpeckers so far studied, putting the downy and hairy
woodpeckers (together with half a dozen other species very much like
them) into one group and the three flickers together into another
group. Each of these groups is called a _genus_, and genus is thus the
name of the next group above the species. A genus usually includes
several, or if there be such, many, similar species. Sometimes it
includes but a single known species. That is, a species may not have
any other species resembling it sufficiently to group with it, and so
it constitutes a genus by itself. If later naturalists should find
other species resembling it they would put these new species into the
genus with the solitary species. Each genus of animals is given a
Greek or Latin name, of a single word. Thus the genus including the
hairy and downy woodpeckers is called _Dryobates_; and the genus
including the flickers is called _Colaptes_. But it is necessary to
distinguish the various species which compose the genus _Colaptes_,
and so each species is given a name which is composed of two words,
first the word which is the name of the genus to which it belongs,
and, second, a word which may be called the species word. The species
word of the Yellow-shafted Flicker is _auratus_ (the Latin word for
golden), so that its scientific name is _Colaptes auratus_. The
natural question, Why not have a single word for the name of each
species? may be answered thus: There are already known more than
500,000 distinct species of living animals; it is certain that there
are no less than several millions of species of living animals; new
species are being found, described and named constantly; with all the
possible ingenuity of the word-makers it would be an extremely
difficult task to find or to build up enough words to give each of
these species a separate name. This is not attempted. The same species
word is often used for several different species of animals, but never
for more than one species belonging to a given genus. And the names of
the genera are never duplicated. (There are, of course, much fewer
genera than species, and the difficulty of finding words for them is
not so serious.) Thus the genus word in the two-word name of a species
indicates at once to just what particular genus in the whole animal
kingdom the species belongs, while the second or species word
distinguishes it from the few or many other species which are included
in the same genus. This manner of naming species of animals and plants
(for plants are given their scientific names according to the same
plan) was devised by the great Swedish naturalist Linnæus in the
middle of the eighteenth century and has been in use ever since.
=Family.=--Examine a red-headed woodpecker (_Melanerpes
erythrocephalus_) and a sapsucker (_Sphyrapicus varius_) and any other
kinds of woodpeckers which can be got. Find out in what ways the hairy
and downy woodpeckers (genus _Dryobates_), the flickers (genus
_Colaptes_) and the other woodpeckers resemble each other. Examine
especially the bill, feet, wings and tail. These birds differ in size,
color and markings, but they are obviously all alike in certain
important structural respects. We recognize them all as woodpeckers.
We can group all the woodpeckers together, including several different
genera, to form a group which is called a _family_. A family is a
group of genera which have a considerable number of common structural
features. Each family is given a proper name consisting of a single
word. The family of woodpeckers is named _Picidæ_.
We have already learned that resemblances between animals indicate
(usually) relationship, and that classifying animals is simply
expressing or indicating these relationships. When we group several
species together to form a genus we indicate that these species are
closely related. And similarly a family is a group of related genera.
=Order.=--There are other groups[6] higher or more comprehensive than
families, but the principle on which they are constituted is exactly
the same as that already explained. Thus a number of related families
are grouped together to form an _order_. All the fowl-like birds,
including the families of pheasants, turkeys, grouse and quail, all
obviously related, constitute the order of gallinaceous birds called
_Gallinæ_. The families of vultures, hawks and owls constitute the
order of birds of prey, the _Raptores_, and the families of the
thrushes, wrens, warblers, sparrows, black-birds, and many others
constitute the great order of perching birds (including all the
singing birds) called the _Passeres_.
=Class and branch.=--But it is evident that all of these orders,
together with the other bird orders, ought to be combined into a great
group, which shall include all the birds, as distinguished from all
other animals, as the fishes, insects, etc. Such a group of related
orders is called a _class_. The class of birds is named _Aves_. There
is a class of fishes, _Pisces_, and one of frogs and salamanders,
_Batrachia_, one of snakes and lizards called _Reptilia_, and one of
the quadrupeds which give milk to their young called _Mammalia_. Each
of these classes is composed of several orders, each of which includes
several families and so on down. But these five classes of Pisces,
Batrachia, Reptilia, Aves and Mammals agree in being composed of
animals which have a backbone or a backbone-like structure, while
there are many other animals which do not have a backbone, such as the
insects, the starfishes, etc. Hence these five backboned classes may
be brought together into a higher group called a _branch_ or _phylum_.
They compose the branch of backboned animals, the branch _Vertebrata_;
all the animals like the starfishes, sea-urchins and sea-lilies which
have the parts of their body arranged in a radiate manner compose the
branch _Echinodermata_; all the animals like the insects and spiders
and centipedes and crabs and crayfishes which have the body composed
of a series of segments or rings and have legs or appendages each
composed of a series of joints or segments make up the branch
_Arthropoda_. And so might be enumerated all the great branches or
principal groups into which the animal kingdom is divided.
In the remainder of this book the classification of animals is always
kept in sight, and the student will see the terms species, genus,
family, order, etc., practically used. In it all should be kept
constantly in mind the significance of classification, that is, the
existence of actual relationships among animals through descent.
FOOTNOTES:
[4] The lesser group called _variety_, or subspecies, we may leave out
of consideration for the present.
[5] Some species of animals are not represented by male individuals:
and in some all the individuals are hermaphrodites, as explained in
chapter XIV.
[6] Each of these higher groups has a proper name composed of a single
word. In the case of no group except the species is a name-word ever
duplicated. Each genus, family, order, or higher group has a name-word
peculiar to it, and belonging to it alone.
CHAPTER XV
BRANCH PROTOZOA: THE ONE-CELLED
ANIMALS
Of this group the structure and life-history of the Amœba (_Amœba_
sp.) and the Slipper Animalcule (_Paramœcium_ sp.) have already been
treated in Chapter VI. Another example is the
BELL ANIMALCULE (_Vorticella_ sp.)
TECHNICAL NOTE.--Specimens of _Vorticella_ may usually be found in
the same water with _Amœba_ and _Paramœcium_. The individuals live
together in colonies, a single colony appearing to the naked eye
as a tiny whitish mould-like tuft or spot on the surface of some
leaf or stem or root in the water. Touch such a spot with a
needle, and if it is a Vorticellid colony it will contract
instantly. Bring bits of leaves, stems, etc., bearing Vorticellid
colonies into the laboratory and keep in a small stagnant-water
aquarium (a battery-jar of pond-water will do).
Examine a colony of _Vorticella_ in a watch-glass of water or in a
drop of water on a glass slide under the microscope. Note the stemmed
bell-shaped bodies which compose the colony. Each bell and stem
together form an individual _Vorticella_ (fig. 8.) How are the members
of the colony fastened together? Tap the slide and note the sudden
contraction of the animals; also the details of contraction in the
case of an individual. Watch the colony expand; note the details of
this movement in the case of an individual.
Make drawings showing the colony expanded and contracted.
With higher power examine a single individual. Note the thickened,
bent-out, upper margin of the bell. This margin is called the
_peristome_. With what is it fringed? The free end of the bell is
nearly filled by a central disk, the _epistome_, with arched upper
surface and a circlet of _cilia_. Between the epistome and peristome
is a groove, the _mouth_ or _vestibule_, which leads into the body.
Study the internal structure of the transparent, bell-shaped body.
Note the differentiation of the protoplasm comprising the body into an
inner transparent colorless _endosarc_ containing various dark-colored
granules, vacuoles, oil-drops, etc., and an outer uniformly granular
_ectosarc_ not containing vacuoles. Is the stalk formed of ectosarc or
endosarc or of both? Note the curved _nucleus_ lying in the endosarc.
(This may be difficult to distinguish in some specimens.) Note the
numerous large circular granules, the _food vacuoles_. Note the
_contractile vesicle_, larger and clearer than the food vacuoles. Note
the thin _cuticle_ lining the whole body externally. A high
magnification will show fine transverse ridges or rows of dots on the
cuticle.
[Illustration: FIG. 8.--_Vorticella_ sp.; one individual with stalk
coiled, and one with stalk extended. (From life.)]
Make a drawing showing the internal structure.
Observe a living specimen carefully for some time to determine all of
its movements. Note the contraction and extension of the stalk, the
movements of the cilia of peristome and epistome, the flowing or
streaming of the fluid endosarc (indicated by the movements of the
food vacuoles), the behavior of the contractile vesicle.
Make notes and drawings explaining these motions.
Specimens of _Vorticella_ may perhaps be found dividing, or two
bell-shaped bodies may be found on a single stem, one of the bodies
being sometimes smaller than the other. These two bodies have been
produced by the longitudinal division or fission of a single body. In
this process a cleft first appears at the distal end of the
bell-shaped body, and gradually deepens until the original body is
divided quite in two. The stalk divides for a very short distance. One
of the new bell-shaped bodies develops a circlet of cilia near the
stalked end. After a while it breaks away and swims about by means of
this basal circlet of cilia. Later it settles down, becomes attached
by its basal end, loses its basal cilia and develops a stalk.
"Conjugation occurs sometimes, but it is unlike the conjugation of
_Paramœcium_ in two important points: Firstly, the conjugation is
between two dissimilar forms; an ordinary large-stalked form, and a much
smaller free-swimming form which has originated by repeated division of
a large form. Secondly, the union of the two is a complete and permanent
fusion, the smaller being absorbed into the larger. This permanent
fusion of a small active cell with a relatively large fixed cell,
followed by division of the fused mass, presents a striking analogy to
the process of sexual reproduction occurring in higher animals."
OTHER PROTOZOA
Besides the _Amœba_, _Paramœcium_, and _Vorticella_ there are
thousands of other Protozoa. Most of them live in water, but a few
live in damp sand or moss, and some live inside the bodies of other
animals as parasites. Of those which live in water some are marine,
while others are found only in fresh-water streams and lakes.
[Illustration: FIG. 9.--Sun animalcule, a fresh-water protozoan with a
siliceous skeleton, and long thread-like protoplasmic prolongations.
(From life.)]
=Form of body.=--The Protozoa all agree in having the body composed
for its whole lifetime of a single cell,[7] but they differ much in
shape and appearance. Some of them are of the general shape and
character of _Amœba_, sending out and retracting blunt, finger-like
pseudopodia, the body-mass itself having no fixed form or outline but
constantly changing. Others have the body of definite form, spherical,
elliptical, or flattened, enclosed by a thin cuticle, and having a
definite number of fine thread-like or hair-like protoplasmic
prolongations called flagella or cilia. Many of the familiar Protozoa
of the fresh-water ponds always have two whiplash-like flagella
projecting from one end of the body. By means of the lashing of these
flagella in the water the tiny creature swims about. Others have many
hundreds of fine short cilia scattered, sometimes in regular rows,
over the body-surface. The Protozoan swims by the vibration of these
cilia in the water.
[Illustration: FIG. 10.--_Stentor_ sp.; a protozoan which may be
fixed, like _Vorticella_, or free-swimming, at will, and which has the
nucleus in the shape of a string or chain of bead-like bodies. The
figure shows a single individual as it appeared when fixed, with
elongate, stalked body, and as it appeared when swimming about with
contracted body. (From life.)]
There is no stagnant pool, no water standing exposed in watering-trough
or barrel which does not contain thousands of individuals of the
one-celled animals. And in any such stagnant water there may always be
found several or many different kinds or species. A drop of this water
examined with the compound microscope will prove to be a tiny world (all
an ocean) with most of its animals and plants one-celled in structure. A
few many-celled animals will be found in it preying on the one-celled
ones. There are sudden and violent deaths here, and births (by fission
of the parent) and active locomotion and food-getting and growth and all
of the businesses and functions of life which we are accustomed to see
in the more familiar world of larger animals.
=Marine Protozoa.=--One usually thinks of the ocean as the home of the
whales and the seals and the sea-lions, and of the countless fishes,
the cod, and the herring, and the mackerel. Those who have been on the
seashore will recall the sea-urchins and starfishes and the
sea-anemones which live in the tide-pools. On the beach there are the
innumerable shells, too, each representing an animal which has lived
in the ocean. But more abundant than all of these, and in one way more
important than all, are the myriads of the marine Protozoa.
Although the water at the surface of the ocean appears clear and on
superficial examination seems to contain no animals, yet in certain
parts of the ocean (especially in the southern seas) a microscopical
examination of this water shows it to be swarming with Protozoa. And
not only is the water just at the surface inhabited by one-celled
animals, but they can be found in all the water from the surface to a
great depth below it. In a pint of this ocean-water there may be
millions of these minute animals. In the oceans of the world the
number of them is inconceivable. And it is necessary that these
Protozoa exist in such great numbers, for they and the marine
one-celled plants (Protophyta) supply directly or indirectly the food
for all the other animals of the ocean.
Among all these ocean Protozoa none are more interesting than those
belonging to the two orders Foraminifera (fig. 11) and Radiolaria. The
many kinds belonging to these orders secrete a tiny shell (of lime in
the Foraminifera, of silica in the Radiolaria) which encloses most of
the one-celled body. These minute shells present a great variety of
shape and pattern, many being of the most exquisite symmetry and
beauty. The shells are perforated by many small holes through which
project long, delicate, protoplasmic pseudopodia. These fine
pseudopodia often interlace and fuse when they touch each other, thus
forming a sort of protoplasmic network outside of the shell. In some
cases there is a complete layer of protoplasm--part of the body
protoplasm of the Protozoan--surrounding the cell externally.
[Illustration: FIG. 11.--_Rosalina varians_, a marine protozoan
(Foraminifera) with calcareous shell. (After Schultze.)]
When these tiny animals die their hard shells sink to the bottom of
the ocean, and accumulate slowly, in inconceivable numbers, until they
form a thick bed on the ocean floor. Large areas of the bottom of the
Atlantic Ocean are covered with this slimy ooze, called Foraminifera
ooze or Radiolaria ooze, depending on the kinds of animals which have
formed it. Nor is it only in present times that there has been a
forming of such beds by the marine Protozoa. All over the world there
are thick rock strata composed almost exclusively of the fossil shells
of these simplest animals. The chalk-beds and cliffs of England, and
of France, Greece, Spain, and America, were made by Foraminifera.
Where now is land were once oceans the bottoms of which have been
gradually lifted above the water's surface. Similarly the rock called
Tripoli found in Sicily and the Barbadoes earth from the island of
Barbadoes are composed of the shells of ancient Radiolaria.
It is thus evident that the Protozoa is an ancient group of animals.
As a matter of fact zoologists are certain that it is the most ancient
of all animal groups. All of the animals of the ocean depend upon the
marine Protozoa and the marine Protophyta, one-celled plants, for
food. Either they feed on them directly, or prey on animals which in
turn prey on these simplest organisms. A well-known zoologist has
said: "The food-supply of marine animals consists of a few species of
microscopic organisms which are inexhaustible and the only source of
food for all the inhabitants of the ocean. The supply is primeval as
well as inexhaustible, and all the life of the ocean has gradually
taken shape in direct dependence on it." The marine Protozoa are the
only animals which live independently; they alone can live or could
have lived in earlier ages without depending on other animals. They
must therefore be the oldest of marine animals. By oldest is meant
that their kind appeared earliest in the history of the world, and as
it is certain that ocean life is older than terrestrial life--that is,
that the first animals lived in the ocean--it is obvious that the
marine Protozoa are the most ancient of all animal groups.
As already learned in the examination of examples of one-celled
animals, it is evident that life may be successfully maintained
without a complex body composed of many organs performing their
functions in a specialized way. The marine Protozoa illustrate this
fact admirably. Despite their lack of special organs and their
primitive way of performing the life-processes, that they live
successfully is shown by their existence in such extraordinary
numbers. They outnumber all other animals. The conditions of life in
the surface-waters of the ocean are easy and constant, and a simple
structure and simple method of performing the necessary life-processes
are wholly adequate for successful life under these conditions.
FOOTNOTE:
[7] In some Protozoa a number of similar cells temporarily unite to form
a colony, but each cell may still be regarded as an individual animal.
CHAPTER XVI
BRANCH PORIFERA: THE SPONGES
THE FRESH-WATER SPONGE (_Spongilla_ sp.)
TECHNICAL NOTE.--Fresh-water sponges may perhaps not be readily
found in the neighborhood of the school, but they occur over most
of the United States, and careful searching will usually result in
the finding of specimens. They are compact, solid-looking masses,
sometimes lobed, resting on and attached to rocks, logs, timbers,
etc., in clear water in creeks, ponds, or bayous. They are creamy,
yellowish-brown or even greenish in color and resemble some
cushion-like plant far more than any of the familiar animal forms.
They can be distinguished from plants, however, by the fact that
there are no leaves in the mass, nor long thread-like fibres such
as compose the masses of pond algæ (pond scum). When touched with
the fingers a gritty feeling is noticeable, due to the presence of
many small stiff spicules. Sponges should be removed entire from
the substance they are attached to, and may be taken alive to the
laboratory. They die soon, however, and should be put into alcohol
before decay begins.
Note the form of the sponge mass. Is it lobed or branched? Examine the
surface for openings. These are of two sizes; the larger are _osteoles_
or _exhalant openings_, while the smaller and more numerous are _pores_
or _inhalant openings_. The sponge-flesh is called _sarcode_. Examine a
bit of sarcode under the microscope; note the _spicules_. Have these
spicules a regular arrangement? Of what are they composed?
Draw the entire sponge, showing shape and openings; draw some of the
spicules.
Embedded in the body-substance, especially near the base, note (if
present) numerous small, yellowish, sub-spherical or disk-like
bodies, the _gemmules_. These are reproductive bodies. Each gemmule is
a sort of internal bud. It is composed of an interior group of
protoplasmic cells, enclosed by a crust thickly covered with spicules.
In winter the sponge dies down and the gemmules are set free in the
water. In spring the protoplasmic contents issue through an aperture
in the crust, called the _micropyle_ or _foraminal opening_, and
develop and grow into a new sponge.
For a good account of the fresh-water sponge, see Pott's "Fresh-water
Sponges."
A CALCAREOUS OCEAN-SPONGE (_Grantia_ sp.) (fig. 7, D, E, F.)
TECHNICAL NOTE.--For inland schools, specimens preserved in
alcohol or formalin must be used. They may be obtained from
dealers in naturalists' supplies (see p. 453). Specimens of some
species of this genus can be obtained at almost any point on the
Atlantic or Pacific coasts of this country.
Examine the external structure of a specimen. Note the elongate,
sub-cylindrical form, the attached base, the free end. Note the large
exhalant opening, _osteole_ or _osculum_, at the free end; the
numerous small inhalant openings elsewhere on the surface (best seen
in dried specimens). Note the _spicules_ covering the surface of the
body, and the longer ones surrounding the osculum. Cut the sponge in
two longitudinally and note the simple cylindrical body-cavity, the
_gastric cavity_ or _cloaca_. Note the thickness of the body-wall;
note the tubes running through the body-wall from cloaca to external
surface. Through these tubes water laden with food enters the gastric
cavity, where the food is digested, the water and undigested particles
passing out through the osculum. Crush a bit of dried sponge, or boil
a bit of soft sponge in caustic potash and mount on a glass slide.
Examine under a microscope and note the abundance of spicules and the
variety in their form. Two kinds may always be found, and sometimes
three. These spicules are composed of carbonate of lime and can be
dissolved by pouring on to them a drop of hydrochloric acid.
Some of the sponges may have buds growing out from them near the base.
These buds are young sponges developed asexually. If allowed to
develop fully the buds would have detached themselves from the parent
and each would have become a new sponge.
Make drawings showing the form of a whole sponge; the appearance of
the inner face of the sponge bisected longitudinally; the shape of the
spicules.
A COMMERCIAL SPONGE
TECHNICAL NOTE.--For the study of the skeleton of an ocean-sponge
with more complex body buy several common small bath-sponges without
large holes running entirely through them. The teacher should have
also a few specimens of small marine sponges preserved in alcohol or
formalin. Such specimens should be part of the laboratory equipment
(see account of laboratory equipment, p. 450), and can be readily
and cheaply obtained from dealers in naturalists' supplies.
The bath-sponge or slate-sponge consists simply of the hard parts or
skeleton of a sponge animal. In life all of the skeleton is enclosed
or covered by a soft, tough mass composed of layers of cells. Note the
many openings on the surface of the sponge. Crush a bit of the
skeleton and examine it under the microscope. Note that it is composed
of fine fibres of a tough, horny substance called _spongin_, instead
of tiny distinct calcareous spicules.
OTHER SPONGES
The sponges are fixed, plant-like aquatic animals. The members of a
single family live in fresh water, being found in lakes, rivers, and
canals in all parts of the world. All the other sponges, and there are
several thousand species known, live in the ocean. They are to be
found at all depths, some in shallow water near the shore and others
in deeper water, even to the deepest depths yet explored. They are
found in all seas, though especially abundantly in the Atlantic Ocean
and Mediterranean Sea.
=Form and size.=--The shape of the simplest sponges is that of a tiny
vase or nearly cylindrical cup, hollow and attached at its base. At the
free end there is a large opening. But there is a great deal of variety
in the form and size of different sponges. There is, indeed, much
variation in the shape and general character of different individuals of
the same species. Unlike most other animals, sponges are fixed, and the
character of the surface to which a sponge is attached has much
influence upon its shape. If this surface is rough and uneven the sponge
may follow in its growth the sinuosities of the surface and so become
uneven and distorted in shape. At best, only a few kinds of sponges have
any very even and symmetrical shape. Most of them are very unsymmetrical
and grow more like a low compact bushy plant than like the animals we
are familiar with. The smallest sponges are only 1 mm. (1/25 in.) high,
while the largest may be over a meter (39 in.) in height. In color
living sponges may be red, purple, orange, gray, and sometimes blue.
Most sponges have the whole body of one color.
[Illustration: FIG. 12.--The skeleton of a "glass" sponge (skeleton
composed of siliceous spicules) from Japan. (From specimen.)]
=Skeleton.=--A very few sponges have no skeleton at all. The others
have a skeleton or hard parts composed of interwoven fibres of the
tough, horny substance called spongin, or of hosts of fine needles or
spicules of silica or of carbonate of lime. The siliceous skeletons of
some of the so-called glass-sponges (fig. 12) are very beautiful. The
lime and siliceous sponge spicules exhibit a great variety of outline,
some being anchor-shaped, some cross-shaped, and some resembling tiny
spears or javelins.
=Structure of body.=--The skeleton of a sponge whether composed of
interlacing fibres or of short spicules is always invisible from the
outside when the sponge is alive. It is embedded in, or clothed by,
the soft, fleshy part of the body. The soft part of the sponge is
composed simply of two layers of cells, one constituting the external
surface of the body, and the other lining the interior cavities and
canals of the body. Between these two cell-layers there is a mass of
soft gelatinous substance all through which protoplasm ramifies, and
in which are embedded numerous scattered cells. There are, as seen in
the case of _Spongilla_ and _Grantia_, no systems of organs such as
characterize the higher animals. No heart, lungs, alimentary canal,
nervous system, organs of locomotion, eyes, ears, or other organs of
special sense; the sponge has none of these. It is simply an aggregate
of cells, arranged in two layers, and supported usually by a skeleton
of horny fibres or calcareous or siliceous spicules. Its body is
usually shapeless, unsymmetrical and without front or back, right or
left. It is not to be wondered at that sponges were for a long time
believed to be plants.
=Feeding habits.=--The sponges feed on minute bits of animal or plant
substance and on the microscopic unicellular plants or animals which
float in the water which bathes their bodies. The water entering the
sponge-body through the various openings of the surface is moved
along by the waving or lashing of the flagella of the cells which line
the canals, and these currents of water bear with them the tiny
organisms which are taken up by these same cells and digested. The
incoming currents of water meet in the central cavity or cavities of
the body and pass out through the large opening called the osculum at
the free end of the vase-like body, or if the body is branched,
through the large openings at the tips of these branches.
The same currents of water bring also oxygen for the sponge's breathing
and carry away the carbonic acid gas given out by the body-cells.
As a German naturalist has said, the one necessary condition for the
life of a sponge is the streaming of water through its body. All
sponges have a system of canals for this water-current and all have
means, in the waving flagella or cilia with which these canals are
lined, for producing these currents. When a live sponge is put into a
vessel of water, currents are immediately set up, and they always flow
into the body through the many fine openings and out of the body
through the osculum.
=Development and life-history.=--Although the sponge in its adult
condition is permanently attached by its base to the sea-bottom or to
some rock or shell, when it is first born it is an active free-swimming
creature. The sponges reproduce in two ways, asexually and sexually. The
asexual mode of reproduction of the fresh-water sponge by gemmules has
already been described. The ocean sponges also reproduce asexually
either by forming interior gemmules or external buds. In this latter
method a bud forms on the outer surface of the body which increases in
size and finally grows into a new sponge individual. In some species
this new sponge does not become separated from the body of the mother,
but remains attached to it like a branch to a tree-trunk. By the
continued production of such non-separating individuals, a colony of
sponges is formed which has the general appearance of a branching plant.
In other species the new sponge formed by the development and growth of
a bud falls off and becomes a distinct separate individual.
In the sexual mode of reproduction, male or sperm-cells and female or
egg-cells are developed in the same individual. The sperm-cells are
motile and swim about in the cavities and canals of the sponge-body
until they find egg-cells, which they fertilize. The fertilized eggs
begin to develop and pass through their first stages in the
sponge-body. Finally the embryo sponge, which is usually a tiny oval
or egg-shaped mass of cells, escapes from the body of the parent into
the water. The young sponge has some of its outer cells provided with
cilia, and by means of these it swims about. After a while it comes to
rest on the ocean-floor or on some rock or shell, attaches itself, and
begins to take on the form and character of the parent. It leads
hereafter a fixed sedentary life.
=The sponges of commerce.=--The sponge-skeletons which are the
"sponges" that we use all belong to a few species, not more than half
a dozen. Most of the commercial sponges come from the Mediterranean
Sea, though some come from the Bahama Islands, some from the Red Sea,
and a few from the coasts of Greece, Asia Minor, and Africa. The
commercial sponges do not live in very deep water; they are usually
found not deeper than 200 feet. The living sponges are collected by
divers, or are dragged up by men in boats using long-poled hooks, or
dredges. "When secured they are exposed to the air for a limited time,
either in the boats or on shore, and then thrown in heaps into the
water again in pens or tanks built for the purpose. Decay thus takes
place with great rapidity, and when fully decayed they are fished up
again, and the animal matter beaten, squeezed, or washed out, leaving
the cleaned skeleton ready for the market. In this condition after
being dried and sorted, they are sold to the dealers, who have them
trimmed, re-sorted and put up in bales or on strings ready for
exportation. There are many modifications of these processes in
different places, but in a general way these are the essential-steps
through which the sponge passes before it is considered suitable for
domestic purposes. Bleaching-powders or acids are sometimes used to
lighten the color, but these unless very delicately handled injure the
durability of the fibres."
=Classification.=--The sponges are classified according to the character
of the skeleton. In one group are put all those sponges which have a
skeleton of calcareous spicules, and this group is called the Calcarea.
All other sponges are grouped as Non-Calcarea, the members of this group
either having no skeleton at all, or having a skeleton composed of
siliceous spicules or of spongin fibres. According to the absence or
presence of a skeleton and the character of the skeleton when it exists
the Non-Calcarea are subdivided into smaller groups.
CHAPTER XVII
BRANCH CŒLENTERATA: THE POLYPS, SEA-ANEMONES,
CORALS, AND JELLYFISHES
The structure and life-history of an example of the polyps (the
Fresh-water Hydra, _Hydra_ sp.) has been studied in Chapters X and XI.
OTHER POLYPS, SEA-ANEMONES, CORALS, AND
JELLYFISHES
TECHNICAL NOTE.--The teacher should have, if possible, several
pieces of coral and a few specimens of Cœlenterates in alcohol or
formalin, which will show the external character, at least, of
these animals (see account of laboratory equipment, p. 450). If
the school is on the coast, the pupils should be shown the
sea-anemones of the tide-pools.
The animals which are included in the branch Cœlenterata are, at least
in living condition, unfamiliar to most of us. Like the sponges, they
are almost all inhabitants of the ocean; a few, like _Hydra_, live in
fresh water. Like the sponges, too, most of the members of this branch
are fixed, and in their general appearance suggest a plant rather than
an animal. The name zoophytes, or plant-animals, which is often
applied to these animals is based on this superficial resemblance. But
many of the Cœlenterates lead an active free-swimming life. This is
true of the jellyfishes which float or swim about on or near the
surface of the ocean. Many of the zoophytes spend part of their life
in an active free-swimming condition before settling down, becoming
attached and thereafter remaining fixed. In localities near the
seashore many animals belonging to this great group can be readily
found and observed. The beautiful sea-anemones with their
slowly-waving tentacles, the fine many-branched truly plant-like
hydroids with their hosts of little buds, and the soft colorless
masses of jelly, the jellyfishes, which are cast up on to the beaches
by the waves are all animals belonging to the branch Cœlenterata.
=General form and organization of body.=--The general or typical plan of
body-structure for the Cœlenterata, these animals which come next to the
sponges in degree of complexity, can best be understood by imagining the
typical cylindrical or vase-like body of the simple sponges to be
modified in the following way: The middle one of the three layers of the
body-wall not to be composed of scattered cells in a gelatinous matrix,
but to be simply a thin non-cellular membrane; the body-wall not to be
pierced by fine openings or pores, but connected with the outside only
by the single large opening at the free end, and this opening to be
surrounded by a circlet of arm-like processes or tentacles, which are
continuations of the body-wall and similarly composed. Such a
body-structure, which we saw well shown by _Hydra_, is the fundamental
one for all polyps, sea-anemones, corals, and jellyfishes. The variety
in shape of the body and the superficial modifications of this type-plan
are many and striking, but after all the type-plan is recognizable
throughout the whole of this great group of animals.
The two chief body-shapes represented in the branch are those of the
polyps on the one hand, and the jellyfishes or medusæ on the other. The
polyp-shape is that of a tube with a basal end blind or closed, attached
to some firm object in the water and with the free end with an opening,
the mouth-opening. At this mouth-end there is a circlet of movable, very
contractile tentacles. The mouth may open directly into the interior of
the body, which interior may be called the digestive cavity, or it may
lead into a simple short tube produced by the invagination or bending in
of the body-wall, which may be looked on as the simplest kind of
œsophagus. This œsophageal tube opens into the body-cavity or digestive
cavity. This cavity may be incompletely divided by longitudinal
partitions which project from the sides into the cavity.
The jellyfish or medusoid body-form corresponds in general to an
umbrella or bell. Around the edge of this umbrella are disposed
numerous threads or tentacles (corresponding to the circlet of
tentacles in the polyp). The mouth-opening is at the end of a longer
or shorter projection which hangs down from the middle of the under
side of the umbrella. The interior body-cavity or digestive cavity
extends out into the umbrella-shaped part of the body, usually in the
condition of canals radiating from the centre and a connecting canal
running around the margin of the umbrella.
=Structure.=--Although the Cœlenterata show little indication of the
complex composition of the body out of organs, as it exists among the
higher animals, yet they do show an unmistakable advance on the simple,
almost organless body of the sponges. This is chiefly shown by the
differentiation among the cells which compose the body. In the polyps
and jellyfishes some of the cells are specialized to be unmistakable
muscle-cells, some to be nerve-cells and fibres, and so on. A very
simple nervous system consisting of small groups of nerve-cells
connected by nerve-fibres exists. Some very simple special sense-organs
may occur. The digestive system, although in the simpler Cœlenterates
consisting merely of the cylindrical body-cavity enclosed by the
body-wall and opening by the single hole at the free end of the body, in
some is rather complex and is composed of different parts. Those
Cœlenterates which are not fixed but lead an active, free-swimming life,
viz., the jellyfishes or medusæ, are the most highly organized.
The tentacles which surround the mouth-opening and serve to grasp food
and carry it into the mouth, and the stinging or lasso threads with
which these tentacles are provided are special organs possessed by
most of these animals.
=Skeleton.=--Like the sponges, some of the Cœlenterata possess a hard
skeleton. This skeleton is always composed of calcium carbonate and is
called _coral_. Those polyps which form such a skeleton are called the
corals. Coral will be described in connection with the account of the
coral-polyps.
=Development and life-history.=--The polyps and jellyfishes reproduce
both asexually and sexually. The asexual mode is usually that of
budding. On a polyp a bud is formed by a hollow outgrowth of the
body-wall. The bud grows, an opening appears at its distal end, a
circlet of tentacles arises about this mouth-opening and a new polyp
individual is formed. This individual may separate from the parent or
it may remain attached to it. By the development of numerous buds, and
the remaining attached of all of the individuals developing from these
buds, a colony of polyp individuals may be formed, plant-like in
appearance. The various polyp individuals of a colony may differ
somewhat among themselves, and these differences are correlated with a
division of labor. Thus some of the individuals may devote themselves
to getting food for the colony, and these have mouth and tentacles.
Others may be devoted to the production of new individuals by budding
or by producing germ-cells, and may not have any mouth-opening or any
food-grasping tentacles.
In case of many polyps all or some of the new individuals which arise
by budding do not become polyps, but develop into medusæ or jellyfish,
which separate from the fixed polyp and swim off through the water.
These medusæ or jellyfish produce sperm-cells and egg-cells. The
sperm-cells fertilize the egg-cells and a new individual develops from
each fertilized egg. This new individual is at first an active
free-swimming larva called a _planula_, which does not resemble either
a medusa or polyp. After a while it settles down, becomes fixed and
develops into a polyp. Thus a polyp may produce a medusa or jellyfish
which, however, produces not a new jellyfish, but a polyp. This is
called an _alternation of generations_, and is not an uncommon
phenomenon among the lower animals. It results from such an
alternation of generations that a single species of animal may have
two distinct forms. This having two different forms is called
_dimorphism_. Sometimes, indeed, a species may appear in more than two
different forms; such a condition is called _polymorphism_.
Not all medusæ or jellyfish are produced by polyp individuals, nor do
jellyfish always produce polyps and not jellyfishes. There are some
jellyfishes (we might call them the true jellyfishes) which always
have the jellyfish form, producing new jellyfishes either by budding
or by eggs, and there are some polyps which always have the true polyp
form, producing new individuals, either by budding or by eggs, always
of polyp form and never of jellyfish form. That is, some species of
Cœlenterata exist only in polyp form, some species exist only in
jellyfish form, while some species (those having an alternation of
generations) exist in both polyp and jellyfish form, these two forms
appearing as alternate generations.
=Classification.=--The branch Cœlenterata is divided into four
classes: (1) the Hydrozoa, including the fresh-water polyps, numerous
marine polyps, many small jellyfishes and a few corals; (2) the
Scyphozoa, including most of the large jellyfishes; (3) the Actinozoa,
including the sea-anemones and most of the stony corals; (4) the
Ctenophora, including certain peculiar jellyfishes.
[Illustration: FIG. 13.--The Portuguese Man-of-War (_Physalia_ sp.).
(From specimen from Atlantic Coast.)]
=The polyps, colonial jellyfishes, etc. (Hydrozoa).=--To the class
Hydrozoa belongs the _Hydra_ already studied. There are a few other
fresh-water polyps and they all belong to this class. The most
interesting members of the class are the "colonial jellyfishes,"
constituting the order Siphonophora. These colonial jellyfishes are
floating or swimming colonies of polypoid and medusoid individuals in
which there is a marked division of labor among the individuals,
accompanied by marked differences in structural character. The
individuals are accordingly polymorphic, that is, appear in various
forms, although all belong to the same species. Because these various
individuals forming a colony have given up very largely their
individuality, combining together and acting together like the organs
of a complex animal, they are usually not called individuals, nor on
the other hand organs, but _zooids_, or animal-like structures. The
beautiful "Portuguese man-of-war" (fig. 13) is one of these colonial
jellyfishes. It appears as a delicate bladder-like float, brilliant
blue or orange in color, usually about six inches long, and bearing on
its upper surface which projects above the water a raised
parti-colored crest, and on its under surface a tangle of various
appendages, thread-like with grape-like clusters of little bell- or
pear-shaped bodies. Each of these parts is a peculiarly modified
polyp- or medusa-zooid produced by budding from an original central
zooid. The Portuguese man-of-war is very common in tropical oceans,
and sometimes vast numbers swimming together make the surface of the
ocean look like a splendid flower-garden.
[Illustration: FIG. 14.--A colonial jellyfish (Siphonophora). (After
Haeckel.)]
Usually the central zooid in a Siphonophore to which the other zooids
are attached is not a bladder-like float, but is an upright tube of
greater or less length. In the Siphonophore shown in figure 14, the
compound body is composed of a long central hollow stem with hundreds
or thousands of variously shaped parts, each of which is reducible to
either a polyp or medusazooid, attached around it. The upper end is
enlarged to form an air-filled chamber, a sac-like boat, by means of
which the whole colony is kept afloat. Around the upper end of the
central stem are many medusoid structures, the swimming-bells, by
means of whose opening and closing the whole colony is made to swim
through the water. Each swimming-bell is a modified medusa-zooid,
without tentacles, without digestive or reproductive organs, but
exercising the power of swimming by contracting and forcing the water
out of the hollow bell just as is done by the free medusæ. Below the
swimming-bells, at the lower end of the central stem, are grouped many
structures presenting at first sight a confusion of variety and
complexity, but on careful examination revealing themselves to be
polyp- and medusa-zooids modified to form at least five kinds of
particularly functioning structures. There are many flattened
scale-like parts whose function is simply that of affording a passive
protection, in times of danger, to the other structures. These
protecting-scales are greatly modified medusa-zooids, each consisting
of a simple cartilage-like gelatinous mass penetrated by a
food-carrying canal. Under the broad leaves of these protecting-zooids
are a number of pear-shaped bodies which have a wide octagonal
mouth-opening at their free end, and possess in their interior certain
digestive glands. Each one is provided with a very long flexible
tentacle which bears many fine stinging-threads. The tentacle waves
back and forth in the water, and on coming in contact with an enemy or
with prey its poisonous stinging-threads shoot out and paralyze or
wound the unfortunate animal. These pear-shaped bodies are the feeding
structures, each being a modified polyp-zooid. Scattered among these
dangerous structures are many somewhat similarly shaped but wholly
harmless structures, the sense-structures. Each of these has a
pear-shaped body but without mouth-opening, and also a long, very
sensitive, tentacle-like process. The sense of feeling is highly
developed in these tentacles, and they discover for the colony the
presence of any strange body. These sense-structures are modified
polyp-zooids. Finally there are two other kinds of structures, usually
arranged in groups like bunches of grapes, which are the reproductive
structures, male and female. They are modified medusa-zooids grown
together and without tentacles. This whole colony, or this compound
animal, floats or swims about at the surface of the ocean, and
performs all of the necessary functions of life as a single animal
composed of organs might. Yet the Siphonophore is more truly to be
regarded as a community in which the hundreds or thousands of animals,
representing five or six kinds of individuals, all of one species, are
fastened together. Each individual performs the particular duties
devolving upon its kind or class. Thus there are food-gathering
individuals, locomotor individuals, sense individuals, and
reproductive individuals. The modifications of the various kinds of
individuals are more extreme than in the case of the various kinds of
individuals composing a bee-community, for example, but the holding
together or fusing of all into one body or corporation is a condition
which makes this greater modification necessary and not unexpected.
And there is no difficulty in seeing that each of these parts is
really, structurally considered, a modified polyp or medusa.
[Illustration: FIG. 15.--A jellyfish or medusa, _Gonionema vertens_,
eating two small fishes. (From specimen from Atlantic Coast.)]
=The large jellyfishes, etc. (Scyphozoa).=--To the class Scyphozoa
belong most of the common large jellyfishes. When one walks along the
sea-beach soon after a storm one may find many shapeless masses of a
clear jelly-like substance scattered here and there on the sand. These
are the bodies or parts of bodies of jellyfishes which have been cast up
by the waves. Exposed to the sun and wind the jelly-like mass soon dries
or evaporates away to a small shrivelled mass. The body-substance of a
jellyfish contains a very large proportion of water; in fact there is
hardly more than 1 per cent of solid matter in it.
The jellyfishes occur in great numbers on the surface of the ocean and
are familiar to sailors under the name of "sea-bulbs." Some live in
the deeper waters; a few specimens have been dredged up from depths of
a mile below the surface. They range in size from "umbrellas" or disks
a few millimeters in diameter to disks of a diameter of two meters
(2-1/6 yards). They are all carnivorous, preying on other small ocean
animals which they catch by means of their tentacles provided with
stinging-threads. The tentacles of some of the largest jellyfishes
"reach the astonishing length of 40 meters, or about 130 feet." Many
of the jellyfishes are beautifully colored, although all are nearly
transparent. Almost all of them are phosphorescent, and when irritated
some emit a very strong light.
=The sea-anemones and corals (Actinozoa).=--Almost everywhere along
the seashore where there are rocks and tide-pools a host of various
kinds of sea-anemones can be found. When the tide is out, exposing the
dripping seaweed-covered rocks, and the little sand- or stone-floored
basins are left filled with clear sea-water, the brown and green and
purple "sea-flowers" may be found fixed to the rocks by the base with
the mouth-opening and circlet of slowly-moving tentacles hungrily
ready for food (fig. 16). Touch the fringe of tentacles with your
fingertip and feel how they cling to it and see how they close in so
as to carry what they feel into the mouth-opening. A host of
individuals there are, and scores of different kinds; some small,
some large, some with the body covered outside with tiny bits of stone
and shell so that they are hardly to be distinguished from the rock to
which they cling; some of bright and showy colors. These are the most
familiar members of the class Actinozoa.
[Illustration: FIG. 16.--Sea anemones, _Bunodes californica_, open and
closed individuals. The closed individuals in upper right-hand corner
show the external covering of small bits of rock and shell,
characteristic of most individuals of this species. (From living
specimens in a tide-pool on the Bay of Monterey, California.)]
But in other oceans, along the coasts of other lands, especially those
of the tropics and sub-tropics, there are some other members of the
class which are of unusual interest. They are the corals, or coral
polyps. We know these animals chiefly by their skeletons (fig. 17).
The specimens of corals which one sees in collections, or made into
ornaments, are the calcareous skeletons of various kinds of the coral
polyps. Some of the corals live together in enormous numbers, forming
branching colonies fixed as closely together as possible, and secrete
while living a stony skeleton of carbonate of lime. These skeletons
persist after the death of the animals, and because of their abundance
and close massing form great reefs or banks and islands. These coral
reefs and islands occur only in the warmer oceans. In the Atlantic
they are found along the coasts of Southern Florida, Brazil and the
West Indies; in the Pacific and Indian Oceans there are great coral
reefs on the coast of Australia, Madagascar and elsewhere, and certain
large groups of inhabited islands like the Fiji, Society, and Friendly
Islands are exclusively of coral formation. Coral islands have a great
variety of form, although the elongated, circular, ring-shaped and
crescent forms predominate. How such islands are first formed is
described as follows by a well-known student of corals:
[Illustration: FIG. 17.--Skeleton of a branching coral, _Madrepora
cervicornis_. (From specimen.)]
"A growing coral plantation, with its multitudinous life, oftentimes
arises from great depths of the ocean, and the sea-bed upon which it
rests is probably a submarine bank or mountain, upon which have lodged
and slowly aggregated the hard skeletons of pelagic forms of life.
When, through various sources of increase, this submarine bank
approaches the depth of from one hundred to one hundred and fifty feet
from the surface of the water, there begins on its top a most
wonderful vital activity. It is then within the bathymetric zone of
the reef-building corals. Of the many groups of marine life which then
take possession of the bank, corals are not the only animals, but
they are the most important, as far as its subsequent history goes. As
the bank slowly rises by their growth, it at last approaches the
surface of the water, and at low tide the tips of the growing branches
of coral are exposed to the air. This, however, only takes place in
sheltered localities, for long before it has reached this elevation it
has begun to be more or less changed and broken by the force of the
waves. As the submarine bank approaches the tide level, the delicate
branching forms have to meet a terrific wave-action. Fragments of the
branching corals are broken off from the bank by force of the waves,
and falling down into the midst of the growing coral below fill up the
interstices, and thus render the whole mass more compact. At the same
time larger fragments are broken and rolled about by the waves and are
eventually washed up into banks upon the coral plantation, so that the
island now appears slightly elevated above the tides. This may be
called a first stage in the development of a coral island. It is,
however, little more than a low ridge of worn fragments of coral
washed by the high tides and swept by the larger waves--a low, narrow
island resting on a large submarine bank."
When the coral island rises thus a little above the surface of the
water, the waves break up some of the coral into fine sand, which fills
in the interstices, and offers a sort of soil in which may germinate
seeds brought in the dried mud on the feet of ocean birds or carried by
the ocean currents. With the beginning of vegetable growth the soil is
more firmly held, is fertilized and ready for the seeds of plants which
need a better soil than lime sand. Flying insects find their way to the
island, especially if it be near the mainland, birds begin to nest on
it, and soon it may be the seat of a luxuriant plant and animal life.
For an account of coral islands see Darwin's "The Structure and
Distribution of Coral Reefs."
There are over 2000 kinds of coral polyp known, and their skeletons
vary much in appearance. Because of the appearance of the skeleton
certain corals have received common names, as the organ-pipe coral,
brain coral, etc. The red coral, of which jewelry is made, grows
chiefly in the Mediterranean. It is gathered especially on the western
coast of Italy, and on the coasts of Sicily and Sardinia. Most of this
coral is sent to Naples, where it is cut into ornaments.
There are other interesting members of the class Actinozoa like the
beautiful sea-pens, sea-feathers and sea-fans, delicate, branching,
tree-like forms found all over the world.
=Ctenophora.=--The members of this class are mostly small, peculiar
jellyfishes which do not form colonies, and are extremely delicate,
being usually perfectly transparent. They swim by means of cilia. They
never appear in a polyp condition, but are always medusoid in shape.
CHAPTER XVIII
BRANCH ECHINODERMATA: STARFISHES,
SEA-URCHINS, SEA-CUCUMBERS
STARFISH (_Asterias_ sp.)
TECHNICAL NOTE.--The species of _Asterias_ are widely distributed
on both coasts of the United States and may be procured on almost
any rocky shore at low tide. Teachers in inland schools can obtain
preserved material from the dealers mentioned on p. 453. Most of
the specimens should be placed in alcohol or 4% formalin. If fresh
material can be had it is well to place at least one specimen for
each student in a 20% solution of nitric acid in water for two or
three hours, when all of the calcareous parts will have been
dissolved, and after a thorough washing the specimen will be ready
for use.
=External structure= (figs. 18 and 19.)--In a fresh specimen or one
which has been preserved in alcohol or formalin note the raying out of
parts of the body from a common centre. This is characteristic of the
body organization of all Echinoderms, and is known as _radial symmetry_.
The lower surface of the body is called the _oral_ (because the mouth is
on this surface), while the upper is called the _aboral_ surface. The
central part of the body is called the _disk_. Note on the aboral
surface of the disk a small striated calcareous plate, the _madreporite_
or _madreporic plate_. In the middle (or very nearly in the middle) of
this surface of the disk there is a small pore, the _anal opening_. The
entire aboral surface as well as a greater part of the oral side is
thickly studded with the calcareous _ossicles_ of the body-wall. These
ossicles support numerous short stout _spines_ arranged in irregular
rows. Note that some of the ossicles support certain very small
pincer-like processes, the _pedicellariæ_. In the interspaces between
the calcareous plates are soft fringe-like projections of the inner
body-lining, the _respiratory cæca_. Note at the tip of each arm or ray
a cluster of small calcareous ossicles and within each cluster a small
speck of red pigment, the eye-spot or _ocellus_.
[Illustration: FIG. 18.--Dissection of a starfish (_Asterias_ sp.).]
Make a drawing of the aboral surface showing all these parts.
On the oral surface note the centrally-located _mouth_, the
_ambulacral grooves_, one running longitudinally along each ray, and
in each groove two double rows of soft tubular bodies with sucker-like
tips. These are called the _tube-feet_ and are organs of locomotion.
Make a drawing of the oral surface.
=Internal structure= (figs. 18 and 19).--TECHNICAL NOTE.--Take a
specimen which has been immersed for some time in the nitric acid
solution, and with a strong pair of scissors, or better,
bone-cutters, cut away all the aboral wall of the disk except that
immediately around the madreporite and the anus. Now begin at the
tip of each ray and cut away the aboral wall of each, leaving,
however, a single arm intact. When the roof of each arm has been
carefully dissected away the specimen should appear as in fig. 18.
Note the large _alimentary canal_, which is divided into several
regions. Note the short _œsophagus_ leading from the _mouth_ on the
oral surface directly into a large membranous pouch, the _cardiac_
portion of the _stomach_. By a short constriction the cardiac portion
is separated from the part which lies just above, i.e., the _pyloric_
portion of the stomach. From the pyloric portion large, pointed,
paired glandular appendages extend into each ray. These are the
_pyloric cæca_. Their function is digestive, and oftentimes they are
spoken of as the _digestive glands_ or "livers." The pyloric cæca, as
well as the cardiac portion of the stomach, are held in place by
paired muscles which extend into each arm. Note two sets of these
muscles, one set for thrusting the cardiac portion of the stomach out
through the mouth and another for pulling it back, the _protractor
muscles_ and _retractor muscles_, respectively. The starfish obtains
its food by enclosing it in its everted stomach and then withdrawing
stomach and food into the body. Note that the pyloric portion of the
stomach opens above into a short _intestine_ terminating in the
_anus_, and observe that there is attached to the intestine a
convoluted many-branched tube, the _intestinal cæcum_.
Carefully remove a pair of pyloric cæca from one of the rays and note
the short duct which connects them with the pyloric chamber of the
stomach. Note in the angle of each two adjoining rays paired glandular
masses which empty by a common duct on the aboral surface. These
glands are the _reproductive organs_. Note the small bulb-like
bladders extending in two double rows on the floor of each ray. These
are the water-sacs or _ampullæ_, and each one is connected directly
with one of the locomotor organs, the tube-feet.
Make a drawing of the organs in the dissection which have so far been
studied.
TECHNICAL NOTE.--For a careful study of the locomotor organs a fresh
starfish should be injected. This can usually be accomplished by
cutting one ray off squarely, and inserting the needle of a
hypodermic syringe (which has been previously filled with a watery
solution of carmine or Berlin blue), into the end of the radial
water-tube which runs along the floor of the ray. By injecting here,
the whole system of vessels, tube-feet, and ampullæ are filled.
Note a ring-shaped canal which passes around the alimentary canal near
the mouth from which radial vessels run out beneath the floor of each
ray and from which a hard tube extends to the madreporite. This hard
tube is the _stone canal_, so called because its walls contain a series
of calcareous rings, while the circular tube is the _ring canal_ or
_circum-oral water-ring_ from which radiate the _radial canals_. In some
species of starfish there are bladder-like reservoirs, _Polian
vesicles_, which extend interradially from the ring canal.
Note that the ampullæ and tube-feet are all connected with the radial
canals. By a contraction of the delicate muscles in the walls of the
ampullæ the fluid in the cavity is compressed, thereby forcing the
tube-feet out. By the contraction of muscles in the tube-feet they are
again shortened while the small disk-like terminal sucker clings to
some firm object. In this way the animal pulls itself along by
successive "steps." This entire system, called the _water-vascular
system_, is characteristic of the branch Echinodermata. In addition to
the fluid in the water-vascular system there is yet another
body-fluid, the _perivisceral fluid_, which bathes all of the tissues
and fills the body-cavity.
[Illustration: FIG. 19.--Semi-diagrammatic figure of cross-section of
the ray of a starfish, _Asterias_ sp.]
TECHNICAL NOTE.--Take a drop of the perivisceral fluid from a
living starfish and examine under high power of microscope, noting
the amœboid cells it contains.
The perivisceral fluid is aerated through outpocketings of the thin
body-wall which extend outward between the calcareous plates of the
body. These outpocketings have already been mentioned as the
respiratory cæca (see p. 109). Surrounding the stone canal is a thin
membranous tube, and within it and by the side of the stone canal is a
soft tubular sac. The function of these organs is not certainly known.
Work out the _nervous system_; note, as its principal parts, a
nerve-ring about the mouth, and nerves running from this ring beneath
the radial canals along each arm.
=Life-history and habits.=--The starfishes are all marine forms. They
hatch from eggs, and in their early stages are very different in
appearance from the adults. At first they are bilaterally symmetrical,
their radial symmetry being acquired later. Thousands of eggs and
sperm-cells are extruded into the sea-water, where fertilization and
development take place. The young swim freely in the open sea, feeding
on microscopic organisms, and then undergo very radical changes in the
course of their development. The adults are for the most part
carnivorous, feeding on crabs, snails, and the like. The live prey is
surrounded by the extruded stomach which secretes fluids that kill it,
after which the soft parts are digested. (See general account of the
life-history of Echinoderms on p. 119.)
THE SEA-URCHIN (_Strongylocentrotus_ sp.)
=External structure.=--TECHNICAL NOTE.--If fresh or alcoholic
specimens or even the dry "tests" of the sea-urchin (fig. 20) are
to be had, the general characteristics of the external structure
can be made out.
How does the external surface of the sea-urchin differ from that of
the starfish? Can you find the very long _tube-feet_? Where is the
mouth-opening? With what is it surrounded? Each tooth is enclosed in a
calcareous framework. The whole structure is known as "_Aristotle's
lantern_."
TECHNICAL NOTE.--Remove the spines from the underlying shell or
test (fig. 21) and wash the test until perfectly clean, or place
in a solution of lye for a short time and then wash.
[Illustration: FIG. 20.--A sea-urchin, _Strongylocentrotus
franciscanus_. (From specimen from Bay of Monterey, Calif.)]
Note the characteristic radial symmetry of the _shell_ or _test_. Note
on the aboral aspect, diverging from the medial anal aperture, five
double rows of pores. What are these for? Each of the five divisions
set with pores is called an _ambulacral area_, while the intervening
segments which support the long spines are called the _interambulacral
areas_. Note on the aboral surface, surrounding the median-placed anal
aperture, a series of small plates. Those which are located in the
interambulacral areas are the _genital plates_. Through these plates
the ducts from the reproductive organs open by small pores. Note a
very much enlarged plate with a striated appearance. This is the
_madreporite_, which, as in the starfish, is the external opening of
the stone canal and water-vascular system. Note the small _ocular
plate_ at the tip of each ambulacral area. The ocular plates contain
small pigment-cells and communicate with the nervous system.
[Illustration: FIG. 21.--"Test" of sea-urchin, _Strongylocentrotus
franciscanus_, with spines removed. (From specimen.)]
From a general inspection of the sea-urchin's shell the Echinoderm
characteristics, namely, radial symmetry and the presence of the
water-vascular system, are readily seen. While at first glance there
is apparent little similarity between the starfish and sea-urchin,
nevertheless careful examination shows that the two animals are alike
in their fundamental structure. Both are radially symmetrical. The
position of the anal opening makes both starfish and sea-urchin
slightly asymmetrical. In both the madreporite and anus are on the
aboral side, while the mouth is centrally located on the oral side. In
the starfish we noted five ambulacral areas, one on the under side of
each arm; similarly we find five in the sea-urchin. In both cases also
we find the ocular spots at the tips of the ambulacral areas. The
genital apertures are situated interradially in the starfish. In the
sea-urchin they are similarly placed. The dissimilarity between the
two forms is largely due to the very much developed outer spines and
the dorso-ventral thickening of the disk in the sea-urchin. The
starfish is carnivorous, while the sea-urchin lives on vegetable
matter consisting for the most part of green algæ and the red
sea-weeds. Correlated with this difference in food-habits there are
certain differences in the structure of the internal organs. For
example, the alimentary canal in the sea-urchin winds in about two and
one-half turns within the body-cavity before it reaches the anus.
OTHER STARFISHES, SEA-URCHINS, SEA-CUCUMBERS,
ETC.
Without exception all the Echinoderms, under which term are included
the starfishes, sea-urchins, brittle-stars, feather-stars, and
sea-cucumbers, live in the ocean. Some of them, the starfishes and
sea-urchins, are among the most common and familiar animals of the
seashore. Most of them are not fixed, but can move about freely,
though slowly. Some of the feather-stars are fixed, as the sponges and
polyps are.
=Shape and organization of body.=--The body-shape of the Echinoderm
varies from the flat, rayed body of the starfish to the thick,
flattened egg-shape of the sea-urchin, the melon-like sac of the
sea-cucumber and the delicate many-branched head of the sea-lily
sometimes borne on a slender stalk. But in all these shapes can be
seen more or less plainly a symmetrical, radiate arrangement of the
parts of the body. The Echinoderm body has a central portion from
which radiate separate arm or branch-like parts, as in the starfishes
and sea-lilies, or about which are arranged radiately the internal
body-parts, although the external appearance may at first sight give
no plain indication of the radiate arrangement. This is the case with
the sea-urchins and sea-cucumbers, yet, as has been seen in the
sea-urchin, the radiate arrangement can be readily perceived by closer
examination of the surface of the egg- or sac-like body. The radiating
parts of the body are usually five. In the body of an Echinoderm can
be usually recognized an upper or dorsal surface and a lower or
ventral surface. The mouth is usually situated on the ventral side and
the anal opening on the dorsal. Echinoderms agree also in having a
calcareous outer skeleton or body-wall usually in the condition of
definitely-shaped plates or spicules fitted either movably or rigidly
together. This outer body-wall or exoskeleton may bear many tubercles
or spines. These spines are sometimes movable. The body-wall of the
sea-urchin shows very well the exoskeleton composed of plates on which
are borne movable strong spines.
=Structure and organs.=--As has been learned from the dissection of
the starfish, the Echinoderms have well-developed systems of organs.
The body-structure in its complex organization presents a marked
advance beyond the structural condition of the polyps and jellyfishes.
There is a well-organized digestive system with mouth, alimentary
canal, and anal opening. The alimentary canal is either a simple
spiral or coiled tube, or it is a tube in which can be recognized
different parts, namely, œsophagus, stomach, intestine, cæca, and
special glands secreting digestive fluids. This alimentary canal is
not, as in the polyps, simply the body-cavity, but it is an inclosed
tubular cavity lying within the general body-cavity. At the
mouth-opening there is in some Echinoderms, notably the sea-urchins, a
strong masticating apparatus consisting of five pointed teeth which
are arranged in a circle about the opening. The nervous system
consists of a central ring around the œsophagus or mouth, from which
branches extend into the radiately arranged arms or regions of the
body. There is no brain as in the higher animals, but the central
nerve-ring is composed of both nerve-cells and nerve-fibres as in the
nerve-centres of higher forms. Of organs of special sense there are
special tactile or touch organs in all the Echinoderms, and the
starfishes have very simply composed eyes or eye-like organs at the
tips of the rays.
While some of the Echinoderms breathe simply through the outer
body-wall, taking up by osmosis the air mixed with the water, some of
them have special, though very simple, gill-like respiratory organs.
These organs consist of small membranous sacs which are either pushed
out from the body into the water, or lie in cavities in the body to
which the water has access. There is also a distinct circulatory
system, but the "blood" which is carried by these organs and which
fills the body-cavity consists mainly of sea-water, although
containing a number of amœboid corpuscles containing a brown pigment.
There is no organ really corresponding to the heart of the higher
animals. There are distinct organs for the production of the germ or
reproductive cells. The sexes are distinct (except in a few species),
each individual producing only sperm-cells or egg-cells, but the
organs or glands which produce the germ-cells are very much alike in
both sexes. There is no apparent difference between male and female
Echinoderms except in the character or rather in the product of the
germ-cell producing organs. A few species are exceptions, certain
starfishes showing a difference in color between males and females.
As all of the Echinoderms except some of the feather-stars can move
about, they have organs of locomotion, and well-defined muscles for
the movement of the locomotory organs. The external organs of
locomotion, the tube-feet (in the sea-urchins the dermal spines aid
also in locomotion), are parts of a peculiar system of organs
characteristic of the Echinoderms, called the ambulacral or the
water-vascular system. This system is composed of a series of radial
tubular vessels which rise from a central circular or ring vessel and
which give off branches to each of the tube-feet. The water from the
outside enters the ambulacral system through a special opening, the
madreporic opening, and flowing to the tube-feet helps extend them.
The tube-feet usually have a tiny sucking disk at the tip, and by
means of them the Echinoderm can cling very firmly to rocks.
=Development and life-history.=--Differing from the sponges and the
polyps and jellyfishes, the reproduction of the Echinoderms is always
sexual; young or new individuals are never produced by budding, or in
any other asexual way. The new individual is always developed from an
egg produced by a female and fertilized by the sperm of a male. The eggs
are usually red or yellow, are very small (about 1/50 in. in diameter in
certain starfishes), and are fertilized by the sperm-cells of the males
after leaving the body of the female. That is, both sperm-cells and
unfertilized egg-cells are poured out into the water by the adults, and
the motile sperm-cells in some way find and fertilize the egg-cells.
From the egg there hatches a tiny larva which does not at all resemble
the parent starfish or sea-urchin. It is an active free-swimming
creature, more or less ellipsoidal in shape and provided with cilia
for swimming. Soon its body changes form and assumes a very curious
shape with prominent projections. The larvæ of the various kinds of
Echinoderms, as the starfishes, sea-urchins, sea-cucumbers, etc., are
of different characteristic shapes. The naturalists who first
discovered these odd little animals did not associate them in their
minds with the very differently shaped starfishes and sea-urchins, but
believed them new kinds of fully developed marine animals, and gave
them names. Thus the larvæ of the starfishes were called Bipinnaria,
the larvæ of the sea-urchins Pluteus, and so on. These names are still
used to designate the larvæ, but with the knowledge that Bipinnaria
are simply young starfishes, and that a Pluteus is simply a young
sea-urchin. From these larval stages the adult or fully developed
starfish or sea-urchin develops by very great changes or
metamorphoses. The Echinoderms have in their life-history a
metamorphosis as striking as the butterflies and moths, which are
crawling worm-like caterpillars in their young or larval condition.
Most of the Echinoderms have the power of regenerating lost parts.
That is, if a starfish loses an arm (ray) through accident, a new ray
will grow out to replace the old. And this power of regeneration
extends so far in the case of some starfishes that if very badly
mutilated they can practically regenerate the whole body. This amounts
to a kind of asexual reproduction. Some species, too, have the
peculiar habit of self-mutilation. "Many brittle stars and some
starfishes when removed from the water, or when molested in any way,
break off portions of their arms piece by piece, until, it may be, the
whole of them are thrown off to the very bases, leaving the central
disc entirely bereft of arms. A central disc thus partly or completely
deprived of its arms is capable in many cases of developing a new set;
and a separated arm is capable in many cases of developing a new disc
and a completed series of arms." In some of the sea-cucumbers "it is
the internal organs, or rather portions of them, that are capable of
being thrown off and replaced, the œsophagus ... or the entire
alimentary canal, being ejected from the body by strong contractions
of the muscular fibres of the body-wall, and in some cases, at least,
afterwards becoming completely renewed."
=Classification.=--The Echinodermata are divided into five classes,
viz., the Asteroidea or starfishes, "free Echinoderms with star-shaped
or pentagonal body, in which a central disc and usually five arms are
more or less readily distinguishable, the arms being hollow and each
containing a prolongation of the body-cavity and contained organs";
the Ophiuroidea, or brittle-stars, "star-shaped free Echinoderms, with
a central disc and five arms, which are more sharply marked off from
the disc than in the Asteroidea and which contain no spacious
prolongations of the body-cavity"; the Echinoidea, or sea-urchins,
"free Echinoderms with globular, heart-shaped, or disc-shaped body
enclosed in a shell or corona of close-fitting, firmly united
calcareous plates"; the Holothuroidea, or sea-cucumbers, "free
Echinoderms with elongated cylindrical or five-sided body, ... with a
circlet of large oral tentacles"; and the Crinoidea, or feather-stars,
"temporarily or permanently stalked Echinoderms with star-shaped body,
consisting of a central disc, and a series of five bifurcate or more
completely branched arms, bordered with pinnules."
=Starfishes (Asteroidea).=--The starfishes feed on other marine
animals, especially shell-fish and crabs. They are also reputed to
destroy young fish. By means of their sucking-tubes, or tube-feet with
sucker tips, they can seize and hold their prey firmly. They do much
injury to oyster-beds by attacking and devouring the oysters. When
attacking prey too large to be taken into the mouth the starfish
everts its stomach over the prey and devours it. The stomach is
afterward drawn back into the body-cavity by special muscles.
Starfishes vary much in size, color and general appearance, although
all are readily recognizable as starfishes (fig. 22). The number of
arms or rays varies from five to thirty or more in different species;
some have the interradial spaces filled out nearly to the tips of the
rays, making the animal simply a pentagonal disc. In size starfishes
vary from a fraction of an inch in diameter to three feet; in color
they are yellow or red or brown or purple.
[Illustration: FIG. 22.--A group of Echinoderms; the upper one, a
starfish, _Asterina mineata_, the one at the right a starfish,
_Asterias ocracia_, at the left a brittle-star, species unknown, and
at bottom two sea-urchins, _Strongylocentrotus franciscanus_. (From
living specimens in a tide-pool on the Bay of Monterey, California.)]
=Brittle-stars (Ophiuroidea).=--The brittle-stars, or serpent-stars
(fig. 22) as they are also called, resemble the starfishes in external
appearance, that is, they are flat and composed of a central disc with
radiating arms (always five in number, although each arm may be
several times branched). The central disc is always sharply
distinguished from the arms, and the arms are usually slender and more
or less cylindrical. The distinguishing difference between the
brittle-stars and the starfishes is that the body-cavity and the
stomach which extend out into the arms in the starfishes are in the
brittle-stars limited to the central disc, or to the disc and bases of
the arms. The tube-feet also have no suckers at the tips. More than
700 species of brittle-stars are known. They feed on marine
shell-fish, crabs and worms.
=Sea-urchins (Echinoidea).=--The sea-urchins (figs. 20, 21 and 22) of
which more than 300 species are known, have no arms or rays, and they
are usually not flat like the starfishes but globular, with poles more
or less flattened. As has been noted in the examination of the body-wall
or "shell," the radiate character of the body is shown by the five
radiating zones of tube-feet. The mouth, with its five strong "teeth,"
is on the ventral surface, and the anal opening and madreporic opening
are on the dorsal surface. The calcareous plates (seen distinctly in a
specimen from which the spines have been removed) which constitute the
firm part of the body-wall, are more or less pentagonal in shape and are
usually firmly united at the edges. The spines which are so
characteristic of the sea-urchins vary much in size and number and
firmness, but are present in some form on all of them.
While most of the sea-urchins live near the shore, being very common
in tide-pools, some live only on the bottom of the ocean at great
depths. Their food consists of small marine animals and of bits of
organic matter which they collect from the sand and débris of the
ocean floor. Many of the sea-urchins are gregarious, living together
in great numbers. Some have the habit of boring into the rocks of the
shore between tide-lines. I have seen thousands of small beautifully
colored purple sea-urchins lying each in a spherical pit or hole in
hard conglomerate rock on the California coast. How they are enabled
to bore these holes is not yet known. There is great variety in size
and color among the sea-urchins. The colors are brown, olive, purple
red, greenish blue, etc.
A few kinds of sea-urchins have a flexible shell or test. The
Challenger expedition dredged up from sea-bottom some sea-urchins, and
when placed on the ship's deck "the test moved and shrank from touch
when handled, and felt like a starfish." The cake-urchins or
sand-dollars are sea-urchins having a very flat body with short
spines. They lie buried in the sand, and are often very brightly
colored. Their hollow bleached tests with the spines all rubbed off
are common on the sands of both the Atlantic and Pacific coasts.
=Sea-cucumbers (Holothuroidea).=--The sea-cucumbers (fig. 23) show at
first glance little resemblance to the other radiate animals. The body
is an elongate, sub-cylindrical sac, resembling a thick worm or
sausage or cucumber in shape. At one end it bears a group of branched
tentacles which are set in a ring around the mouth-opening. The
body-wall is muscular and leathery, but contains many small separated
calcareous spicules. There are usually five longitudinal rows of
tube-feet. In some species, however, tube feet are wholly wanting; in
others they are scattered over the surface.
Although there are known about five hundred species of sea-cucumbers
many of which live along the shores, they are much less familiar to us
than the starfishes and sea-urchins. They usually rest buried in the
sand by day, feeding at night. Some of them attain a large size. A
great orange-red species of the genus _Cucumaria_, which is found in
the Bay of Monterey, California, is three feet long.
The people of some nations use sea-cucumbers as food. They are called
"trepang" in the orient. The trade of preparing the trepang is almost
entirely in the hands of the Malays, and every year large fleets set
sail from Macassar and the Philippines to the south seas to catch
sea-cucumbers.
[Illustration: FIG. 23.--A sea-cucumber, _Pentacta frondosa_. (After
Emerton.)]
=Feather-stars (Crinoidea).=--The feather-stars or sea-lilies or
crinoids (fig. 24), as they are variously called, differ from the
other Echinoderms in having the mouth on the upper side of the central
disc, and in the fact that all of the species are fixed, either
permanently or for a part of their life, being attached to rocks on
the sea-bottom by a longer or shorter stalk which is composed of a
series of rings or segments. The central disc is small and the
radiating arms are long, slender, sometimes repeatedly branched, and
all the branches bear fine lateral projections called pinnulæ. Most of
the feather-stars live in deep water and are thus only seen after
being dredged up. They feed on small crab-like animals, and on the
marine unicellular animals and plants.
[Illustration: FIG. 24.--A crinoid or feather-star, _Pentacrinus_ sp.
(After Brehm.)]
CHAPTER XIX
BRANCH VERMES:[8] THE WORMS
THE EARTHWORM (_Lumbricus_ sp.).
TECHNICAL NOTE.--Obtain live earthworms of large size, killing
some in 30% alcohol and hardening and preserving them in 80%
alcohol, and bringing others alive to the laboratory. The worms
may be found during the daytime by digging, or at night by
searching with a lantern. They often come above ground in the
daytime after a heavy rain. Live specimens may be kept in the
laboratory in flower-pots filled with soil. "They may be fed on
bits of raw meat, preferably fat, bits of onion, celery, cabbage,
etc., thrown on the soil."
=External structure= (fig. 25).--Examine the external structure of
live and dead specimens. Which is the ventral and which the dorsal
surface? Which the anterior and which the posterior end? Note the
segmented condition of the body; the number of _segments_ or
_somites_, and their relative size and shape. Note absence of
appendages such as limbs and the presence of _locomotor setæ_ (short
bristles). How many setæ are there on each segment and what is their
disposition? The _mouth_ is covered by a dorsal projection called the
_prostomium_. The _anal opening_ is situated in the posterior segment
of the body. The broad thickened ring or girdle including several
segments near the anterior end of the body is the _clitellum_, a
glandular structure which secretes the cases in which the eggs are
laid. On the ventral surface of the fourteenth and fifteenth segments
(in most species) are two pairs of small pores; two other pairs of
small openings (usually difficult to find), one between segments 9 and
10, and one between segments 10 and 11, are present. All these are the
external openings of the reproductive organs.
[Illustration: FIG. 25.--Dissection of the earthworm, _Lumbricus_ sp.]
Make drawings showing the external structure of the earthworm.
Examine a live specimen placed on moist paper or wood. Note the
characteristics of its locomotion, and the movements of its
body-parts. How do the setæ aid in locomotion?
=Internal structure= (figs. 25, 26 and 28).--TECHNICAL NOTE.--With
a fine-pointed pair of scissors make a dorsal median incision, not
too deep, behind the clitellum and cut forward as far as the first
segment. Put the specimen into dissecting-dish, carefully pin back
the edges of the cut and cover with clear water or, better, 50%
alcohol.
Note the long body-cavity divided by the thin _septa_ which have been
torn away for the most part by the pinning process. Note the thin
transparent covering of the body, the _cuticle_. Just beneath this
note a less transparent layer, the _epidermis_, and underneath this a
layer of muscles. The _muscular layer_ is made up of two clearly
recognizable sets, an outer circular layer and an inner longitudinal
layer the fibres of which are continuous with the septa.
Note, as the most conspicuous internal organ, the long _alimentary
canal_, of which a number of distinct parts may be recognized. Most
anteriorly is a muscular _pharynx_, which is followed by a narrow
_œsophagus_, leading directly into the thin-walled _crop_; next comes
the muscular _gizzard_, and next the _intestine_ which opens externally
in the terminal segment through the _anus_. The anterior end of the
alimentary canal is more or less protrusible, while the posterior
portion is held more firmly in place by the septa which act as
mesenteries. Surrounding the narrow œsophagus are the reproductive
organs, three pairs of large white bodies and two pairs of smaller sacs.
Note the _dorsal blood-vessel_ lying along the dorsal surface of the
alimentary canal, from the anterior portion of which arise several
_circumœsophageal rings_ or "hearts." These hearts are contractile and
serve to keep the blood in motion through the blood-vessels (see
later). In the most anterior of the body segments note the pear-shaped
_brain_ or _cerebral ganglion_.
TECHNICAL NOTE.--Lift carefully to right and left the reproductive
organs, thus exposing the œsophagus.
Note three pairs of bag-like structures projecting from the œsophagus.
The front pair is the _œsophageal pouches_; the next two pairs are the
_œsophageal_ or _calciferous glands_. They communicate with the
alimentary canal, and their secretion is a milky calcareous fluid.
Make a drawing that will show all the parts so far studied.
TECHNICAL NOTE.--Cut transversely through the alimentary canal in
the region of the clitellum and carefully dissect the anterior
portion of the canal away from the surrounding organs.
Note the dorsal fold of the intestine, _typhlosole_, extending into
the lumen. This fold gives a greater surface for digestion, and in it
are a great many _hepatic_ or special _digestive cells_. The entire
alimentary canal is lined with _epithelium_. Observe just beneath the
alimentary canal the _ventral blood-vessel_, and still beneath this
blood-vessel the _ventral nerve-cord_. There is a slight swelling on
the nerve-cord in each segment of the body. These swellings are the
_ganglia_. How many pairs of nerves are given off from each ganglion?
Observe in each segment, posterior to the first three or four, the
successive pairs of convoluted tubes, the _nephridia_, or organs of
excretion. Each nephridium opens internally through a ciliated funnel,
the _nephrostome_, within the body-cavity, while it opens externally
by a small excretory pore between the setæ on the ventral surface of
the segment behind that in which the nephridium chiefly lies. The
function of the nephridia is to carry off waste matter from the fluid
which fills the body-cavity.
[Illustration: FIG. 26.--Dissection to show alimentary canal in
section and nephridia of earthworm.]
Trace the ventral nerve-cord forward to its connection with the
cerebral ganglion. Note the throat nerve-ring or _circumœsophageal
collar_ connecting the ventral cord with the brain.
Make a drawing of the nervous system showing its relation to other
organs.
[Illustration: FIG. 28.--Cross-section of earthworm.]
=Life-history and habits.=--The earthworm lives in soft moist soil
which is rich in organic matter. Its food is taken into the mouth
mixed with dirt and sand. As this mixture passes through the long
alimentary canal the organic particles are taken up and digested. As
we have already seen, there are in each worm two sets of reproductive
glands, namely, male and female organs. Each earthworm produces both
egg-cells and sperm-cells, but the sperm-cells of one worm are not
used to fertilize the eggs of the individual producing them. When the
eggs are ready to be discharged from the body, the clitellum becomes
very much swollen and its glands begin an active secretion which
hardens and forms a collar-like structure about the body of the worm.
As this collar moves forward toward the anterior end of the body it
collects the eggs and also the sperm-cells previously received from
another worm, and finally slips off the head end of the animal. The
entire structure with the contained eggs and sperm-cells as it passes
off from the body becomes closed at both ends, thus forming a horny
capsule which lies in the earth until the young worms emerge. Only a
part of the eggs develop in each capsule, the rest being used as food
for the growing young. The young earthworms, though of very small
size, are fully formed before they leave the egg-capsule. Earthworms
are more or less gregarious, large numbers often being found together.
For an interesting account of the habits of earthworms see Darwin's
"The Formation of Vegetable Mold."
OTHER WORMS.
The branch Vermes comprises so large a number of kinds of animals
presenting such great differences in structure and habit that it is
impossible to give a brief statement in general or summary terms of
their external body-characters, of the structural and functional
condition of their various organs and systems of organs, and of the
course of their development and life-history as has been done for the
preceding branches. Many zoologists, indeed, do not include all the
worms or worm-like animals in one branch, but consider them to form
several distinct branches.
[Illustration: FIG. 29.--A group of marine worms: at the left a
gephyrean, _Dendrostomum cronjhelmi_, the upper right-hand one a nereid,
_Nereis_ sp., the lower right-hand one, _Polynoe brevisetosa_. (From
living specimens in a tide-pool on the Bay of Monterey, California.)]
In certain very general characters all of the animals which compose the
branch Vermes do agree. All, or nearly all, have an elongate body which
is bilaterally symmetrical, that is, which could be cut by a median
longitudinal cutting in two similar halves. In most of them also the
body is composed of a number of successive segments or somites which are
more or less alike. This kind of segmented or articulated body is also
possessed by the insects and crabs. Almost all of the worms have the
power of locomotion; usually that of crawling. For this crawling they do
not have legs composed of separate segments or joints as do the higher
articulated animals, the crabs and insects, but either have fleshy
unjointed legs, or various kinds of bristles or spines, or suckers, or
even no external organs of locomotion at all. As regards their internal
structure they have well-organized systems of organs, which show great
variety in character and degree of complexity. The special sense-organs
are usually of simple character and low degree of functional
development. Reproduction occurs both sexually and asexually; in some
species the sexes are distinct, while in others both sperm-cells and
egg-cells are produced by the same individual. Asexual reproduction is
by budding or by a kind of simple division or fission. The worms live
either in salt or fresh water, or in moist, muddy or slimy places or as
parasites in the bodies of other animals or in plants. While most worms
feed on animal substance either living or dead, some feed on living or
decaying plant matter.
=Classification.=--There is great lack of agreement among zoologists in
the matter of the classification of the worms. Not only are the various
groups which by some are called classes held by others to be distinct
branches, co-ordinate in rank with the Echinodermata, Cœlenterata, etc.,
but the limits of these groups are also constantly called in question.
It will require a great deal better knowledge of the structure and
life-history of these diverse animals before the matter of their
classification is satisfactorily settled. We shall consider briefly four
of the various groups (which we may consider as classes) which include
worms either specially familiar to us or of special interest or
importance. One or two examples of each group (the groups being selected
primarily because of the examples) will be described in some detail. By
this means we may get an idea of the extremely diverse character of the
animals which are included in the heterogeneous branch Vermes.
=Earthworms and leeches (Oligochætæ).=--The various species of
earthworms, an example of which has been studied are found in all parts
of the world; they occur in Siberia and south to the Kerguelen Islands.
They are absent from desert or arid regions, and some can live
indifferently either in soil or in water. Some near allies of the
earthworms are aquatic, living in fresh or brackish water, some in salt
water near the shore. In size earthworms vary from 1 mm. (1/25 in.) to 2
metres (2-1/6 yds.) in length. All show the distinct segmentation of the
body noticeable in the common earthworm already studied.
The leeches, some of which are familiar animals, are closely related
to the earthworms, although at first glance the similarity in
structure is not very noticeable.
TECHNICAL NOTE.--Some common water-leeches, alive or preserved in
alcohol, should be examined by the class. The animals are not
unfamiliar to boys who "go in swimming" in the small streams of
the country. The body of a leech should be examined carefully, and
drawings of it showing the external structural characters should
be made.
The body of a leech is flattened dorso-ventrally, instead of being
cylindrical as in the earthworm, and tapers at both ends. In the live
animal the body can be greatly elongated and narrowed or much
shortened and broadened. It is composed of many segments (not as many
as there are cross-lines however; each segment is transversely
annulated), and bears at each end on the ventral surface a sucker, the
one at the posterior end being the larger. These suckers enable the
leech to cling firmly to other animals. The mouth is at the front end
of the body on the ventral surface and is provided with sharp jaws.
Leeches live mostly on the blood of other animals which they suck from
the body. The common leech "fastens itself upon its victim by means of
its suckers, then cuts the skin, fastens its oral sucker over the
wound and pumps away until it has completely gorged itself with blood,
distending enormously its elastic body, when it loosens its hold and
drops off." Its biting and sucking cause very little pain, and in
olden days physicians used the leeches when they wanted to "bleed" a
person. A common European species of leech much used for this purpose
is known as the "medicinal leech." All leeches are hermaphroditic,
that is, the sexes are not distinct, but each individual produces both
sperm-cells and egg-cells. Most of the leeches lay their eggs in small
packets or cocoons. This cocoon is dropped in soil on the banks of a
pond or stream so that the young may have a moist but not too wet
environment. The young issue from the eggs in four or five weeks, but
they grow very slowly and it is several years before they attain
their full size. Leeches are long-lived animals, some being said to
live for twenty years.
=Flatworms (Platyhelminthes).=--TECHNICAL NOTE.--Collect some live
fresh-water planarians (see fig. 30), which are to be found on the
muddy bottom of most fresh-water ponds, and examine them while
alive in watch-glasses of water. Make drawings showing the
external appearance, and as much of the internal anatomy as can be
seen. The branching alimentary canal can be seen in more or less
detail, and with higher power of the microscope parts of the
nervous system can be seen also. Have also a tapeworm preserved in
alcohol or formalin to show the very flat and many-segmented body.
The flatworms include a large number of forms which vary much in shape
and habits. They are all, however, characteristically flat; in some
this condition is very marked. Some are active free-living animals, as
the planarians (figs. 30 and 31), while many live as parasites in the
alimentary canal of other animals, as do the sheep-fluke and the
tapeworms.
[Illustration: FIG. 30.--A fresh water planarian, _Planaria_ sp. (From
a living specimen.)]
The fresh-water planarians (fig. 30), which live commonly in the mud
of the bottom of ponds, are small, being less than half an inch long.
They are very thin and rather broad, tapering from in front backwards.
On the upper surface near the front they have a pair of eyes; the
mouth is on the under surface a little behind the middle of the body.
The alimentary canal is composed of three main branches, each with
numerous small side branches. One main branch runs forward from the
mouth, and the other two run backwards, one on each side of the body.
There is no anal opening, and the alimentary canal thus forms a
system of fine branches closed at the tips, and extending all through
the body. The nervous system is composed of a ganglion or brain in the
front end of the body from which two main branches extend back
throughout its whole length. From these main longitudinal branches
arise many fine lateral branches.
[Illustration: FIG. 31.--A marine planarian, _Leptoplana californica_.
(From a living specimen.)]
Of the parasitic flatworms the tapeworms are the best known. There are
numerous species of them, all of which live in the bodies of vertebrate
animals. In the adult or fully developed stage the tapeworms live in the
alimentary canal, holding on to its inner surface by hook-like clinging
organs and being nourished by the already digested food by which they
are bathed. In the young or larval stage tapeworms live in other parts
of the body of the host, and usually, indeed, in other hosts not of the
same species as the host of the adult worm.
The common tapeworm of man, _Tænia solium_ (there are several other
species of _Tænia_ which infest man, but _solium_ is the common one),
may serve as an example of the group. In the adult condition its body,
which is found attached to the inner wall of the intestine, is like a
long narrow ribbon: it may be two or three metres long. It is attached
by one end, the head, which is very small and provided with a score of
fine hooks. Behind the head the thin ribbon-like body grows wider. The
body is composed of many (about 850) joints called proglottids. There
is no mouth or alimentary canal, the liquid food being simply taken in
through the skin. Each proglottid produces both sperm-cells and
egg-cells; one by one these proglottids or joints with their supply of
fertilized eggs break off and pass from the alimentary canal with the
excreta. If now one of these escaped proglottids or the eggs from it
are eaten by a pig, the embryos issue from the eggs in the alimentary
canal of the pig, bore through the walls of the canal and lodge in the
muscles. Here they increase greatly in size and develop into a sort of
rounded sac filled with liquid. If the flesh of the pig be eaten by a
man, without its being first cooked sufficiently to kill the larval
sac-like tapeworms, these young tapeworms lodge in the alimentary
canal of the man and develop and grow into the long ribbon-like
many-jointed adult stage.
The life-history of the other tapeworms which infest the various
vertebrate animals is of this general type. There is almost always an
alternation of hosts, the larval tapeworm living in a so-called
intermediate host, and the adult in a final host. Of the domestic
animals the dog is the most frequently attacked. At least ten
different species of tapeworms have been found in the dog. The
intermediate hosts of these dog tapeworms include rabbits, sheep,
mice, etc. Some of the domestic fowl, ducks, geese and chickens, for
instance, are also infested by tapeworms, and the intermediate hosts
in these cases are usually insects or small aquatic crustaceans like
the familiar _Cyclops_.
=Roundworms (Nemathelminthes).=--TECHNICAL NOTE.--Vinegar-eels
from mouldy vinegar, and hair-worms from fresh-water pools, can
usually be readily obtained. They should be examined, and drawings
should be made of them, showing their shape and simple external
structural character. If a specimen of trichinosed pork be
obtained, the encysted stage of the _Trichina_, described in the
following account, can be shown.
The roundworms are slender, smooth, cylindrical worms pointed at both
ends. They are all very long in proportion to their diameter, although
their actual length may be short. Some species are of microscopic
size; as the _Trichina_ worm, which is about 1/20 in. long; while the
guinea-worm, one of the worst parasites of man, may reach a length of
six feet. Many of the roundworms are parasites living in the various
organs of other animals. Some, however, lead an independent free life
in water or in damp earth.
[Illustration: FIG. 32.--A vinegar eel, _Anguillula_ sp. (From a
living specimen.)]
Familiar examples of roundworms are the so-called vinegar-eels
(_Anguillula_) (fig. 32) to be found in weak vinegar, and other
species of this same genus which live in water or moist ground or in
the tissues of plants, doing much injury. The hair-worms (_Gordius_)
or horse-hair snakes, which are believed by some people, to be
horse-hairs dropped into water and turned into these animals, are also
familiar examples of roundworms. They are often found abundantly in
little pools after a rain, and it is sometimes said that these worms
come down with the rain. They have in reality come from the bodies of
insects in which they pass their young or larval stages as parasites.
The hair-worms all live as parasites during their larval stage, and as
free independent animals in their adult stage. Some of them require
two distinct hosts for the completion of their larval life, living for
a while in the body of one, and later in the body of another. The
first host is usually a kind of insect which is eaten by the second
host. The eggs are deposited by the free adult female in slender
strings twisted around the stems of water-plants. The young hair-worm
on hatching sinks to the bottom of the pond, where it moves about
hunting for a host in which to take up its abode.
[Illustration: FIG. 33.--_Trichina spiralis_, encysted in muscle of a
pig. (From specimen.)]
The terrible _Trichina spiralis_ (fig. 33), which produces the disease
called trichinosis, is another roundworm of which much is heard. This
is a very small worm which in its adult condition lives in the
intestine of man as well as in the pig and other mammals. The young,
which are borne alive, burrow through the walls of the intestine, and
are either carried by the blood, or force their way, all over the
body, lodging usually in muscles. Here they form for themselves little
cells or cysts in which they lie. The forming of these thousands of
tiny cysts injures the muscles and causes great pain, sometimes
death, to the host. Such infested muscle or flesh is said to be
"trichinosed," and the flesh of a trichinosed human subject has been
estimated to contain 100,000,000 encysted worms. To complete the
development of the encysted and sexless _Trichinæ_ the infested flesh
of the host must be eaten by another animal in which the worm can
live, e.g. the flesh of man by a pig or rat, and that of a pig by man.
In such a case the cysts are dissolved by the digestive juices, the
worms escape, develop reproductive organs and produce young, which
then migrate into the muscles and induce trichinosis as before. But
however badly trichinosed a piece of pork may be, thorough cooking of
it will kill the encysted _Trichinæ_, so that it may then be eaten
with impunity. Some people, however, are accustomed to eat ham, which
is simply smoked pork, without cooking it, and in such cases there is
always great danger of trichinosis.
=Wheel animalcules (Rotifera).=--TECHNICAL NOTE.--Live specimens of
Rotifers can be found in almost any stagnant water. Examine a drop
of such water with the compound microscope, and find in it a few
small, active, transparent creatures, larger than the _Paramœcium_
and other Protozoa in the water and which have the appearance shown
in fig. 34. They may be known by the constant whirling, or rather
vibrating, circlet or wheel of cilia at the larger or head end of
the body. These wheel animalcules may be studied alive by the class.
Although usually darting about, the animalcules occasionally cease
to move, when, because of their transparency, almost the whole of
their anatomy can be made out. Their feeding habits can also be
readily observed, and the food itself watched as it moves through
the body. Make drawings showing as much of the anatomy as can be
worked out. Note especially the "mastax" or gizzard-like masticating
apparatus in the alimentary canal.
The wheel animalcules (fig. 34) or Rotifers look little like the other
worms we have studied. But they are nevertheless more nearly related
to the worms than to any other branch of animals. They are all small,
about 1/3 mm. long, and have a compact body. They are aquatic and feed
on smaller animals and plants or on bits of organic matter which they
capture by means of the currents produced by the vibrating cilia of
the "wheel." Small as they are they have a complex body-structure,
with well-organized systems of organs. For a long time, however, they
were classed by naturalists with the Protozoa on account of their
size. They are found all over the world, mostly in fresh water; a few
are marine. More than 700 species of them are known.
[Illustration: FIG. 34.--A wheel animalcule, _Rotifer_ sp. (From
living specimen, Stanford University.)]
An interesting thing about the Rotifers is their remarkable power to
withstand drying-up. When the water in a pond or ditch evaporates some
of the Rotifers do not die, but simply dry up and lie in the dust,
shrivelled and apparently lifeless, yet really in a state of suspended
animation. On being put into water they will gradually fill out to their
full size and shape, and finally resume all their normal activities. In
this dried-up condition Rotifers may persist for a long time, several
years even, although otherwise their natural life is short, being
probably of not over two weeks' duration. Certain other of the lower
animals have this same power of withstanding desiccation.
FOOTNOTE:
[8] The author recognizes the untenability of the group Vermes as a
group co-ordinate with the other branches of the animal kingdom, and
that "Vermes" has been discarded in modern text-books. But because of
the very scant consideration which can be given the various kinds of
worm-like animals the course of the older text-books will be followed,
and all of the worm-like animals, as far as referred to in this book,
be considered under the group name Vermes.
CHAPTER XX
BRANCH ARTHROPODA: CRUSTACEANS, CENTIPEDS,
INSECTS, AND SPIDERS
The great branch Arthropoda includes a host of familiar animals. It
contains more species than any other branch of the animal kingdom. To it
belong the crayfishes, shrimps, crabs, lobsters, water-fleas, and other
animals which compose the class Crustacea; the centipeds and
thousand-legged worms which compose the class Myriapoda; the true or
six-footed insects forming the class Insecta, which includes nearly
two-thirds of all the known species of animals; and the scorpions,
mites, ticks, and spiders which constitute the class Arachnida. There is
also a fifth class in the branch Arthropoda which includes a few species
of animals unfamiliar to us but of great interest to zoologists.
All these varied kinds of animals have a body on the annulate or
segmented type-plan, like that shown by most worms, but they differ from
the worms in possessing jointed appendages, used for locomotion or food
taking. There is typically or racially one pair of these jointed or
segmented appendages on each segment of the body, but in all of the
Arthropoda some of the segments have lost their appendages. The body is
covered by a firm cuticle or outer body-wall called the exoskeleton.
This exoskeleton serves not only to enclose and protect the soft parts
of the body but also for the attachment of the body muscles. It may be
flexible as in the sutures between the body-segments in most insects, or
hard and rigid as in the sclerites of the segments. The firmness is due
primarily, and in the insects usually solely, to a deposit in the
cuticle of _chitin_, a substance probably secreted by the underlying
cells of the true skin, or it may be due chiefly, as in the crabs, to a
calcareous deposit. In such cases it becomes a veritable armor. The
internal organs of the Arthropods show a more or less obvious
segmentation corresponding with the segmentation of the body-wall. The
alimentary canal runs longitudinally through the center of the body from
mouth to anal opening. The nervous system consists of a brain lying
above the œsophagus and a double nerve-chain running backward from
beneath the œsophagus, along the median line of the ventral wall, to the
posterior extremity of the body. This ventral nerve-chain consists of a
pair of longitudinal commissures or cords and a series of pairs of
ganglia, arranged segmentally. The two ganglia of each pair are fused
more or less nearly completely to form a single ganglion, and the
nerve-cords are partially fused, or at least lie close together. In
addition there is a smaller sympathetic system composed of a few small
ganglia and certain nerves running from them to the viscera, this system
being connected with the main or central nervous system. In this group
the organs of special sense reach for the first time a high stage of
development. Compound eyes are peculiar to Arthropoda. The heart lies
above the alimentary canal. Respiration is carried on by gills in the
aquatic forms, and by a remarkable system of air-tubes or tracheæ in the
land forms (insects). The sexes are usually distinct, and reproduction
is almost universally sexual. Most of the species lay eggs.
The Arthropods are animals of a high degree of organization. The
extremely diverse life-habits of the various kinds among them have
led to much modification and to great specialization of structure. The
course of development, too, is made very complicated by the elaborate
metamorphosis undergone by many of the members of the branch.
We shall study the Arthropoda by getting acquainted with a few examples
of each class and thus learning the special class characteristics.
CLASS CRUSTACEA: CRAYFISHES, CRABS, LOBSTERS,
ETC.
THE CRAYFISH (_Cambarus_ sp.)
=Structure.=--The structure of the crayfish has been already studied
(see Chapter IV and figs. 3 and 4).
=Life-history and habits.=--Crayfish frequent fresh-water lakes, rivers,
and springs in most parts of the United States. Many of them perish
whenever the small prairie ponds dry up. But some burrow into the earth
when the dry season comes. There may be noticed in meadows where water
stands for certain seasons of the year many scattered holes with slight
elevations of mud about them. These are mostly the burrows of crayfish.
During the dry season the crayfish digs down until it reaches water, or
at least a damp place, where it rests until wet weather brings it to the
surface once more. One of these burrows, followed in digging a mining
shaft, extended vertically down to a distance of twenty-six feet, where
the crayfish was found tucked snugly away.
The eggs are carried by the female on her abdominal appendages. Previous
to the laying of the eggs the female rubs off all foreign matter from
the appendages, thus preparing them for the reception of the eggs. This
cleaning is done with the fifth pair of legs. When the eggs are ready
to be laid, which is during the last of March or in April in the Central
States, a sticky secretion passes out of the openings at the base of the
walking legs and smears the pleopods of the abdomen. The eggs as they
pass out are fertilized and caught on the pleopods, where they remain
attached in clusters. After some weeks the young crayfishes issue from
the eggs. In general appearance they are not very unlike the adults.
They grow very rapidly at this stage. As the animal is enclosed in a
hard shell, growth can only take place during the period just following
the molt, for the crayfish casts its skin periodically, and it is while
the new shell is forming that the animal does its growing. The crayfish
when it molts casts not only the exoskeleton, but also the lining of
part of the alimentary canal. After the females have hatched their young
many die in the shallow pools, in which places the dried-up skeletons
are noticeable during the summer months.
OTHER CRUSTACEANS.
Most of the crustaceans live in water, a few being found in damp soil
or in other moist places. Some are fresh-water animals and some
marine. They vary in size from the tiny water-fleas, a millimeter
long, to crabs two feet across the shell or sixteen feet from tip to
tip of legs. They present great differences in form and general
appearance of body, being adapted for various conditions of life. Some
crustaceans live as parasites on other animals, in some cases on other
crustaceans. Such parasitic species have the body much modified and
are hardly to be recognized as members of the class.
=Body form and structure.=--In structural character and body
organization the Crustaceans show, of course, the general
characteristics already attributed to the Arthropoda, the branch to
which they belong. The characteristics which distinguish them from
other Arthropods are the possession of gills for respiration (some
insects have gills, but of a very different kind as will be seen
later), and the bi-ramose condition of the body appendages, each
appendage (excepting the antennules) consisting of a single basal
segment from which arise two branches made up of one or more segments.
Of the form of the crustacean body few generalizations can be made.
"There is no [other] class in the animal kingdom which presents so
wide a range of organization as the Crustacea, or in which the
deviations in structure from the 'type form' are so striking and so
interesting from their obvious adaptation to the mode of life." For
this reason no attempt will be made to discuss in general terms the
form of the crustacean body, but brief accounts will be given of a few
of the more familiar kinds of Crustacea which will serve to illustrate
this remarkable diversity of body form.
Similarly impossible is it also to give a general account of the
development of the crustaceans. The sexes are distinct in most
Crustacea, and there is often great difference in form between the
male and female. A certain amount of metamorphosis takes place in the
development of all crustaceans; that is, the young when hatched from
the egg differs, often decidedly, in appearance and structure from the
parent, and in the course of its post-embryonic development undergoes
more or less striking change or metamorphosis. This metamorphosis is
often very marked.
=Water-fleas (Cyclops).=--TECHNICAL NOTE.--The water-fleas are
common in the water of ponds or of slow streams; they may often be
found in the school aquarium. They are, though small (about 1 mm.
long), readily seen with the unaided eye; they are white, rather
elongate, and have a rapid jerky movement. Examine specimens alive
in water in a watch glass. Note the "split pear" shape, broadest
near the front, tapering posteriorly, flat beneath, convex above;
note the forked stylets at tip of abdomen; also the two pairs of
antennæ, the single median eye, the mandibles, two pairs of
maxillæ, and five pairs of legs (last pair very small). There are
no gills. Some of the specimens, females, may have attached to the
first abdominal segment on either side an egg sac. Make drawings
showing all these structural details. Watch the _Cyclops_
capturing and feeding on _Paramœcium_ or other small animals.
[Illustration: FIG. 35.--A water-flea, _Cyclops_ sp. Female with
egg-masses. (From living specimen.)]
The water-fleas (_Cyclops_) (fig. 35) are among the smallest of the
Crustacea. They are extremely abundant, having great power of
multiplication. "An old _Cyclops_ may produce forty or fifty eggs at
once, and may give birth to eight or ten broods of children living
five to six months. As the young begin to reproduce at an early age,
the rate of multiplication is astonishing. The descendants of one
_Cyclops_ may number in one year nearly 4,500,000,000, or more than
three times the total population of the earth, provided that all the
young reach maturity and produce the full number of offspring." The
_Cyclops_ feed on smaller aquatic animals such as Protozoa, Rotifera,
etc. They in turn serve as food for fishes; and because of their
immense numbers and occurrence in all except the swiftest fresh waters
"they form the main food of most of our fresh-water fishes while
young." Many aquatic insect larvæ feed almost exclusively on them.
Related to the _Cyclops_ are a host of other kinds of minute
Crustaceans. Among these the so-called fish-lice are specially
interesting because of their parasitic habits and greatly modified and
degenerate structure. There are many kinds of these parasitic
crustaceans infesting fishes, whales, molluscs, and worms. "As on land
almost every species of bird or mammal has its own parasitic insects, so
in the water almost every species of fish or larger invertebrate has its
parasitic crustaceans." Some of the most common of these parasites
attach themselves to the gills of fishes. Here they cling, sucking the
blood or animal juices from the host. In form of body they do not at all
resemble other Crustaceans, but are strangely misshapen. They are often
worm-like, or sac-like, without legs or other locomotory appendages. As
with other parasites (see Chapter XXX) an inactive dependent life
results in the atrophy and loss by degeneration of the body-parts
concerned with locomotion and orientation.
=Wood lice (Isopoda).=--TECHNICAL NOTE.--Specimens of wood lice,
pill bugs, or damp bugs, as they are variously called, may be
readily found in concealed moist places, as under stones or boards
on damp soil. They are often common in houses, near drains or in
dark, damp places. Examine some live wood lice, and some dead
specimens (killed by chloroform or in an insect-killing bottle).
Note the division of the body into the head, thorax, and abdomen;
find the eyes, the antennæ and the mouthparts (mandibles and
maxillæ are usually pressed closely together). All the locomotory
appendages are adapted for walking or running, not swimming. Note
the number of pairs of legs; the structure of a leg; find gills
and gill-covers. Some females may be found with eggs on the under
side of the thorax near the bases of the legs, the eggs being
covered by thin membranous plates. Make drawings showing the
general form and character of body and details of legs, gills,
etc. Compare with the crayfish and _Cyclops_.
The wood-lice (fig. 36) are among the few Crustacea which have a
wholly terrestrial life. They run about quickly and feed chiefly on
decaying vegetable matter. They are night scavengers. They have the
body oval and convex above, rather purplish or grayish brown, and
smooth. Although they do not live in the water they breathe partly at
least by means of gills (though they may breathe partly through the
skin). It is therefore necessary for them to live in a damp atmosphere
so that the gill membranes may be kept damp. If not kept moist they
could not serve as osmotic membranes.
[Illustration: FIG. 36.--A damp bug, Isopod, species not determined.
(From specimen.)]
=Lobsters, Shrimps and Crabs (Decapoda).=--TECHNICAL
NOTE.--Teachers living near the sea-shore can get specimens of
live and dead lobsters, shrimps, and crabs in the markets. Schools
in the interior should have a few preserved specimens for
examination. These specimens should be compared with the crayfish;
although differences in shape of body are evident, the character
and arrangement of body parts will be found to be very similar.
The largest and most familiar Crustaceans, as the crayfishes,
lobsters, shrimps, prawns and crabs, all belong to the order Decapoda,
or ten-legged Crustacea. The members of this order have, including the
large claws, ten walking feet; they all have eyes on movable stalks,
and the front portion of the body is covered by a horny fold of the
body-wall called the carapace.
The lobsters are large ocean-inhabiting crustaceans which are very like
the fresh-water crayfish in all structural characters. They live on the
rocky or sandy ocean-bottom at shallow depths. They feed largely on
decaying animal matter. They are caught in great numbers in so-called
"lobster pots," a kind of wooden trap baited with refuse. "The number
thus taken upon the shores of New England and Canada amounts to between
twenty and thirty million annually." Live lobsters are brownish or
greenish with bluish mottling; they turn red when boiled. A single
female will lay several thousand eggs. The eggs are greenish and are
carried about by the mother until the young hatch. The young are
free-swimming larvæ, until they reach a length of half an inch.
The shrimps and prawns are mostly marine, though some species live in
fresh water. They are, like the lobsters, used for food. Some of the
species are gregarious in habit, occurring in great "schools" of
individuals. Like the lobsters they crawl about on the sea-bottom
feeding on decaying animal matter. Shrimps are very abundant near San
Francisco, where extensive "shrimp fishing" is done by the Chinese.
[Illustration: FIG. 37.--Some crabs and barnacles of the Pacific
coast; the short sessile acorn barnacles in the upper left-hand corner
belong to the genus _Balanus_; the stalked barnacles in the upper
right-hand corner are of the species _Pollicipes polymenus_; the
largest crab (upper left-hand) is _Brachynotus nudus_; the one in
left-hand lower corner is a young rock-crab, _Cancer productus_; the
crab in the sea-weed at the right is a kelp-crab, _Epialtus
productus_, while the two in snail-shells in lower corner are
hermit-crabs, _Pagurus samuelis_. (From living specimens in a
tide-pool on the Bay of Monterey, California.)]
The crabs (fig. 37) differ from the lobsters and crayfishes and
shrimps in having the body short and broad, instead of elongate. This
is due to the special widening of the carapace and the marked
shortening of the abdomen. The abdomen, moreover, is permanently bent
underneath the body, so that but little of it is visible from the
dorsal aspect. The number of abdominal legs or appendages is reduced.
When the tide is out the rocks and tide-pools of the ocean shore are
alive with crabs. They "scuttle" about noisily over the rocks,
withdrawing into crevices or sinking to the bottom of the pools when
disturbed. They move as readily backward or sidewise, "crab-fashion,"
as forward. They are of various colors and markings, often so
patterned as to harmonize very perfectly with the general color and
appearance of the rocks and sea-weeds among which they live. The
spider-crabs are especially strange-looking creatures with unusually
long and slender legs and a comparatively small body-trunk. They
include the _Macrocheira_ of Japan, the largest of the crustaceans.
Specimens of this crab are known measuring twelve to sixteen feet from
tip to tip of extended legs; the carapace is only as many inches in
width or length. The soft-shelled crab is a species common along our
Atlantic coast. It is "soft-shelled" only at the time of molting, and
has to be caught in the few days intervening between the shedding of
the old hard shell and the hardening of the new body-wall. The little
oyster-crabs (_Pinnotheres_) which live with the live oyster in the
cavity enclosed by the oyster shell are well-known and interesting
crabs. They are not parasites preying on the body of the oyster, but
are simply messmates feeding on particles of food brought into the
shell by the currents of water created by the oysters.
Among the most interesting crabs are the hermit crabs (fig. 37),
familiar to all who know the seashore. There are numerous species of
these crabs, all of which have the habit of carrying about with them, as
a protective covering into which to withdraw, the spiral shell of some
gastropod mollusc. The abdomen of the crab remains always in the cavity
of the shell; the head and thorax and legs project from the opening of
the shell, to be withdrawn into it when the animal is alarmed or at
rest. The abdomen being always in the shell and thus protected loses the
hard body-wall, and is soft, often curiously shaped and twisted to
correspond to the cavity of the shell. It has on it no legs or
appendages except a pair for the hindmost segment which are modified
into hooks for holding fast to the interior of the shell. As the hermit
crab grows it takes up its abode in larger and larger shells, sometimes
killing and removing piece-meal the original inhabitant. Some hermit
crabs always have attached to the shell certain kinds of sea-anemones.
It is believed that both crab and sea-anemone derive advantage from this
arrangement. The sea-anemone, which otherwise cannot move, is carried
from place to place by the crab and so may get a larger supply of food,
while the crab is protected from its enemies, the predaceous fishes, by
the stinging threads of the sea-anemone, and also perhaps by the
concealment of the shell its presence affords. This living together by
two kinds of animals to their mutual advantage is called commensalism or
symbiosis (see Chapter XXX). The hermit crabs are not true crabs, but
are more nearly related to the crayfishes and shrimps than to the true
broad-bodied, short-tailed crabs.
=Barnacles.=--TECHNICAL NOTE.--Specimens of barnacles may be got
readily from the tide rocks or from piles in a harbor. Interior
schools should have, if possible, specimens preserved in alcohol or
formalin for examination. The "shells" of acorn (sessile) barnacles
may often be found on oyster shells (get at restaurants).
Crustaceans which at first glance are hardly recognizable as such are
the stalked or sessile barnacles (fig. 37) which live fixed in great
numbers on the rocks between the tide lines, or on the piles
supporting wharves, or on the bottom of ships or even on the body-wall
of whales and other ocean animals. In the stalked forms the stalk is a
flexible stem or peduncle covered with a blackish finely-wrinkled skin
bearing at its free end the greatly modified body of the barnacle.
This body is enclosed in a sort of bivalved shell or carapace formed
by a fold of the skin and stiffened by five calcareous plates. Within
this curious shell is the compact, rather worm-like body-mass, showing
little or no indication of segmentation. The legs, of which there are
usually six pairs, are much modified, being long, feathery, and
divided nearly to the base. These feathery feet project from the
opened shell when the animal is undisturbed, and waving about in the
water catch small animals which serve as the barnacle's food. When
disturbed the barnacle withdraws its feet and closes tightly its
strong protecting shell. The acorn-barnacles have no stalk, but look
like a low bluntly-pointed pyramid, this appearance being due to the
converging arrangement of six calcareous plates in its body-wall.
The barnacles present several unusual conditions with regard to the
internal organs. They have no heart nor any blood-vessels; most of the
species are hermaphroditic; and there are other indications of a
degenerate condition. This degeneration of the barnacles is due to
their fixed life, the results of which are like those of a parasitic
life. The young barnacles when hatched from the egg are free-swimming
larvæ as with the other Crustacea. They finally attach themselves and
undergo the changes, some of them of degenerative nature, which
produce the body-structure of the adult. It was long a belief among
many people that the barnacle produced the barnacle goose. Pictures in
ancient books show the young barnacle geese issuing from the opened
shell of the barnacle. The early naturalists believed barnacles, on
account of the shell, to be a kind of shell-fish or mollusc, but when
their development was thoroughly worked out, it became evident that
they belong to the Crustacea.
CHAPTER XXI
BRANCH ARTHROPODA (_continued_); CLASS INSECTA:
THE INSECTS
THE LOCUST (_Melanoplus_ sp.)
TECHNICAL NOTE.--Locusts or grasshoppers are common and familiar
insects all over the country. The genus _Melanoplus_ includes
numerous species, one or more of which are to be found in almost any
locality. The common red-legged locust (_M. femur-rubrum_) of the
East, the Rocky Mountain migratory locust (_M. spretus_), of the
West, the large differential (_M. differentialis_) and two-striped
(_M. bivittatus_) locusts of the Southwest, are especially common
species. All the members of the genus have their hind wings
uncolored, and the front wings marked with a longitudinal series of
small dots more or less distinct, or with a longitudinal line. There
is a small blunt spine or process projecting from the ventral aspect
of the prothorax. If a species of _Melanoplus_ cannot be found, any
other locust may be used, although there are some slight variations
in the external structure of the various species. Fresh specimens
killed in a cyanide bottle (for preparing see p. 463) are preferable
in the study of the external structure, but specimens preserved in
alcohol will do.
=External structure= (fig. 38).--Note that the body of the grass-hopper
is composed of successive rings or _segments_ grouped into three
regions, the _head_ (anterior), _thorax_ (median), and _abdomen_
(posterior). In which region of the body are the segments most readily
distinguished? Of how many segments does the head appear to be composed?
The thorax is composed of three segments of which the most anterior, to
which is attached the front pair of legs, differs from the succeeding
two, being freely movable and bearing a large hood- or saddle-shaped
piece on its dorsal aspect. To the other two thoracic segments the
second and third pair of legs are attached, as are also the two pairs
of wings. The remaining segments of the body compose the abdomen.
[Illustration: FIG. 38.--The red-legged locust, _Melanoplus
femur-rubrum_, to show external structure.]
Note the smooth, rather firm and horny character of the body. This is
due to the fact that the skin is everywhere covered with a cuticle in
which is deposited a horny substance called _chitin_. The cuticle is not
uniformly firm over the body. At the junction of the body segments in
the abdomen, in the neck and between the segments of the legs, in fact,
wherever motion is desirable, the cuticle is flexible, thus making
bending of the body-wall possible. Elsewhere, however, it is hard and
stiff, serving not only as a protective coat or armor over the body, but
also affording firm places for the attachment of muscles.
Insects (and all other Arthropods) have no[9] internal skeleton, but,
in this firm cuticle, an _exoskeleton_.
Although the head is apparently a single segment, it is really
composed of six or seven body segments greatly modified and firmly
fused together. Note that it bears a pair of large _compound eyes_ and
three much smaller _simple eyes_ or _ocelli_.
TECHNICAL NOTE.--Strip off a bit of the outer covering of a compound
eye, mount on a glass slide and examine under the microscope.
Note that, as in the crayfish, each compound eye is composed
externally of many small hexagonal _facets_, the outer covering, the
_cornea_, being simply the cuticular covering of the body, in this
place transparent and divided into small facets. Besides the eyes, the
head bears also several movable appendages, namely the _antennæ_, and
the _mouth-parts_. Note the number, place of insertion, and segmented
character of the antennæ. These antennæ are sense-organs and are used
for feeling, smelling, and, in some insects, for hearing. Note that
the mouth-parts consist of an upper, broad, flap-like piece,
the[10]_labrum_; of a pair of brown, strongly chitinized, toothed jaws
or _mandibles_; of a second pair of jaw-like structures, the
_maxillæ_, each of which is composed of several parts; and of an
under, freely-movable flap, the _labium_, also composed of several
pieces. Each maxilla bears a slender feeler or _palpus_ composed of
five segments. The labium bears a pair of similar palpi, which are,
however, only three-segmented. The mandibles and maxillæ, which are
the insect jaws, move laterally, not vertically as with most animals.
Make drawings of the lateral aspect of the head; of a bit of the
cornea; of the dissected out mouth-parts.
Of the three segments of the thoracic region of the body, the most
anterior one is called the _prothorax_. It is freely movable and has a
large hood or saddle-shaped piece, the _pronotum_, on its dorsal
aspect, and a blunt-pointed tubercle on the ventral aspect. The
foremost pair of legs is attached to the prothorax. The next segment
is the _mesothorax_, which is immovably fused to the next thoracic
segment. What appendages does it bear? The third segment is the
_metathorax_, which besides being fused with the mesothorax in front,
is similarly fused with the foremost abdominal segment behind. What
appendages does the metathorax bear?
Examine one of the fore legs and note that it is composed of a series
of unequal parts or segments. The segment nearest the body is
sub-globular and is called the _coxa_; the second segment is smaller
than the coxa and is called the _trochanter_; the third, known as the
_femur_, is the largest of all; the fourth, _tibia_, is long and
slender; and the next three, the last of which is the terminal one and
bears a pair of claws and between them a little pad, the _pulvillus_,
are called the _tarsal segments_. Most insects have five tarsal
segments. Note the great size of the hindmost or leaping legs.
Determine the segments of the middle and hindmost legs. Make a drawing
of a fore leg.
Examine the wings. In what ways do the front wings differ from the
hind wings? The front wings are known as the wing covers or _tegmina_.
Note how the hind wings fold up like a fan, and are covered and
protected by the wing covers. Draw the wings.
The abdomen is composed of a number of segments most of which resemble
each other. The first segment (immediately behind the metathorax) has
its dorsal and ventral parts widely separated by the cavities for the
insertion of the hindmost legs. The ventral part of this segment is
dovetailed into the ventral part of the metathorax and appears to be
part of it. In the dorsal part of this segment there is on each side a
spot where the cuticle is only a thin membrane. At these places are
the auditory organs or ears of the locust. The thin membranes are the
_tympana_. Only the various kinds of locusts and those insects closely
related to them have ears of this kind. Most other insects are
believed to have the sense of hearing situated in the antennæ.
The abdominal segments from second to eighth are ring-like in form and
are without appendages. There is on the side of each of these segments
near its front margin a tiny opening or pore called a _spiracle_.
These spiracles are the breathing pores of the locust, which does not
take in air through its mouth or any other opening in the head. There
is a spiracle near each ear in the first abdominal segment, and one on
each side of the mesothorax near the insertion of the middle legs.
The terminal segments of the abdomen are provided with certain processes
which are different in male and female. The female has at the tip of its
abdomen two pairs of strong, curved pointed pieces which compose the
_ovipositor_, or egg-laying organ. The opening of the oviduct lies
between the pieces. The male has a swollen rounded abdominal tip, with
three short inconspicuous pieces on the dorsal surface.
Make a drawing of the lateral aspect of the abdomen of a female
locust; also, of a male.
For a more detailed account of the external anatomy of a locust see
Comstock and Kellogg's "Elements of Insect Anatomy," chap. II.
The external structure of the grasshopper should be carefully compared
with that of the crayfish; pay special attention to the mouth-parts
and legs.
The teacher should point out the homologies and modifications.
=Life-history and habits.=--The eggs of the locust are laid in the
autumn in the ground in bare dry places, as roadsides, closely-grazed
pastures, etc. The female thrusts her strong ovipositor into the soil,
and by opening and shutting it, thus boring, pushes in the abdomen for
about two thirds its length. The eggs, about one hundred, are then
deposited in a capsule or pod. The young locusts hatch in the
following spring. When just hatched they resemble the parent locust in
general appearance and structure except that they lack wings, and are
of course very small. The young locusts are gregarious, congregating
in warm and sunny places. They feed on green plants and travel about
by walking and hopping. At night they try to find shelter under
rubbish in the fields. They feed voraciously and grow rapidly,
reaching maturity in about two months. During this post-embryonic
development and growth they molt (shed the chitinous exoskeleton) five
times. After the first molt indications of the wings appear in the
shape of small backward and downward prolongations of the posterior
margins of the dorsum of the mesothorax and metathorax. With each
succeeding molt these wing-pads, or developing wings, are larger and
more wing-like, until after the last molting they appear fully
developed. With each molting, too, there is a marked increase in size
of the locust, the average length of the body just before the first
moult being 4.3 mm., before the second 6.8 mm., before the third 9
mm., before the fourth 14 mm., before the fifth 17 mm., and after the
fifth (the full-grown stage) about 26 mm.
The molting is an interesting process, and can be readily observed.
The young locust ready for its last molt crawls up some post, weed,
grass stalk, or other object, and clutches this object securely with
the hind feet. The head is generally downward. The locust remains
motionless in this position for several hours, when the skin suddenly
splits along the back from the middle of the head to the base of the
abdomen. By steady swelling and contracting and slight wriggling,
lasting for half an hour to three-fourths of an hour, the old skin is
completely shed, and the wings spread out. In an hour the wings are
dry and the new chitinized exoskeleton firm enough for flying, or
crawling about, and in another hour the locust begins to eat.
The red-legged locust does considerable damage to cultivated crops,
but its injuries are insignificant compared with the tremendous losses
occasioned by a near relative, the Rocky Mountain Locust (_Melanoplus
spretus_). This locust has its breeding-grounds on the high plateaus
of the Rocky Mountain region, but it sometimes migrates in countless
numbers southeast over the plains and into the great grain-fields of
the Mississippi valley. Such migrations occurred in 1866, 1867, 1874
(in this year eighteen hundred and forty two families in Kansas were
reduced to destitution by the utter wiping out of their crops by the
locusts) and 1876. With the settling-up of the regions in which the
Rocky Mountain locust breeds, there seems to have come a change of
conditions, so that no great migrations have occurred since 1876.
THE GREAT WATER-SCAVENGER BEETLE (_Hydrophilus_ sp.)
TECHNICAL NOTE.--The great water-scavenger beetles are large,
black, elliptical insects common in quiet pools where they may be
found swimming through the water, or crawling among the plants
growing on the bottom. They are an inch and a half long and are
readily distinguishable from all other water insects except the
predaceous diving beetles (_Dyticus_). The antennæ of
_Hydrophilus_, however, are thickened (clavate) at the tip, while
those of _Dyticus_ are thread-like for their whole length. The
beetles may be readily collected with a water-net, and kept alive
in glass jars or aquaria in water containing decaying vegetation.
=External structure= (fig. 39).--Is the body of the water-beetle
composed of segments? Can you make out three body-regions, _head_,
_thorax_ and _abdomen_? As in the locust the _metathorax_ is fused
with the first abdominal segment and with the _mesothorax_, while
the _prothorax_ is freely movable, and is covered above by a strong
shield. The chitin armor of the whole body is specially heavy and
strong, affording a great protection to the insect.
[Illustration: FIG. 39.--Ventral aspect of male great water-scavenger
beetle, _Hydrophilus_ sp.]
On the flattened head note the _compound eyes_ and the peculiarly-shaped
nine-segmented _antennæ_. Are there any _ocelli_? Dissect out the
mouth-parts. The beetle's mouth is fitted for biting, the mouth-parts
being in general character like those of the locust, with distinct
flap-like _labrum_, dentate _mandibles_, jaw-like _maxillæ_ with long,
slender, four-segmented _palpi_ and lip-like _labium_ with
three-segmented _palpi_. Make drawings of the antennæ and mouth-parts.
Note the character of the thoracic segments. Examine the wings and
legs. The fore wings are modified into strong horny sheaths, or
_elytra_, which completely cover and protect the folded hind wings.
The hind wings are large and membranous. How are they folded? Note the
adaptation of the middle and hind legs for swimming. Determine the
various segments of the legs, i.e. _coxa_, _trochanter_, _femur_,
_tibia and tarsus_. Note the long longitudinal median keel on the
ventral aspect of the thorax.
The abdomen articulates with the metathorax by the full width of the
broad first abdominal segment. It is composed of a series of segments
without appendages, of about equal length but decreasing in width from
in front backwards. Of how many segments does the abdomen seem to be
composed when viewed from the ventral aspect? From the dorsal?
Make a drawing of the ventral aspect of the whole body.
TECHNICAL NOTE.--After examining the abdomen thus far, remove it
from the rest of the body, and boil it in dilute potassium hydrate
(KOH) in a test-tube. This will soften and partially bleach the
body wall.
Examine the softened specimen, and note that at least two additional
segments are to be found retracted or telescoped into the apparently
last segment. The character of these terminal abdominal segments differs
in male and female individuals, and specimens of both sexes should be
examined. (The males can be distinguished from the females by the
peculiar pad-like expansion of the last tarsal segment of the fore
legs.) Pull out the retracted segments, and note that they are unevenly
chitinized, parts of their surface being simply membranous. Projecting
backwards are several long-pointed processes. The female has but one
retracted segment. Though the females of many insects possess more or
less elaborately developed egg-laying organs, this is not the case with
the beetles. Look for _spiracles_ near the lateral margins of the dorsal
surface of the abdomen. How many pairs are present?
=Internal structure= (fig. 40).--TECHNICAL NOTE.--If fresh
specimens are to be had, kill by dropping into the cyanide bottle
(see p. 463). Specimens preserved in a 5% solution of chloral
hydrate may be used if necessary. When putting specimens into this
solution a small slit should be cut through the body wall to allow
the preservative to enter the body cavity. When ready to dissect a
specimen cut off the elytra and wings close to the base, and
carefully remove all of the dorsal wall of the abdomen and thorax
and the median portion of the dorsal wall of the head. Pin out,
ventral side down, under water in a dissecting-dish.
[Illustration: FIG. 40.--Dissection of female great water-scavenger
beetle, _Hydrophilus_ sp., the heart and tracheæ being cut away.]
Note in the median dorsal line of the abdomen a pale transparent
longitudinal vessel, the _heart_ or _dorsal vessel_. Note on each side
of it six prominent triangles or "Vs" with apex of each directed
laterally, the posterior three smaller than the anterior three of each
side. These triangles are formed by respiratory tubes or _tracheæ_.
From each spiracle or breathing-pore there extends into the body a
respiratory tube or trachea. These lateral tracheæ join a main
longitudinal trachea on each side, from which are given off branches,
which in turn repeatedly subdivide, until all parts of the body are
ramified by tracheæ, large and small, bringing air to all the tissues.
The oxygen is taken up from this air, and carbonic-acid gas is given
up to it, when it passes out of the body again through the spiracles.
Thus in the insects oxygen and carbonic-acid gas are not carried by
the blood but by special air-tubes. The respiratory system of insects
is very different from that of other animals.
Mount a bit of trachea in glycerine on a glass slide and examine under
the microscope. Note the fine spiral line (looking like transverse
annular striations) which is a thickening of the chitinous inner wall
of the tube and which by its elasticity keeps the tracheal tubes open.
The heart, already noted, is composed of a longitudinal series of very
thin-walled chambers, each with a pair of lateral openings into the
body-cavity and with terminal openings into the adjacent chambers. The
blood, which is colorless or greenish or yellowish, is sent forward
through the successive heart chambers by regular contractions until it
finally pours from the most anterior chamber freely into the
body-cavity. Here it bathes the body-tissues, flowing perhaps in
regular paths, giving up food to the tissues and taking up food from
the alimentary canal, until it finds its way through the lateral
openings into the heart chamber again. There are no arteries or veins.
Note the large mass of _muscles_ in the metathorax. Note, by attempting
to remove it, that the anterior part of the muscle mass is attached to a
chitinous partition-wall between the meso- and meta-thorax. Remove this
partition-wall (and one between the metathorax and abdomen) and note
that certain muscles run deeply down into the body. By pulling on the
bits of chitin to which the muscles are attached, the muscles (if they
have not been cut) can be stretched to the length of three-quarters of
an inch. When released they will contract. (This stretching and
contracting takes place only in fresh specimens.) What are these large
and numerous muscles of the thorax for?
Remove the thin membrane stretching over the abdomen and in which the
heart and tracheal "Vs" lie, and note immediately underneath it the
large coiled _intestine_ with a knot of greenish yellow threads in the
centre. Carefully uncoil and pin out the intestine, cutting away the
tying tracheæ, but being careful not to cut other structures. Work out
the full length of the alimentary canal, noting the _œsophagus_, the
widened _crop_ behind it, and the long intestine. From the intestine
arise several greenish yellow threads, the _Malpighian tubules_. These
are the excretory organs of the insect. What is the total length of
the alimentary canal?
The reproductive organs, consisting of a pair of glands (egg-glands or
sperm-glands) with a pair of tubes which unite before reaching the
body-wall and have a common external opening, may now be seen. These
should be removed, thus exposing the _ventral nerve-chain_ in the
abdomen. To expose the chain in the thorax it will be necessary to pick
away carefully the muscles. As in the crayfish, the central nervous
system in the beetle consists of a ventral nerve-chain, a _brain_ or
_supra-œsophageal ganglion_ and a pair of _circum-œsophageal
commissures_ connecting the brain and the foremost ganglion
(_infra-œsophageal_) in the ventral chain. There are, in the ventral
chain, four ganglia in the thorax and four in the abdomen. The large
nerves running from the brain to the compound eyes and to the antennæ
can be traced.
Make a drawing showing the nervous system.
=Life-history and habits.=--The eggs, usually about one hundred, are
deposited in a silken sac or case which is spun by the female, and
either floats freely or is attached to the under sides of the leaves
of aquatic plants. This egg-case is not wholly filled with eggs but
has a considerable air-chamber in it, causing it to float. It is oval
in shape, and has a peculiar curved horn-like projection at the upper
end. In sixteen or eighteen days the young water-scavenger beetles
hatch as elongate, wingless, active larvæ, provided with three pairs
of legs and strong jaws. They remain for a short time after hatching
in the egg-case, feeding on each other! After they issue from the case
they feed on flies or other insects which fall into the water, and on
snails. They breathe through a pair of spiracles situated at the
posterior tip of the abdomen, coming to the surface and thrusting this
tip up so that the spiracles are out of water. They grow rapidly,
molting three times before becoming full grown. They attain a length
of nearly three inches. When full grown they leave the water, crawling
out on the damp shore of the pond or stream, and burrow into the soil
for a few inches. Here they molt again, or pupate as it is called,
changing to a non-feeding, quiescent stage called the pupal stage. The
pupa is the stage in which the great changes from wingless, crawling
and swimming, short-legged, long, slender-bodied larva to winged,
swimming and flying, long-legged, compact, broad-bodied adult are
completed. Late in the summer or in the fall the pupal skin breaks and
the adult issues. It works its way to the surface of the ground, and
betakes itself to the nearest water.
The water-scavenger beetle shows in its post-embryonal development a
"complete metamorphosis" as contrasted with the "incomplete
metamorphosis" of the locust. Wherever among insects similar changes
occur, the young issuing from eggs as larvæ only remotely resembling
the parent, and these active feeding larvæ changing finally into more
or less quiescent, strictly non-feeding pupæ, which finally change
into the active adults, a complete metamorphosis is said to exist. All
the beetles, the butterflies and moths, the two-winged flies, the
ants, bees and wasps, and certain other groups of insects undergo in
their post-embryonic development a complete metamorphosis. The
crickets, katydids, the sucking bugs, the May-flies, the white ants
and numerous other insects have, like the locust, an incomplete
metamorphosis, that is, the young when hatched resemble in most
respects, except in the absence of wings, their parents.
The adult water-scavenger beetle feeds chiefly on decaying vegetation
in the water, but instances of the taking of other insects and of
snails have been noted. Although an aquatic insect the beetle, like
its larva, has no gills for breathing the air which is mixed with the
water, but has to come to the surface occasionally to obtain air. This
it does in an interesting way, which should be carefully observed by
the pupils. The air is received and held by a covering of fine hairs
on the ventral surface of the body, so that a considerable supply may
be carried about by the beetle while underneath the surface. The
beetles often leave the water by night, flying abroad to other ponds
or streams. In winter the beetles hibernate, burying themselves in the
banks of the ponds which they inhabit.
For a good account, with illustrations, of the water-scavenger
beetle's life-history see Miall's "Natural History of Aquatic
Insects," pp. 61-87.
THE MONARCH BUTTERFLY (_Anosia plexippus_)
TECHNICAL NOTE.--The Monarch or Milkweed butterfly is distributed
all over the country. It is large, and red-brown in color, and
lays its eggs on milk weeds where the greenish yellow and
black-banded larvæ (caterpillars) may be found feeding. The
covering of scales conceals the outlines of the various external
parts, but these scales may be easily removed with dissecting
needle and a small brush. In brushing the scales from the head
care must be taken not to break off the mouth-parts.
=External structure= (fig. 41).--Note the three body-regions, _head_,
_thorax_ and _abdomen_. Is the body segmented? Note the dark color and
firm character of the chitinized cuticle.
[Illustration: FIG. 41.--Body of the monarch butterfly, _Anosia
plexippus_, with scales removed to show the external parts.]
Note on the head the large _compound eyes_. Note the tumid convex
_clypeus_ which composes most of the anterior aspect of the head. Are
_ocelli_ present? Compare the _antennæ_ with those of the locust and
water-beetle. Compare also the _mouth-parts_ and note that they differ
radically from those of the locust and beetle. They are not fitted for
biting, but for sucking up liquid food (the nectar of flowers). Note
the absence of a movable flap-like _labrum_ (a minute narrow stiff
piece, bearing at each latera end a small group of fine brown hairs,
represents the labrum), the entire absence of _mandibles_, and the
absence of a movable flap-like labium. The _labium_ is a fixed
chitinized triangular piece forming part of the floor of the head.
Note the long slender _proboscis_ coiled up like a watch-spring. (In
fresh specimens this proboscis can be uncoiled and will be found
flexible. If dried or alcoholic specimens are being studied, the head
of the butterfly should be removed and softened in warm water before
the mouth-parts are examined.) On either side of this proboscis is a
peculiar pointed process which rises from the under side of the head.
These processes are the _labial palpi_ and serve to protect the
sucking proboscis. The proboscis itself is composed of the two greatly
modified _maxillæ_. Instead of being short, jaw-like and composed of
several pieces as in the locust, in the butterfly each maxilla is a
slender, flexible half tube applied against its mate on the opposite
side in such a way as to form a perfect tube long enough to reach into
the nectaries of flowers when in use and capable of being compactly
coiled up at other times. Cut across the proboscis and note the canal
in the centre. Try to separate the two maxillæ which compose it.
Make a drawing of the frontal aspect of the head with the eyes and
appendages.
Compare the thorax with that of the beetle and that of the locust. The
_prothorax_ is a freely movable narrow ring or collar. The _mesothorax_
and _metathorax_ are fused to form a large convex mass, of which fully
five-sixths is mesothorax and only one-sixth metathorax. Try to
distinguish the boundaries of the two segments. Note the three pairs of
legs; the differences in size among them, and the differences between
them and the legs of the locust and water-beetle. In one of the legs
determine the _coxa_, _trochanter_, _femur_, _tibia_ and _tarsal
segments_. Note the differences between the wings of the butterfly and
those of the locust and beetle. Note that the wings are membranous, but
are covered with many fine _scales_ (fig. 42), as is, indeed, the whole
body. Rub off some of these scales on a glass slide and examine; note
shape, little stem or pedicel of insertion, and longitudinal striations.
Examine under microscope a bit of wing from which some of the scales
have been rubbed. How are the scales attached to the wing membranes?
How are the scales arranged? Note that the wing is colorless where the
scales have been removed. All the colors and patterns of the wings of
butterflies are produced by the scales.
Make drawings of scales; of parts of denuded wings, and of bit of wing
covered with scales.
Remove all or nearly all the scales from a wing and note the
arrangement of the _veins_ (_venation_). Compare with venation in
wings of locust.
Make drawing showing venation in the butterfly's wings.
[Illustration: FIG. 42.--Bit of wing of Monarch butterfly, _Anosia
plexippus_, magnified to show the scales; some scales removed to show
the insertion-pits and their regular arrangement. (From specimen.)]
The venation of insects' wings is much used in insect classification,
and the various veins have been given names. The names of the veins in
the butterfly's wings are given in fig. 43. When the veins in the wings
of all the various groups of insects are studied, it is evident that the
principal ones are the same in all insects, so that the costa,
sub-costa, radius, media, cubitus and anal veins of the butterfly's
wings can be compared with the corresponding veins in the wings of a
beetle or wasp or fly. Noting the differences in the number and
character of branching of these principal veins, and the number and
disposition of the cross-veins which connect the longitudinal veins, the
various kinds of insects can be to a large extent properly grouped or
classified. A detailed account of the wing-veins of insects is given in
Comstock and Kellogg's "Elements of Insect Anatomy," chap. VII.
Of how many segments is the abdomen composed? The first or basal segment
is depressed, while the others are more or less compressed. The
_spiracles_ are, as in the locust, situated on the lateral aspects of
the abdominal segments. What segments bear spiracles? The terminal
segments of the abdomen differ in the two species. In the female the
dorsal part of the (apparently) last segment is longer than the ventral
part and is bent down over it forming a sort of hood over a space
enclosed partly by this hood, partly by a bluntly-pointed projection
from the ventral surface, and partly by the lateral margins of the
segment. In this chamber lies the opening from which the eggs issue. In
the male there are several backward-projecting, horny, thin processes.
[Illustration: FIG. 43.--Wings of monarch butterfly, _Anosia
plexippus_, to show venation; _c_, costal vein; _sc_, sub-costal vein;
_r_, radial vein; _cu_, cubital vein; _a_, anal veins. In addition
most insects have a vein lying between the sub-costal and radial veins
called the median vein.]
Make a drawing of the lateral aspect of the whole body.
=Life-history and habits.=--The tiny, conical, yellowish-green eggs of
the monarch butterfly are deposited on the under side of the leaves of
milkweeds (_Asclepias_) and when examined under the microscope are
seen to be very beautiful little objects finely ribbed with
longitudinal and transverse striæ. The eggs are laid in April and May
(depending on the latitude and season) by females which have
hibernated in the adult condition. From the eggs the minute,
cylindrical, pale-green, black-headed larvæ hatch in four or five
days. As soon as hatched the larva devours the eggshell from which it
has escaped and then feeds voraciously on the milkweed leaves. It
grows rapidly, and in three or four days a blackish band or ring
appears on each segment, and for the rest of its life it is very
conspicuously colored with its black rings on a yellowish-green
background. It molts three times, and in from twelve to twenty days is
ready to pupate, or change to a chrysalis.
When ready to pupate the larva usually leaves the milkweed plant, and
seeks some such protected place as the under side of a fence-rail or
jutting rock. Here it attaches its posterior extremity by a small
silken web to the rail or rock, and casting its larval skin appears as
a beautiful pale-green chrysalis with ivory black and golden spots. It
hangs motionless, and of course without taking food, for from a week
to two weeks (according to season and temperature), when the pupal
cuticle breaks and the great red-brown butterfly (fig. 165) issues.
The butterfly feeds (as is indicated by the structure of its
mouth-parts) very differently from the larva; it sucks up by means of
its long tubular proboscis the nectar of flowers, nor does it confine
itself at all to the flowers of milkweeds. It is a fine flyer and a
great traveller. Many thousands of these butterflies often make long
flights or migrations together. At other times tens of thousands of
these butterflies congregate in a certain limited area, clinging
sometimes to the branches of a few trees in such numbers and so
closely together as to give the tree a brown color. Such a "sembling"
of monarch butterflies occurs every year near the Point Pinos
lighthouse on the Bay of Monterey, California. The object of this
assembling together is not understood. Both the larvæ and adults of
the monarch butterfly are distasteful to birds, by their possession
of an acrid body-fluid. The species is thus protected against the most
dangerous enemies of butterflies, a fact which chiefly accounts for
the great abundance and wide distribution of the monarch (see p. 137).
For a full account of the life-history of the monarch butterfly, see
"Scudder's Life of a Butterfly."
LARVA OF MONARCH BUTTERFLY (_Anosia plexippus_)
TECHNICAL NOTE.--For directions for finding and identifying the
larvæ of the monarch butterfly see p. 171. If larvæ (caterpillars)
of _Anosia_ cannot be found, those of any other butterfly or moth
will do. Use naked, smooth kinds like cutworms, cabbage worms and
the like, rather than hairy or spiny ones. Use large specimens.
Kill the caterpillar with ether or in a cyanide bottle.
=Structure= (fig. 44).--As we have learned from the study of the
life-history of the locust, water-beetle and butterfly, some insects
are hatched from the egg in a condition resembling that of the parents
in most structural characters. This is true of the locust. Other
insects, as the beetle and butterfly, are hatched in a form and
condition apparently very different from that of the parents. The
external appearance of a beetle or butterfly larva differs much from
that of the adult or imago of the same individual. It will be of
interest to examine more particularly the structural condition of one
of these larvæ and to compare it with the structure of the adult.
[Illustration: FIG. 44.--Dissection of the silkworm, larva of the moth
_Bombyx mori_.]
Is the body segmented? Is the body composed of _head_, _thorax_ and
_abdomen_? Note the soft, flexible, weakly-chitinized condition of the
_body-wall_. How many pairs of legs are there? Where are they
situated? Is there any difference in the various legs? If so, what is
the difference? Which of the legs of the larva correspond with the
legs of the butterfly? Why? The prothoracic segment and the abdominal
segments 1 to 8 each bear a pair of _spiracles_ (small blackish spots
on the sides). Are both _compound_ and _simple eyes_ present? How many
eyes are there? Are there _antennæ_? Dissect out the _mouth-parts_.
How do they differ from those of the butterfly? Are they more like the
mouth-parts of the butterfly or more like those of the locust?
With fine sharp-pointed scissors make a shallow longitudinal incision
along the whole length of the dorsal wall. In a freshly-killed
specimen a drop of pale greenish blood will issue as the scissors'
point is first thrust through the skin. Put a droplet of this blood on
a glass slide, cover with cover glass and examine with high power of
the microscope. Note that the blood is a fluid containing numerous
sub-circular or elliptical bodies, the _blood-corpuscles_. Note at
least two kinds of corpuscles: most abundant a granular, circular
kind, the true blood-corpuscles; and rarer, a larger, clear, usually
elliptical or oval, but sometimes irregular and amœbiform kind,
generally spoken of as _fat-cells_.
Make a drawing of the corpuscles in the field of the microscope.
After making the dorsal longitudinal incision pin out the caterpillar in
the dissecting-dish with dorsal aspect uppermost. When the edges of the
skin are pinned back, the organs most conspicuous in the body-cavity
will be the flocculent masses of _adipose tissue_, the large, simple,
tubular _alimentary canal_ usually dark or greenish because of the color
of its contents, and the numerous silvery _tracheal tubes_. In those
caterpillars which spin a silken cocoon, the _silk_ or _spinning-glands_
are usually long and prominent. They lie on either side of the anterior
part of the alimentary canal, and open by a common duct on the labium.
Rising from behind the middle of the alimentary canal may be found the
long, whitish, folded and twisted _Malpighian tubules_. By picking away
the fat masses, expose the full length of the alimentary canal. Note its
great size (large diameter). Is it divided into distinct regions such
as crop, proventriculus, stomach, intestine, etc.? How is it held in
place? Trace the principal longitudinal tracheal trunks. Find, if you
can, a pair of small compact bodies usually somewhat elongate, one lying
on each side of the posterior part of the alimentary canal. These are
the rudimentary reproductive organs.
Remove the alimentary canal by cutting it off at its posterior tip and
also in the prothoracic segment. Work out now the _ventral nerve-cord_
and _ganglia_, and the _supra-œsophageal_ (brain) and _infra-œsophageal
ganglia_ and the _commissures_ in the head.
In the body of the caterpillar we have found the same general
disposition of organs as in the body of an adult insect, but several
differences are nevertheless noticeable, viz., the presence of a large
quantity of fatty tissue, the great size and simple character of the
alimentary canal, and the undeveloped condition of the reproductive
organs.
OTHER INSECTS
The class Insecta includes those Arthropods which have one pair of
antennæ (sense appendages), three pairs of mouth-parts (oral
appendages), and three pairs of legs (locomotory appendages). The
insects, in further contradistinction to the crustaceans, are mostly
land animals and breathe by means of tracheæ or tracheal gills. They
are the most familiar of land invertebrates, and, as already
mentioned, include more species than are comprised in all the other
groups of animals taken together. Beetles, moths and butterflies,
flies, wasps and bees, dragonflies and grasshoppers are familiar
members of the class of insects, but spiders, mites, scorpions,
centipeds and thousand-legged worms are not true insects and should
not be so miscalled. These last belong to the branch Arthropoda but to
other classes than the class Insecta. While insects are found living
under most diverse conditions on land, that is, on the ground, in the
leaves, fruits and stems of plants, in the trunks of trees or in dead
wood, in the soil, in decaying animal or plant matter, as parasites on
or in other animals, and in all fresh-water ponds and streams, they do
not live in ocean water. A few species live habitually on the surface
of the ocean, and a few other forms are found habitually on the
water-drenched rocks and seaweeds between tide lines. The varied
habits of insects, their economic relations with man, the beauty and
grace of many of them, and the readiness with which they may be
collected, reared and studied, renders them unusually fit animals for
the special attention of beginning students of zoology.
[Illustration: FIG. 45.--A wingless insect; the American spring-tail.
_Lepidocyrtus americanus_, common in dwelling-houses. The short line
at the right indicates the natural size. (From Marlatt.)]
=Body form and structure.=--The segments composing the body of an insect
are grouped to form three body-regions, the head, thorax, and abdomen.
The head of an adult insect appears to be a single segment or body-ring,
but in reality it is composed of several segments, probably seven,
completely fused. The head bears the eyes, antennæ and the mouth-parts.
The thorax is made up of three segments, each segment bearing a pair of
legs. From the dorsal side of the hinder two thoracic segments arise
the two pairs of wings which are the most striking structural features
of insects. Not all insects are winged, (fig. 45), and of those which
are a few have only one pair of wings, but the great majority of them
have two pairs of well-developed wings (fig. 46), which give them, as
compared with the other animals we have studied, a new and most
effective means of locomotion. The great numbers of insects and their
preponderance among living animals is undoubtedly largely due to the
advantage derived from their power of flight. The hindmost part of the
body, the abdomen, is composed of from seven to eleven segments, only
the last one or two of which are ever provided with appendages. When
such posterior abdominal appendages are present they form egg-laying or
stinging or clasping organs.
[Illustration: FIG. 46.--A four-winged insect; a stone fly, _Perla_
sp., common about brooks. (From Jenkins and Kellogg.)]
The body-wall is usually firm and rigid, with thinner flexible places
between the segments and body-parts for the sake of motion. The
body-wall is composed of a cellular skin or hypoderm, and an outer
non-cellular cuticle in which is deposited a horny substance called
chitin. This chitinous cuticle or exoskeleton serves as an armor or
protective covering for the soft body within, and also as a point of
attachment for the many muscles of the body.
Insects vary a great deal in regard to shape and appearance of the body,
and certain of the external organs are greatly modified in different
insects to adapt them to the varied conditions under which they live.
Especially interesting and important are the variations in the character
of the mouth-parts and wings, the organs of food-getting and locomotion.
In our consideration later of some of the more important groups of
insects the modification of these parts will be specially referred to.
Despite the great number of insects, however, and their varied habits of
life, a strong uniformity of body-structure is noticeable, all of them
holding pretty closely to the typical body-plan.
The most interesting feature of the internal anatomy of the insect
body is the respiratory system. Insects breathe through tiny paired
openings, called spiracles, in the sides of the abdominal (and
sometimes the thoracic) segments (the number and disposition of the
pairs of spiracles varying much in different insects). These spiracles
are the external openings of an elaborate system of air-tubes or
tracheæ (fig. 47) which ramify throughout the whole body and carry air
to all the organs and tissues. The blood has apparently nothing to do
with respiration as it has in the vertebrate animals, where it carries
oxygen to all the body tissues.
[Illustration: FIG. 47.--Piece of trachea (air-tube) from the larva of
the giant-cranefly. (Photo-micrograph by Geo. O. Mitchell.)]
[Illustration: FIG. 48.--The antenna of a carrion beetle, with the
terminal three segments enlarged and flattened, and bearing many
"smelling-pits", the antenna thus serving as an olfactory organ.
(Photo-micrograph by Geo. O. Mitchell.)]
The other systems of organs are well developed and in many respects
more complex and elaborate than those of any of the other
invertebrates. The muscular system comprises a large number of
distinct muscles, usually small and short, which are disposed so as to
make very effective the various complex motions of antennæ,
mouth-parts, legs, wings, and egg-laying organs. The muscles appear to
be very delicate, being almost colorless when fresh, but they have a
high contractile power. The alimentary canal is divided into various
special regions, as pharynx, œsophagus, crop, fore stomach or gizzard,
digesting stomach, and small and large intestine. From the canal just
at the point of union of the digesting stomach (ventriculus) and the
small intestine rise the so-called Malpighian tubules, which are
excretory organs. They are long slender diverticula of the alimentary
canal, and are typically six (three pairs) in number. The circulatory
system is composed of a tubular vessel running longitudinally through
the body in the median line just under the dorsal wall. It is composed
of a series of chambers or segmental parts, which by a rhythmic
contraction and expansion propel the blood anteriorly and into a
short, narrow, unsegmented anterior portion of the vessel which may
be called the aorta. There are no other arteries or veins, the blood
simply pouring out of the anterior end of the dorsal vessel into the
body-cavity. It bathes the body tissues, flowing usually in regular
channels without walls. It re-enters the dorsal vessel through paired
lateral openings in the chambers.
[Illustration: FIG. 49.--A section through the compound eye (in late
pupal stage) of the blow-fly, _Calliphora romitoria_. In the centre is
the brain, with optic lobe, and on the right-hand margin are the many
ommatidia in longitudinal section. (Photo-micrograph by Geo. O.
Mitchell.)]
The main or central nervous system consists of a large ganglion, the
"brain," situated in the head above the œsophagus, which sends nerves to
the antennæ and eyes, a ganglion in the head below the œsophagus
connected with the brain by a short commissure on each side of the
œsophagus, and sending nerves to the mouth-parts; and a ventral
nerve-chain composed of a pair of longitudinal commissures lying close
together and running from the head to the next to the last abdominal
segment, which bears a series of segmentally disposed ganglia, each
ganglion being composed of two ganglia more or less nearly completely
fused. There is, in addition, a lesser system called the sympathetic
system, which comprises a few small ganglia and certain nerves which run
from them to the viscera. The function of the nervous system of insects
reaches a very high development among the so-called "intelligent
insects" and certain extraordinarily complex and interesting instincts
are possessed by many forms. The social or communal habits of the ants,
bees, and wasps and the habits connected with the deposition of the eggs
and the care of the young exhibited by the digger wasps and other
insects are of extreme specialization. The organs of special sense are
highly specialized, the sense of smell (fig. 48) reaching in particular
a high degree of perfection. One of the compound eyes (figs. 49 and 50)
may contain as many as 30,000 distinct eye-elements or ommatidia, but
the sight is probably in no insect very sharp or clear. Among insects
there are organs of hearing of two principal kinds. In one kind the
organ for taking up the sound-waves is a group of vibratile hairs
usually situated on the antennæ, as is the case with the mosquito; in
the other kind, it is a stretched membrane or tympanum such as is found
in the fore leg of a cricket or katydid or on the first abdominal
segment of the locust (fig. 51).
[Illustration: FIG. 50.--Part of cornea, showing facets, of the
compound eye of a horse-fly (_Therioplectes_ sp.). (Photo-micrograph
by Geo. O. Mitchell.)]
[Illustration: FIG. 51.--The auditory organ of a locust (_Melanoplus_
sp.). The large clear part in centre of the figure is the thin
tympanum, with the auditory vesicle (small black pear-shaped spot) and
auditory ganglion (at left of vesicle and connected with it by a
nerve) on its inner surface. (Photo-micrograph by Geo. O. Mitchell.)]
The sexes are distinct in insects, and there is often a marked sex
dimorphism; in numerous species the males are winged while the females
are wingless, and in a few cases this condition is reversed. Where there
is a difference in size between male and female, the females are usually
the larger. Fertilization of the egg takes place in the body of the
female and, strangely, this fertilization is effected after the eggshell
has been formed. In all insect eggs there is a minute opening in one
pole of the eggshell called the micropyle through which the sperm-cells
enter. In a few cases the young are born alive, but such a viviparous
condition is exceptional. In a few species, too, young are produced
parthenogenetically, that is, are produced from unfertilized eggs. And
in the case of a few insect species male individuals are not known.
[Illustration: FIG. 52.--The young (at left) and adult (at right) of
the bed-bug, _Acanthia lectularia_, a wingless insect with incomplete
metamorphosis. (After Riley.)]
=Development and life-history.=--The young insect when just hatched
from the egg either resembles, except for the absence of wings, its
parent in general appearance as in the case of the locust, or it may,
as in the butterfly, emerge in a form very unlike the parent. In the
first case the young has simply to grow, that is, to increase in size,
to develop wings, and to make some other not very obvious
developmental changes in order to become fully grown. But in the case
of the butterfly, and similarly in the case of all other insects as
the flies, beetles, bees _et al._, whose young hatch in a larval
condition differing markedly from the adult, some radical and striking
developmental changes occur before maturity is reached. Such insects
are said to undergo complete metamorphosis in their development, while
those insects like the locusts, the sucking-bugs, white ants, and
others, the just hatched young of which resemble their parents, are
said to have an incomplete metamorphosis (fig. 52).
[Illustration: FIG. 53.--The larva of the violet tip butterfly,
_Polygonia interragationis_, making its last molt, i.e. pupating.
(Photograph from life.)]
In the case of insects with complete metamorphosis, the young hatches
as an active grub or worm-like feeding larva which increases in size,
casting its skin or molting several times in its growth. Finally after
the last larval molt (fig. 53) called pupation the insect appears in a
quiescent non-feeding stage called the pupa (fig. 54), and encased in
an extra thick and firm chitinous exoskeleton. The immovable pupa is
sometimes concealed underground, sometimes enclosed in a silken cocoon
spun by the larva just before pupation, or is in some other way
specially protected. It is in this pupal condition that the great
changes from wingless, often legless, worm-like larva to winged,
six-legged, graceful imago of adult stage are completed, and with the
molting of the chitinous pupal cuticle the metamorphosis or
development of the insect is completed. As a matter of fact many of
the special organs of the adult, the legs and wings, for example,
begin to develop as little buds or groups of cells in the body of the
larva, and when the larva is ready to pupate these imaginal wings and
legs are drawn out to the external surface of the body, and may be
readily recognized as they lie on the ventral surface of the pupa
folded and closely pressed to the body surface. In recent years the
study of the post-embryonic development of insects with complete
metamorphosis has revealed some remarkable changes of the internal
organs which result in a nearly complete disintegration or breaking
down of most of the internal organs of the larva (fig. 55) and a
rebuilding of the organs of the adult from primitive beginnings.
[Illustration: FIG. 54.--Chrysalid (pupa) of the violet tip butterfly,
_Polygonia interragationis_. From this chrysalid issues the full
fledged butterfly. (Photograph from life.)]
[Illustration: FIG. 55.--A cross-section of the body of the pupa of a
honey-bee, showing the body cavity filled with disintegrated tissues,
and (at the bottom) a budding pair of legs of the adult, the larva
being wholly legless. (Photo-micrograph by Geo. O. Mitchell.)]
The habits of the larvæ of insects with complete metamorphosis and of
the young of some insects with incomplete metamorphosis often differ
markedly from the habits of the adults, and as the habits and
instincts of insects are remarkably specialized, the study of their
behavior and of the structural and physiological modification which
their varied habits of life have brought about is of much interest and
significance. In later paragraphs this phase of insect study will be
again referred to.
=Classification.=--Much attention has been paid to the classification of
insects and the 300,000 (approximately) known species have been
variously grouped together into orders by different entomologists. A
subdivision of the class Insecta into five orders was proposed by
Linnæus about 1750 and was used until comparatively recently. Since
then, however, numerous other arrangements have been proposed, all of
them agreeing in increasing the number of orders by breaking up some of
the old ones into two or more new ones. The classification adopted in
the text-book[11] of zoology which we have made our reference in
classification is an 8-order system. The latest English[12] text-book in
entomology adopts a 9-order system, while the principal American[13]
text-book on this subject divides the insects into nineteen orders.
The classification depends chiefly on the character of the
post-embryonic development, that is, on whether the metamorphosis is
complete or incomplete, and on the structural character of the
mouth-parts and wings. In the following paragraphs a few of the larger
insect orders, with some special representatives of each, will be
briefly considered.
The best American text-book of the classification and habits of
insects is Comstocks' "Manual of Insects." For an account of the
structure of the wings and mouth-parts of various insects see Comstock
and Kellogg's "Elements of Insect Anatomy."
=Orthoptera: the locusts, cockroaches, crickets, katydids,
etc.=--TECHNICAL NOTE.--Obtain specimens of crickets or katydids,
and cockroaches, and compare the external body structure with that
of the grasshopper; examine especially the wings, mouth-parts,
legs, and egg-laying organs. Note that the hindmost legs of the
cockroach are not fitted for leaping but for running. Note the
sound-making (stridulating) organs on the bases of the fore wings
of the male katydids and crickets. Note the auditory organs
(tympana) in the fore tibiæ of the katydids and crickets. Crickets
can be easily kept alive in breeding-cages in the laboratory and
their feeding habits and much of their life-history observed. The
growth of the young and the development of the wings can be noted,
and will be found to be essentially similar to the conditions
already found in the case of the locust.
[Illustration: FIG. 56.--The house cricket, male (_a_) and female
(_b_). (From Marlatt.)]
The locust studied as one of the examples of the class Insecta belongs
to the order Orthoptera, which also includes the cockroaches, crickets
(fig. 56), katydids and green grasshoppers, the walking-stick or twig
insects, the praying mantis and others. The members of this order all
have an incomplete metamorphosis, and in all the mouth-parts are fitted
for biting and the fore wings are more or less thickened and modified to
serve as covers or protecting organs for the broad, plaited, membranous
hind wings, which are the true flight organs. The hind legs of locusts,
grasshoppers, crickets, and katydids are very large, and enable the
insects to leap; the legs of the cockroaches are fitted for swift
running; the fore legs of the praying mantis are fitted for grasping
other insects which serve as their food, and the legs of the
walking-stick (fig. 162) are long and slender and fitted for slow
walking. The shrill singing of the crickets and katydids and the loud
"clacking" of the locusts are all made by stridulation, that is, by
rubbing two roughened parts of the body together. The sounds of insects
are not made by vocal cords in the throat. The male crickets and
katydids (for only the males sing) have the veins of the fore wings
modified so that when the bases of the wings are rubbed together (and
when the cricket or katydid is at rest the base of one fore wing
overlaps the base of the other) a part of one wing called the "scraper"
rubs against a part of the other called the "file" and the shrilling is
produced. The sounds of locusts are produced by the rubbing of the
inside of the hind leg against the outside of the fore wing when the
insect is at rest, or by striking the front margin of each hind wing
against the hind margin of each fore wing when the locust is flying. For
hearing the Orthoptera are provided with auditory organs having the
character of tympana or vibrating membranes. In the locusts these ears
(fig. 51) are situated on the dorsal surface of the first abdominal
segment; in the katydids and crickets they are in the tibiæ of the fore
legs. The food of locusts, crickets, and katydids is vegetable, being
usually green leaves; the cockroaches eat either plant or animal
substances fresh or dry, while the praying mantis is predaceous, feeding
on other insects which it catches in its strong grasping fore legs. The
walking-stick or twig insect is an excellent example of what is called
"protective resemblance" among animals. Indeed most of the Orthoptera
are so colored and patterned as to be almost indistinguishable when on
their usual resting- or feeding-grounds. Some of the tropical Orthoptera
carry to a marvelous degree this modification for the sake of
protection. (In this connection read Chapter XXXI referring to
"Protective Resemblances".)
[Illustration: FIG. 57.--A bird louse, _Nirmus præstans_, from a tern,
_Sterna maxima_. Most birds are infested with small, wingless, biting
insects, called bird-lice, which are external parasites feeding on the
feathers of the bird host. The bird louse figured is about 1/12 in.
long. (Photo-micrograph by Geo. O. Mitchell.)]
=Odonata and Ephemerida: the dragon-flies and
May-flies.=--TECHNICAL NOTE.--Obtain specimens of adult and
immature dragon-flies. The young dragon-flies (fig. 59) may be got
by raking out some of the slime and aquatic vegetation from the
bottom of a small pond. Compare the external structure of the
adult dragonflies with that of the grasshopper; note the large
eyes, the narrow nerve-veined wings, the biting mouth-parts, and
the short antennæ. Compare the young dragon-flies with the
adults; note the developing wings and the peculiar modification of
the lower lip into a protrusible, grasping organ which when at
rest is folded like a mask over the face. Examine the interior of
the posterior part of the alimentary canal to find the rectal
gills. Obtain specimens of adult and young May-flies. The young
may be found on the under side of stones in a "riffle" in almost
any stream. They live also in ponds. They may be recognized by
reference to fig. 61. Compare adult May-flies with the
dragon-flies; note the weakly chitinized, delicate body-wall, and
the difference in size between fore and hind wings; note the
biting mouth-parts of the young and their absence or presence in
vestigial condition only in the adults.
The young of both dragon-flies and May-flies may easily be kept
alive in the laboratory aquarium (fruit-jars or battery-jars with
pond water in), and their feeding habits, their swimming, their
respiration, and much of their development observed. The young
May-flies should be got from ponds, not running streams. Put one
of these semi-transparent May-fly nymphs into a watch-glass of
water, and examine under the microscope. The movements of the
gills, heart, and alimentary canal, and much of the anatomy can
be readily made out. The emergence of the adult from the nymphal
skin can be seen if close watch is kept. The young dragon-flies
may be seen to capture and devour their prey. They may also
transform into adults, but for this it will be necessary to
obtain nymphs nearly ready for transformation.
Among the most familiar and interesting insects are the dragon-flies
(fig. 58), sometimes called "devil's darning-needles." They are
commonly seen flying swiftly about over ponds or streams catching
other flying insects. The dragon-flies are the insect-hawks; they are
predaceous and very voracious, and are probably the most expert flyers
of all insects. There are many species, and their bright iridescent
colors and striking wing-patterns make them very beautiful. The young
dragon-flies (fig. 59) are aquatic, living in streams and ponds, where
they feed on the other aquatic insects in their neighborhood. They
catch their prey by lying in wait until an insect comes close enough
to be reached by the extraordinarily developed protrusible grasping
lower lip (fig. 60). When at rest this lower lip lies folded on the
face so as to conceal the great jaws. The young dragon-flies breathe
by means of gills which do not project from the outside of the body,
as do the gills of other aquatic insects, but line the inner wall of
the posterior or rectal part of the alimentary canal. Water enters the
canal through the anal opening and bathes these gills, bringing oxygen
to them and taking away carbonic acid gas. The aquatic immature life
of the dragon-flies lasts from a few months to two years. When ready
to change to adult, the young crawls out of the water and clinging to
a rock or plant makes its last molt.
[Illustration: FIG. 58.--A dragon-fly, _Sympetrum illotum_, common in
California. (From life.)]
[Illustration: FIG. 59.--The young (nymph) of the dragon-fly,
_Sympetrum illotum_. (From Jenkins and Kellogg.)]
[Illustration: FIG. 60.--Young (nymph) dragon-fly, showing lower lip
folded and extended. (From Jenkins and Kellogg.)]
[Illustration: FIG. 61.--Young (nymph) of May-fly, showing (_g_)
tracheal gills. (From Jenkins and Kellogg.)]
Other abundant and interesting pond and brook insects are the May-flies.
The young May-flies (fig. 61) are aquatic, living in streams and ponds
and feeding on minute organisms such as diatoms and other algæ. The
immature life lasts a year, or even two or three in some species, and
then the May-fly crawls out of the water upon a plant-stem or projecting
rock and, molting, appears as the winged adult. The adult May-fly,
having its mouth-parts atrophied (a few May-flies have functional
mouth-parts), takes no food, and lives only a few hours or at most
perhaps a few days. It has the shortest life (in adult stage) of all
insects. The female drops her eggs into the water.
=Hemiptera: the sucking-bugs.=--TECHNICAL NOTE.--Obtain specimens of
water-striders (narrow elongate-bodied insects with long spider-like
legs which run quickly about on the surface of ponds or quiet pools
in streams), water-boatmen (mottled grayish insects about half an
inch long which swim and dive about in ponds and stream-pools),
back-swimmers (which are usually in company with the water-boatmen,
but which swim with back downwards and are marked with
purplish-black and creamy white patches), cicadas (the dog-day
locusts), and plant-lice (the "green fly" of rose-bushes and other
cultivated plants). Compare the external structure of some of these
Hemiptera with the other insects already examined; note especially
the sucking beak, composed of the elongate tube-like labium in which
lie the greatly modified flexible needle-like maxillæ and mandibles,
the whole forming an equipment for piercing and sucking. Obtain
immature specimens of some of these insects (distinguished by their
smaller size and the wing-pads); note that the metamorphosis is
incomplete, the young resembling the parents in general appearance.
Both immature and adult specimens of water-boatmen (_Corisa_),
back-swimmers (_Notonecta_), and water-striders (_Hygrotrechus_)
can be easily kept in the laboratory aquaria- and their swimming,
breathing, and feeding habits observed. Note especially the carrying
of air down beneath the water.
[Illustration: FIG. 62.--The female red orange scale insect,
_Aspidiotus aurantii_, very injurious to orange-trees. It has no
wings, legs, nor eyes, but remains motionless on a leaf, stem, or
fruit, holding fast by its long slender beak, through which it sucks
up the plant-sap. The male is winged, and has no mouth-parts, taking
no food. (Photo-micrograph by Geo. O. Mitchell.)]
[Illustration: FIG. 63.--The female rose-scale, _Diaspis rosæ_, a pest
of rose-bushes, without eyes, wings, or legs, but with slender sucking
proboscis. The male is winged and without mouth-parts. (Photo-micrograph
by Geo. O. Mitchell.)]
The Hemiptera are characterized particularly by their highly
specialized sucking mouth-parts, no other of the sucking insects
having the proboscis composed in the same manner. The palpi of both
maxillæ and labium are wholly wanting in Hemiptera and the flexible
needle-like maxillæ and mandibles are enclosed in the tubular labium.
This order is a large one and includes many well-known injurious
species, as the chinch-bug (_Blissus leucopterus_), which occurs in
immense numbers in the grain-fields of the Mississippi valley, sucking
the juices from the leaves of corn and wheat, the grape Phylloxera
(_Phylloxera vastatrix_), so destructive to the vines of Europe and
California, the scale insects (_Coccidæ_) (figs. 62 and 63), the
worst insect pests of oranges, the squash-bugs and cabbage-bug and a
host of others. Some of the Hemiptera, for example, the lice and
bed-bugs, are predaceous, sucking the blood of other animals.
[Illustration: FIG. 64.--A water-strider, _Hygrotrechus_ sp. (From
Jenkins and Kellogg.)]
The water-striders (fig. 64) catch other insects, both those that live
in the water and those which fall on to its surface, and holding the
prey with their seizing fore legs they pierce its body with their
sharp beak and suck its blood. They lay their eggs in the spring glued
fast to water-plants. The young water-striders are shorter and stouter
in shape than the adults.
[Illustration: FIG. 65.--A water-boatman, _Corisa_ sp. (From Jenkins
and Kellogg.)]
The water-boatmen (fig. 65) and back-swimmers swim and dive about in the
water, coming more or less frequently to the surface to get a supply of
air. This air they hold under the wings, or on the sides and under part
of the body entangled in the fine hairs on the surface. The insects
appear to have silvery spots on the body, due to the presence of this
air. The "rowing" legs of the water-boatmen (_Corisa_) are the hindmost
pair; in the back-swimmers (_Notonecta_) they are the middle legs.
The cicadas (fig. 66) are the familiar insects of summer which sing
so shrilly from the trees, the seventeen-year cicada (_Cicada
septendecim_) (oftentimes called locust) being the best known of this
family. Its eggs are laid in slits cut by the female in live twigs.
The young, which hatch in about six weeks, do not feed on the green
foliage, but fall to the ground, burrow down to the roots of the tree
and there live, sucking the juices from the roots, for sixteen years
and ten or eleven months. When about to become adult, the young cicada
crawls up out of the ground and clinging to the tree-trunk molts for
the last time, and flies to the tree-tops.
[Illustration: FIG. 66.--The seventeen-year cicada, _Cicada
septendecim_; the specimen at left showing sound-making organ, _v.
p._, ventral plate; _t_, tympanum. (From specimen.)]
The plant-lice (_Aphididæ_) are small soft-bodied Hemiptera which have
both winged and wingless individuals. In the early spring a wingless
female hatches from an egg which, laid in the preceding fall, has
passed the winter in slow development. This wingless female, called
the stem-mother, lays unfertilized eggs or more often perhaps gives
birth to live young, all of which are similarly wingless females which
reproduce parthenogenetically. This reproduction goes on so rapidly
that the plant-lice become overcrowded on the food-plant and then a
generation of winged[14] individuals is produced from time to time.
These winged plant-lice fly away to new plants. In the autumn a
generation of males and females is produced; these individuals mate
and each female lays a single large egg which goes over the winter,
and produces in the spring the wingless agamic stem-mother. Plant-lice
produce honey-dew, a sweetish substance much liked by ants, and the
lice are often visited, and sometimes specially cared for, by the ants
for the sake of this honey-dew. Small as they are, plant-lice occur in
such numbers as to do great damage to the plants on which they feed.
The apple-aphis, cherry-aphis, pear-aphis, cabbage-aphis and others
are well-known pests. The most notoriously destructive plant-louse is
the grape _Phylloxera_, which lives on the roots and leaves of the
grape-vine. Immense losses have been caused by this pest, especially
in the wine-producing countries of southern Europe.
=Diptera: the flies.=--TECHNICAL NOTE.--Obtain specimens of the
adult and young stages of the blowfly and the mosquito. All the
young stages of the blowfly may be obtained, and its life-history
studied, by exposing a piece of meat to decay in an open glass
jar. The larvæ of the mosquito are the familiar wrigglers of
puddles and ponds, and by collecting some of them and keeping them
in a glass jar of water covered with a bit of mosquito-netting,
the life-history of the mosquito is easily studied. If the eggs
can be obtained from the pond so much the better; they are in
little black masses floating on the surface of the water, and
resemble at first glance nothing so much as a floating bit of
soot. The external structure of the adult flies should be compared
with that of the other insects studied, noting especially the
condition of mouth-parts and wings, and the substitution of
balancers for the hind wings. The mouth-parts of the mosquito are
in the form of a long proboscis composed of six slender
needle-like stylets lying in a tube narrowly open along its dorsal
surface. The tube is the labium, and the stylets are the two
maxillæ, two mandibles, and two other parts known as the
epipharynx and the hypopharynx. Two additional thicker elongate
segmented processes lying outside of and parallel with the tube
are the maxillary palpi. The male mosquito (distinguished from the
female by the more hairy or bushier antennæ) lacks the pair of
needle-like mandibles. The mouth-parts of the blowfly are composed
almost exclusively of the thick fleshy proboscis-like labium,
which is expanded at the tip to form a rasping organ.
The Diptera or true flies are readily distinguishable from other
insects by their having a single pair of wings instead of two pairs,
the hind wings being transformed into small knob-headed pedicels
called balancers or halteres. The flies undergo complete
metamorphosis, and their mouth-parts are fitted for piercing and
sucking (as in the mosquito) or for rasping and lapping (as in the
blowfly). Nearly 50,000 species of flies are known, more than 4,000
being known in North America alone.
The blowfly (_Calliphora vomitoria_) is common in houses, but can be
distinguished from the house-fly by its larger size and its steel-blue
abdomen. It lays its eggs on decaying meat (or other organic matter)
and the white footless larvæ (maggots) hatch in about twenty-four
hours. They feed voraciously and become full grown in a few days. They
then change into pupæ which are brown and seed-like, being completely
enclosed in a uniform chitinized case which wholly conceals the form
of the developing fly. The house-fly has a life-history and immature
stages like the blowfly, but its eggs are deposited on manure.
[Illustration: FIG. 67.--The mosquito, _Culex_ sp.; showing eggs (on
surface of water), larvæ (long and slender, in water), pupa (large
headed, at surface), and adult (in air). (From living specimens.)]
The mosquito (_Culex_ sp.) (fig. 67) lays its eggs in a sooty-black
little boat-shaped mass which floats lightly on the surface of the
water. In a few days the larvæ, or "wrigglers," issue and swim about
vigorously by bending the body. The head end of the body is much
broader than the other, the thoracic segments being markedly larger
than the abdominal ones. The head bears a pair of vibrating tufts of
hairs, which set up currents of air that bring microscopic organic
particles in the water into the wriggler's mouth. At the posterior tip
of the body are two projections, one the breathing-tube (the wriggler
coming often to the surface to breathe), and the other the real tip of
the abdomen. The wriggler, although heavier than water, can hang
suspended from the surface film by the tip of its breathing-tube. It
changes in a few days into the pupa, which, instead of being quiescent
as with most flies, can swim about. It has a large bulbous head end
and the posterior end of the body bears a pair of swimming-flaps. It
takes no food. When ready to change to the adult mosquito the pupa
(which, unlike the wriggler, is lighter than water) floats at the
surface of the water, back uppermost. The chitinous cuticle splits
along the back and the delicate mosquito comes out, rests on the
floating pupal skin until its wings are dry, and then flies away. Only
the female mosquitoes suck blood. If they cannot find animals,
mosquitoes live on the juices of plants. They are world-wide in their
distribution, being serious pests even in Arctic regions, where they
are often intolerably numerous and greedy. Recent investigations have
shown that the germs which cause malaria in man live also in the
bodies of mosquitoes, and are introduced into the blood of human
beings by the biting (piercing) of the mosquitoes. It is probable
also that the germs of yellow fever are distributed by mosquitoes in
the same way. By pouring a little kerosene on the surface of a puddle
no mosquitoes will be able to escape from the water.
[Illustration: FIG. 68.--The house-flea, _Pulex irritans_; _a_, larva;
_b_, pupa; _c_, adult. (The fleas are probably more nearly related to
the Diptera than to any other order of insects.) (After Beneden.)]
=Lepidoptera: the moths and butterflies.=--TECHNICAL NOTE.--Obtain
specimens of a few moths, and compare with the butterfly already
studied; note especially the character of antennæ. Obtain
miscellaneous specimens of larvæ, pupæ, and cocoons of any moths or
butterflies. Note the variety in colors, markings, and skin
coverings of the larvæ; note the shape and markings of the pupæ.
Rear from eggs, larvæ, or pupæ in breeding-cages any moths and
butterflies obtainable (for directions for rearing moths and
butterflies see Chapter XXXIV), keeping note of the times of molting
and of the duration of the various immature stages. If the eggs of
silkworms can be obtained the whole life cycle of the silkworm moth
can be observed in the schoolroom. The larvæ (worms) feed on
mulberry or osage orange leaves, feeding voraciously, growing
rapidly and making no attempts to escape. The molting of the larvæ
can be observed, the spinning of the silken cocoon, and the final
emergence of the moth. The moths after emergence will not fly away,
but if put on a bit of cloth will mate, and lay their eggs on it.
From these eggs, which should be kept well aired and dry, larvæ will
hatch in nine or ten months (if the race is an "annual").
The Lepidoptera (figs. 69-74) include all those insects familiarly
known to us as moths and butterflies; they are characterized by their
scale-covered wings (fig. 69) and long nectar-sucking proboscis
composed of the two interlocking maxillæ. They undergo a complete
metamorphosis (fig. 70) and their larvæ are the familiar caterpillars
of garden and field. These larvæ have biting mouth-parts and feed on
vegetation, some of them being very injurious, for example the
army-worms, cut-worms, codlin moth worms, etc. The adult moths and
butterflies take only liquid food, or no food at all, and are wholly
harmless to vegetation. The structure and life-history of a butterfly
has already been studied, and in the more general conditions of
structure and life-history there is much similarity in the many
insects of this order. The eggs are usually laid on the food-plant of
the larva; the larva feeds on the leaves of this plant, grows, molts
several times, and pupates either in the ground or in a silken cocoon
or simply attached to a branch or leaf. There are about six thousand
species of moths and butterflies known in North America, and they are
our most beautiful insects.
[Illustration: FIG. 69.--A small, partly denuded part, much magnified,
of a wing of a "blue" butterfly, _Lycæna_ sp., showing the wing,
scales and the pits in the wing-membrane, in which the tiny stems of
the scales are inserted. (Photo-micrograph by Geo. O. Mitchell.)]
=Coleoptera: the beetles.=--TECHNICAL NOTE.--Obtain specimens of
various beetles, among them some water-beetles and June-beetles
with their young stages, if possible; if not, then the young
stages and adults of any beetle common in the neighborhood of the
school. Of the swimming and diving water-beetles there are three
families, viz., the Gyrinidæ or whirligig beetles, with four eyes
(each compound eye divided in two), the Hydrophilidæ, or
water-scavengers with two eyes and antennæ with the terminal
segments thicker than the others, and the Dytiscidæ or predaceous
water-beetles with two eyes and slender thread-like antennæ. Try
to find Dytiscidæ, large, oval, shining black beetles; the larvæ
are called water-tigers and are long, slim, active creatures with
six legs and slender curving jaws (see fig. 76). The June-beetles
are the heavy brown buzzing "June-bugs" and their larvæ are the
common "white grubs" found underground in lawns and pastures. Have
live water-tigers and predaceous water-beetles in the aquarium.
Note their feeding and breathing. Compare the external structure
of the beetles with that of the other insects, noting especially
the biting mouth-parts, and their thickened horny fore wings
serving as covers for the folded membranous hind wings.
[Illustration: FIG. 70.--The forest tent-caterpillar moth, _Clisiocampa
disstria_, in its various stages; _m_, male moth; _f_, female moth; _p_,
pupa; _e_, eggs (in a ring) recently laid; _g_, eggs hatched; _c_, larva
or caterpillar. Moths and caterpillar are natural size, eggs and pupa
slightly enlarged. (Photograph by M. V. Slingerland.)]
[Illustration: FIG. 71.--A trio of apple tent-caterpillars, _Clisiocampa
americana_, natural size. These caterpillars make the large unsightly
webs or "tents" in apple-trees, a colony of the caterpillars living in
each tent. (Photograph from life by M. V. Slingerland.)]
[Illustration: FIG. 72.--A family of forest tent-caterpillars
(_Clisiocampa disstria_), resting during the day on the bark, about
one-third natural size. (Photograph from life by M. V. Slingerland.)]
The Coleoptera is the largest insect order, probably 100,000 species
of beetles being known, of which 10,000 species are found in North
America. They pass through a complete metamorphosis (figs. 75 and 76),
the larvæ of the various kinds showing much variety in form and
habit. The pupæ are quiescent and are mummy-like in appearance, the
legs and wings being folded and pressed to the ventral surface of the
body. Among the familiar beetles are the lady-birds, which are
beneficial insects feeding on plant-lice and other noxious forms; the
beautifully colored tiger-beetles, predaceous in habit; the
"tumblebugs" and carrion beetles, which feed on decaying organic
matter; the luminous fire-flies with their phosphorescent organs on
the ventral part of the abdomen; the striped Colorado potato-beetle
and the cucumber-beetles and numerous other destructive leaf-eating
kinds; the various weevils (fig. 78) that bore into fruits, nuts and
grains, and the many wood-boring beetles, destructive to fruit-trees
as well as to shade- and forest-trees.
[Illustration: FIG. 73.--Moths of the peach-tree borer, _Sanninoidea
exitiosa_, natural size; the upper one and the one at the right are
females. (Photograph by M. V. Slingerland.)]
The predaceous water-beetles (_Dyticus_ sp.) are common in ponds and
quiet pools in streams. When at rest they hang head downward with the
tip of the abdomen just projecting from the water. Air is taken under
the tips of the folded wing-covers (elytra) and accumulates so that it
can be breathed while the beetle swims and feeds under water. When the
air becomes impure the beetle rises to the surface, forces it out, and
accumulates a fresh supply. The beetles are very voracious, feeding on
other insects, and even on small fish. The eggs are laid promiscuously
in the water, and the elongate spindle-form larvæ (fig. 77) called
water-tigers are also predaceous. They suck the blood from other insects
through their sharp-pointed sickle-shaped hollow mandibles. When a larva
is fully grown it leaves the water, burrows in the ground, and makes a
round cell within which it undergoes its transformations. The pupa state
lasts about three weeks in summer, but the larvæ that transform in
autumn remain in the pupa state all winter.
[Illustration: FIG. 74.--Army-worms, larvæ of the moth, _Leucania
unipuncta_, on corn. (Photograph by M. V. Slingerland.)]
[Illustration: FIG. 75.--The quince-curculio (a beetle),
_Conotrachelus cratægi_, natural size and enlarged. (Photograph by M.
V. Slingerland.)]
The June-beetles (June-bugs) (_Lachnosterna_ sp.) feed on the foliage
of trees. Their eggs are laid among the roots of grass in little
hollow balls of earth, and the fat sluggish white larvæ feed on the
grass-roots. They sometimes occur in such numbers as to injure
seriously lawns and meadows. The larvæ live three years (probably)
before pupating. They pupate underground in an earthen cell, from
which the adult beetle crawls out and flies up to the tree-tops.
=Hymenoptera: the ichneumon flies, ants, wasps, and
bees.=--TECHNICAL NOTE.--Obtain specimens of wasps, both social
(distinguished by having each wing folded longitudinally) and
solitary (wings not folded longitudinally), and if possible of both
queens (larger) and workers (smaller) of the social kinds; of ants
both winged (males or females) and wingless (workers) individuals;
also of honey-bees, including a queen, drones, and workers, and some
brood comb containing eggs, larvæ, and pupæ. The bee specimens can
be got of a bee raiser. Compare the external structure of ants,
bees, and wasps with that of other insects; note the pronounced
division of the body into three regions (head, thorax, abdomen);
note the character of the mouth-parts having mandibles fitted for
biting (ants and wasps) or moulding wax (honey-bees) and having the
other parts adapted for taking both solid and liquid food; note the
sting (possessed by the females and workers only). Observe the
behavior of bees in and about a hive; note the coming and going of
workers for food. Observe bees collecting pollen at flowers; observe
them drinking nectar. Examine the honey-bee in its various stages,
egg, larva, pupa, adult. Note the special structure of the adult
worker fitting it to perform its various special labors; the
pollen-baskets on the hind legs; the wax-plates on the ventral
surface of the abdomen, the wax-shears between tibia and tarsus of
hind legs; the antennæ-cleaners on the fore legs; the hooks on front
margin of hind wings, etc.
[Illustration: FIG. 76.--Immature stages of the quince curculio,
_Conotrachelus cratægi_; at the left, the larva natural size and
enlarged; at the right, the pupa. The beetle lays its eggs in pits on
quinces, and the larva lives inside the quince as a grub; the pupa
lives in the ground. (Photograph by M. V. Slingerland.)]
The Hymenoptera include the familiar ants, bees, and wasps, and also a
host of other four-winged, mostly small, insects, many of which are
parasites in their larval stage on other insects. All Hymenoptera have
a complete metamorphosis, and their habits and instincts are, as a
rule, very highly specialized. The parasitic Hymenoptera such as the
ichneumon flies, chalcid flies, etc., are stingless but have usually a
piercing ovipositor (the sting being only a modified ovipositor). The
general life-history of these ichneumons is as follows: the female
ichneumon fly, finding one of the caterpillars or fly or beetle larvæ
which is its host, settles on it and either lays an egg or several
eggs on it, or thrusting in its ovipositor, lays the eggs in the body;
the young ichneumon hatching as a grub burrows into the body of its
caterpillar host, feeding on the body-tissues, but not attacking the
heart or nervous system, so that the host is not soon killed; the
ichneumon pupates either inside the host, or crawls out and, spinning
a little silken cocoon (fig. 160), pupates on the surface of the body
or elsewhere.
[Illustration: FIG. 77.--Water-tiger, the larva of the predaceous
water-beetle, _Dyticus_ sp. (From specimen.)]
Some of the stingless Hymenoptera are not parasites, but are
gall-producers. The female with its piercing ovipositor lays an egg in
the soft tissue of a leaf or stem, and after the larva hatches the
gall rapidly forms. The larval insect lies in the plant-tissue, having
for food the sap which comes to the rapidly growing gall. It pupates
in the gall, and when adult eats its way out.
[Illustration: FIG. 78.--The plum curculio, _Conotrachelus nenuphar_,
a beetle very injurious to plums. (Photograph by M. V. Slingerland.)]
The ants, bees, and wasps are called the stinging Hymenoptera,
although the ants we have in North America have their sting so reduced
as to be no longer usable. Among these Hymenoptera are the social or
communal insects, viz., all the ants, the bumblebees and honey-bee,
and the few social wasps, as the yellow-jacket and black hornet. There
are many more species of non-social or solitary bees and wasps than
social ones, and their habits and instincts are nearly as remarkable.
[Illustration: FIG. 79.--The currant-stem girdler, _Janus integer_, a
Hymenopteron at work girdling a stem after having deposited an egg in
the stem half an inch lower down. (Photograph by M. V. Slingerland.)]
The solitary and digger wasps do not live in communities as the
hornets do, but each female makes a nest or several nests of her own,
lays eggs and provides for her own young. The nest is usually a short
vertical or inclined burrow in the ground, with the bottom enlarged to
form a cell or chamber. In this chamber a single egg is laid, and some
insects or spiders, captured and so stung by the wasps as to be
paralyzed but not killed, are put in for food. The nest is then closed
up by the female, and the larva hatching from the egg feeds on the
enclosed helpless insects until full grown, when it pupates in the
cell and the issuing adult gnaws and pushes its way out of the ground.
Each species of wasp has habits peculiar to itself, making always the
same kind of nest, and providing always the same kind of food. Some of
these wasps make their nests in twigs of various plants, especially
those with pithy centres in the stems. For interesting accounts of the
habits of several digger wasps see Peckham's "The Solitary Wasps."
The solitary bees, of which there are similarly many kinds, are like
the solitary wasps in general habit, only they provision the nest with
a mixture of pollen and nectar got from flowers instead of with stung
insects. Sometimes many individuals of a single species of solitary
bee will make their nests near together and thus form a sort of
community in which, however, each member has its own nest and rears
its own young. In the case of certain small mining bees of the genus
_Halictus_, a step farther toward true communal life is taken by the
common building and use by several females of a single vertical tunnel
or burrow from which each female makes an individual lateral tunnel,
at the end of which is a brood-chamber. Perhaps half a dozen females
will thus live together, each independent except for the common use of
the vertical tunnel and exit.
The bumblebees (_Bombus_ sp.) are truly communal in habit. All the
eggs are laid by a queen or fertile female, which is the only member
of the colony to live through the winter. In the spring she finds a
deserted mouse's nest or other hole in the ground, gathers a mass of
pollen and lays some eggs on it. The larvæ, hatching, feed on the
pollen, dig out irregular cells for themselves in it, pupate, and soon
issue as workers, or infertile females. These workers gather more
pollen, the queen lays more eggs, and several successive broods of
workers are produced. Finally late in the summer a brood containing
males (drones) and fertile females (queens) is produced, mating takes
place, and then before winter all the workers and drones and some of
the queens die, leaving a few fertilized queens to hibernate and
establish new communities in the spring.
The yellow-jackets and hornets (Vespidæ), the so-called social wasps,
have a life-history very like that of the bumblebees. The communities
of the social wasps are larger and their nests are often made above
ground, being composed of several combs one above the other and all
enclosed in a many-layered covering sac open only by a small hole at
the bottom. This kind of nest hangs from the branch of a tree and is
built of wasp-paper, which is a pulp made from bits of old wood chewed
by the workers. The brood-cells are provisioned with killed and chewed
insects, the larvæ of both solitary and social wasps being given
animal food, while the larvæ of both solitary and social bees are fed
flower-pollen and honey. As in the bumblebees, all the members of the
community except a few fertilized females die in the autumn, the
surviving queens founding new colonies in the spring. The queen builds
a miniature "hornet's nest" in the spring, lays an egg in each cell
and stores the cells with chewed insects. The first brood is composed
of workers, which enlarge the nest, get more food, and relieve the
queen of all labor except that of egg-laying. More broods of workers
follow until the fall brood of males and females appears, after which
the original process is repeated.
The honey-bees and ants show a highly specialized communal life, with
a well-marked division of labor and an individual sacrifice of
independence and personal advantage which is remarkable. Their
communities are large, including thousands of individuals, and the
structural differences among the males, females, and workers are
readily recognizable. With the ants the workers may be of two or more
sorts, a distinction into large and small workers or worker majors and
worker minors being not uncommon.
[Illustration: FIG. 80.--The honey-bee, _Apis mellifica_; _A_, queen,
_B_, drone, _C_, worker. (From specimens.)]
A honey-bee community, living in hollow tree or hive, includes a queen
or fertile female, a few hundred drones or fertile males, and ten to
forty thousand workers, infertile females (fig. 80). The number of
drones and workers varies, being smallest in winter. Each kind of
individual has a certain particular part of the work of the whole
community to do; the queen lays all the eggs, that is, is the mother of
the entire community; the drones act simply as the royal consorts,
fertilizing the eggs; while the workers build the comb, produce the wax
from which the cells are constructed, bring in all the food consisting
of flower-pollen and nectar, care for the young bees, fight off
intruders, and in fact perform all the many labors and industries of the
community except those of reproduction. There is a certain not very well
understood and perhaps not very sharply defined division of these labors
among the worker individuals, the younger ones acting specially as
"nurses," feeding and caring for the young bees (larvæ and pupæ), the
older ones making the food-gathering expeditions. The queen lays her
eggs one in each of many cells (fig. 81). These eggs hatch in three
days, and the young bee appears as a white, soft, footless, helpless
grub or larva that is fed at first by the nurses with a highly
nutritious substance called bee-jelly which the nurses make in their
stomachs and regurgitate for the larva. After two or three days of this
feeding the larvæ are fed pollen and honey. After a few days a small
mass of this food is put into the cell, which is then "capped" or
covered with wax. The larva after using up this food-supply pupates, and
lies quiescent in the pupal stage for thirteen days, when the fully
developed bee issues, and breaking through the wax cap of the cell is
ready for the labors which are immediately assigned it. The bee with the
kind of life-history just described is a worker. It has been
demonstrated that the eggs which produce workers and those which produce
queens do not differ, but if the workers desire to have a queen produced
they tear down two or three cells around some one cell, enlarging this
latter into a large vase-shaped cell. When the larva hatches from the
egg in this cell it is fed for its whole larval life with bee-jelly.
From the pupa into which this larva transforms issues not a worker but a
new queen. The eggs which produce drones or males differ from those
which produce queens and workers in being unfertilized, the queen having
the power to lay either fertilized or unfertilized eggs. When a new
queen appears or when several appear at once there is great excitement
in the community. If several appear they fight among themselves until
only one survives. It is said that a queen never uses its sting except
against another queen. The old queen now leaves the hive accompanied by
many of the workers. She and her followers fly away together, finally
alighting on some tree-branch and massing there in a dense swarm. This
is the familiar act of "swarming." Scouts leave the swarm to find a new
home, to which they finally conduct the whole swarm. Thus is founded a
new colony. "This swarming of the honey-bee is essential to the
continued existence of the species; for in social insects it is as
necessary that the colonies be multiplied as it is that there should be
a reproduction of individuals. Otherwise as the colonies were destroyed
the species would become extinct. With the social wasps and with the
bumblebees the old queen and the young ones remain together peacefully
in the nest; but at the close of the season the nest is abandoned by all
as an unfit place for passing the winter, and in the following spring
each young queen founds a new colony. Thus there is a tendency towards a
great multiplication of colonies. But with the honey-bee the habit of
storing food for winter, and the nature of the habitations of these
insects, render it possible for the colonies to exist indefinitely, and
thus if the old and young queens remained together peacefully there
would be no multiplication of colonies and the species would surely die
out in time. We see, therefore, that what appears to be merely jealousy
on the part of the queen honey-bee is an instinct necessary to the
continuance of the species."
[Illustration: FIG. 81.--Worker brood and queen cells of honey-bee;
beginning at the right end of upper row of cells and going to the left
is a series of egg, young larvæ, old larvæ, pupa, and adult ready to
issue; the large curving cells below are queen cells. (From Benton.)]
[Illustration: FIG. 82.--Honey-bees building comb. (From Benton.)]
For the special labors of gathering food, making wax, building cells,
etc., the workers are provided with special structures, as the
pollen-baskets on the outer surface of the widened tibia of the hind
legs, the wax-shears between the tibia and first tarsal joint of the
hind legs, the wax-plates on the ventral surface of the abdomen, etc.
A great many interesting things connected with the life and
industries of a honey-bee community can be learned by the student from
observation, using for a guide some book such as Cowan's "Natural
History of the Honey-bee."
[Illustration: FIG. 83.--Comb of the tiny East Indian honey-bee, _Apis
florea_, one-third natural size. (From Benton.)]
The gathering of food from long distances, the details of wax-making
and comb-building, of honey-making (for the nectar of flowers is made
into honey by an interesting process), the storing of food, how the
community protects itself from starvation when winter sets in or food
is scarce by killing the useless drones and the immature bees in egg
and larval stage, and many other phenomena of the life of the bee
community present good opportunities for careful observation and field
study. Although the community is a persistent or continuous one, the
individuals do not live long, the workers hatched in the spring
usually not more than two or three months, and those hatched in the
fall not more than six or eight months. But new ones are hatching
while the old ones are dying and the community as a whole always
persists. A queen may live several years, perhaps as many as five. She
lays about one million eggs a year.
There are more than two thousand known species of ants (fig. 84), all
of which live in communities and show a truly communal life. The ant
workers are specially distinguished in structure from the males and
females by being wingless, and in numerous species there are two sizes
or kinds of workers known as worker majors and worker minors. The
life-history and communal habits of ants are not so thoroughly known
as are those of the honey-bee, but they show even more remarkable
specializations. The ant nest or formicary is with most species an
elaborate system of underground galleries and chambers, special rooms
being used exclusively for certain special purposes, as nurse-rooms,
food-storage rooms, etc. The food of ants comprises many animal and
vegetable substances, but the favorite food with many species is the
"honey-dew" secreted by the plant-lice (Aphididæ) and scale insects
(Coccidæ). To obtain this food an ant strokes one of the aphids with
its antennæ, when the fluid is excreted by the insect and drunk by the
ant. In order to have a certain supply of this food some species of
ants care for and defend these defenseless aphids, which have been
called the "cattle" of the ants. In some cases they are even taken
into the ants' nests and food provided for them. "In the Mississippi
Valley a certain kind of plant-louse lives on the roots of corn. Its
eggs are deposited in the ground in the autumn and hatch the following
spring before the corn is planted. Now the common little brown ant
(_Lasius flavus_) lives abundantly in the cornfields, and is
especially fond of the honey secreted by the corn-root louse. So when
the plant-lice hatch in the spring before there are corn-roots for
them to feed on, the little brown ants with great solicitude carefully
place the plant-lice on the roots of a certain kind of knot-weed which
grows in the field and protect them until the corn germinates. Then
the ants remove the plant-lice to the roots of the corn, their
favorite food-plant. In the arid lands of New Mexico and Arizona the
ants rear scale insects on the roots of cactus."
[Illustration: FIG. 84.--The little black ant, _Monomorium minutum_;
_a_, female, _b_, female with wings, _c_, male, _d_, workers, _e_,
pupa, _f_, larva, _g_, egg of worker, all enlarged. (From Marlatt.)]
The ants are among the most warlike of insects. Battles between
communities of different species are numerous, and the victorious
community takes possession of the food-stores of the conquered. Some
species of ants live wholly by war and robbery. In the case of the
remarkable robber-ant (_Eciton_), found in tropical and subtropical
regions, most of the workers are soldiers, and no longer do any work
but fighting. The whole community lives exclusively by pillage. Some
kinds of ants go even farther than mere robbery of food-stores: they
make slaves of the conquered ants. There are numerous species of these
slave-making ants. They attack a nest of another species and carry
into their own nest the eggs and larvæ and pupæ of the conquered
community, and when these come to maturity they act as slaves of the
victors, collecting food, building additions to the nest, and caring
for the young of the slave-makers.
As with the honey-bee the larval ants are helpless grubs and are cared
for and fed by nurses. The so-called "ants' eggs," the little white
oval masses which we often see being carried in the mouths of ants in
and out of an ants' nest, are not eggs, but are the pupæ which are
being brought out to enjoy the warmth and light of the sun or being
taken back into the nest afterward.
There are in this country numerous species of ants showing much variety
of habit and offering excellent opportunities for most interesting field
observations. For an account of several of the common species see
Comstock's "Manual of Insects," pp. 633-643. Ants may be readily kept in
the schoolroom in an artificial nest or formicary and their life-history
and habits closely watched. For full directions for making and keeping a
simple and inexpensive formicary see Comstock's "Insect Life," pp.
278-281. For an interesting account of some of the habits of the social
insects see Lubbock's "Ants, Bees, and Wasps."
CLASS MYRIAPODA: THE MYRIAPODS, OR CENTIPEDS
AND MILLIPEDS.
Belonging to the branch Arthropoda, with the classes Crustacea and
Insecta, are three other classes, of which one, the Onychophora, is
represented by a single genus _Peripatus_ (Fig. 85), of extremely
interesting animals. However, as these animals are not found in the
United States we cannot study them. The other two classes are the
Myriapoda, including the centipeds and millipeds or thousand-legged
worms, and the Arachnida, including the scorpions, spiders, mites, and
ticks. All these animals are often spoken of as insects, but though
related to them they are not true insects.
[Illustration: FIG. 85.--_Peripatus eiseni_ (Mexico). (From specimen.)]
TECHNICAL NOTE.--From under stones or logs obtain specimens of
millipeds, or thousand-legged worms (large blackish, cylindrical,
worm-like animals with each body-segment back of the fourth bearing
two pairs of jointed legs); also specimens of centipeds or
hundred-legged worms (flattened, usually brownish or pale worm-like
animals with the body-segments bearing only one pair of legs each)
in the same places. Examine the external structure; note number of
body-rings; division into body-regions; presence of antennæ;
character and number of eyes; character of mouth-parts; character
and arrangement of legs. In the centipeds the first pair of legs is
modified to form a pair of poison-fangs. They appear to belong to
the mouth-parts. The internal anatomy will be found to be, if
examined, much like that of insects and can be studied from the
account of the anatomy of the water-scavenger beetle and butterfly
larva. Compare the Myriapods with the Hexapods or true insects. What
are the points of resemblance? what are the points of difference?
The Myriapoda are land-animals breathing by means of tracheæ like the
insects. In them the body-segments are nearly uniform in character with
the exception of the head, which, as in the insects, bears the
mouth-parts and antennæ. There is no grouping of the body-segments into
regions except as the head is opposed to the rest of the body. (In a few
myriapods there are indications of a division of the hind body into
thorax and abdomen.) The presence of true legs on all the segments of
the hinder region of the body and the lack of the three-region division
of the body are the principal external structural characteristics which
distinguish myriapods from insects. The internal anatomy corresponds in
general character with that of insects.
[Illustration: FIG. 86.--A galley-worm (milliped), _Julus_ sp. (From
specimen.)]
The most familiar myriapods are the millipeds, and the lithobians and
centipeds. The millipeds are cylindrical in shape, have two pairs of
legs on most of the body-segments and are vegetable feeders, though
some may feed on dead animal matter. The galley-worms (_Julus_) (fig.
86), large, blackish, cylindrical millipeds found under stones and
logs and leaves and in loose soil, are familiar forms. They crawl
slowly and when disturbed curl up and emit a malodorous fluid. They
can easily be kept alive in shallow glass vessels with a layer of
earth in the bottom, and their habits and life-history may thus be
studied. They should be fed sliced apples, green leaves, grass,
strawberries, fresh ears of corn, etc. They are not poisonous and may
be handled with impunity. They lay their eggs in little spherical
cells or nests in the ground. An English species of which the
life-history has been studied lays from 60 to 100 eggs at a time. The
eggs of this species hatch in about twelve days.
[Illustration: FIG. 87.--The skein centiped, _Scutigera forceps_,
natural size, common in houses and conservatories. (From Marlatt.)]
[Illustration: FIG. 88.--A centiped, _Scolopendra_ sp. (From specimen.)]
The lithobians and centipeds are flattened and have but a single pair
of legs on each body-ring. They are predaceous in habit, catching and
killing insects, snails, earthworms, etc. They can run rapidly, and
have the first pair of legs modified into a pair of poison-claws,
which are bent forward so as to lie near the mouth. The common "skein"
centiped (_Scutigera forceps_) (fig. 87) is yellowish and has fifteen
pairs of legs, long 40-segmented antennæ, and nine large and six
smaller dorsal segmental plates. The true centipeds (_Scolopendra_)
(fig. 88) have twenty-one to twenty-three body-rings, each with a pair
of legs, and the antennæ have seventeen to twenty joints. They live in
warm regions, some growing to be very large, as long as twelve inches
or more. The "bite" or wound made by the poison-claws is fatal to
insects and other small animals, their prey, and painful or even
dangerous to man. The popular notion that a centiped "stings" with all
of its feet is fallacious. It is recorded by Humboldt that centipeds
are eaten by some of the South American Indians.
CLASS ARACHNIDA: THE SCORPIONS, SPIDERS, MITES,
AND TICKS.
TECHNICAL NOTE.--Obtain specimens of various spiders; the running
or hunting spiders may be found on the ground, especially under
stones and boards, the web-makers on their snares. Get also
spiders' "cocoons" (egg-sacs). Examine the external structure of
the spider; note the two body-regions; the number and character of
legs; the absence of antennæ; the number and arrangement of the
eyes (which are simple, not compound); the mouth-parts, especially
the large mandibles; the spinnerets at the tip of the abdomen
(examine a cut off spinneret under the microscope to see the
spinning-tubes); note the breathing openings or spiracles on under
side of abdomen. Obtain also a scorpion if possible, and some
ticks and mites. Compare with the spiders and note that in the
scorpion the body is plainly seen (especially in the abdomen) to
be composed of segments. Note the extreme fusion of the segments
and body-regions in the mites and ticks. The common red spider of
hothouses and gardens is a mite; ticks may sometimes be found on
dogs. Observe various kinds of spider-webs, and try to observe the
process of web-making (this can be observed early in the morning
or about dusk) by one of the orb-weaving garden-spiders. Live
spiders can be kept in the schoolroom and their feeding habits and
perhaps web-making habits observed.
The class Arachnida is composed of Arthropods whose body-segments are
grouped into two regions, a cephalothorax bearing the mouth-parts,
eyes, and legs, and an abdomen. The segments composing these two parts
are so fused that, except in the scorpions, they are usually
indistinguishable. There are no antennæ, the eyes are simple, the
mouth-parts fitted for biting, and there are four pairs of legs. In
their internal anatomy the arachnids show in some forms a peculiar
modification of the respiratory organs, the tracheæ being flat and
leaf-like and massed together in a few groups rather than being
tubular and ramifying through the body.
[Illustration: FIG. 89.--A scorpion, _Centrurus_ sp., from California.
(From specimen.)]
The dorsal vessel or heart usually has a few blood-vessels or arteries
running from it. This class is divided into three orders, the
Arthrogastra, or scorpions, the Acarina, or mites and ticks, and the
Araneina, or spiders.
[Illustration: FIG. 90.--The cheese-mite, _Tyroglyphus siro_, greatly
enlarged. (After Berlese.)]
The scorpions (fig. 89) have the posterior six segments of the abdomen
much narrower than the seven anterior segments and forming a tail
which bears at its tip a poison-fang or sting. This sting is used to
kill prey, insects and other small animals. The tail can be darted
forwards over the body to strike prey which has been previously seized
by the large pincer-like maxillary palpi. Scorpions are common in warm
regions, about twenty species being known in southern North America.
Their sting though painful is not dangerous to man. The young are born
alive and are carried about by the mother for some time after birth.
The mites (figs. 90 and 91) and ticks (fig. 92) are mostly small
obscure animals, which live more or less parasitically. The common red
spider of house-plants as well as the sugar- and cheese-mites, the
dreaded itch-mite and the chigger are familiar examples of these
degraded arachnids, and the wood-ticks, dog- and chicken-ticks are
common examples of the larger bloodsucking forms. The body in both
mites and ticks is very compact, the two body-regions, cephalothorax
and abdomen, being closely fused.
[Illustration: FIG. 91.--Bird mite, species undetermined, from the
gnome-owl, Glaucidium gnomus. (Photo-micrograph by Geo. O. Mitchell.)]
The spiders have the abdomen distinctly set off from the
cephalothorax. The eyes (fig. 93) vary in number and arrangement, the
mandibles are large, each being composed of two parts, a basal
hair-covered part, the falx, and a terminal smooth, shining, slender,
sharp-pointed part, the fang, which is movably articulated with the
falx (fig. 93). In the falx is a poison-sac from which poison flows
through the hollow fang and out at its tip. The legs vary in relative
length in different spiders, and each is made up of seven joints. The
spinnerets (fig. 94), which are situated at the tip of the abdomen,
are six in number (a few spiders have only four), and are like little
short fingers. They have at their tips many fine little spinning-tubes
from each of which a fine silken thread issues when the spider is
spinning. These many fine threads fuse as they issue to form a single
strong cable or sometimes a flat rather broad band. The spinnerets are
movable, and by their manipulation the desired kind of line is
produced. The silk comes from many silk-glands in the abdomen, from
each of which a fine duct runs to a spinning-tube.
[Illustration: FIG. 92.--The dog or wood tick, _Dermacentor americanus_
male, the most common tick in the Northern States. (After Osborn.)]
[Illustration: FIG. 93.--The eyes and jaws, showing falx and fang of a
spider. (From Jenkins and Kellogg.)]
[Illustration: FIG. 94.--The six spinnerets (below) of a spider, with
one spinneret enlarged (above) to show the spinning "spools" or tubes.
(From Jenkins and Kellogg.)]
The spiders may be divided into two groups according to their habits,
viz., the wandering or hunting spiders, which do not spin webs to catch
their prey, and the sedentary or web-weaving spiders, which spin snares
to catch their prey. The wandering spiders can spin silk, however, and
often do so to line their burrows, to make nests, or to make egg-sacs.
[Illustration: FIG. 95.--A long-legged spider, _Tetragnatha_ sp., on
its web. (From life.)]
The hairy tarantulas and the trap-door spiders of similar appearance are
among the most interesting of the hunting spiders. They live in
vertical burrows or tunnels in the ground which are lined with silk, and
which in the case of the trap-door spider are covered with a door or lid
made of silk and soil. The top of this door is always covered with soil
or bits of leaves or twigs so that it is nearly indistinguishable from
the surface of the ground about it. When the nest is in ground covered
with moss the spider covers the door with moss. The tarantulas hunt at
night and rest in the burrow in the daytime. They are very large,
sometimes having an expanse of legs of 6 inches.
[Illustration: FIG. 96.--A running spider (Lycosidæ). (From life.)]
[Illustration: FIG. 97.--A female running spider (Lycosidæ) carrying its
egg-sac about attached to its spinnerets. (From Jenkins and Kellogg.)]
The common, rather large swift black spiders found under stones and
boards are hunting spiders, belonging to the family Lycosidæ and are
called the running spiders (fig. 96). They live in burrows in the
ground, coming out to stalk and chase their prey. The eggs are laid in
globular egg-sacs which are often carried about, attached to the
spinnerets, by the female (fig. 97). The young spiderlings after
hatching, in some species, climb on to the mother's back and are carried
by her for some time. Other kinds of wandering or hunting spiders are
the crab-spiders (Thomisidæ) (fig. 98), which run sidewise or backward
as well as forward, and the black and red, fierce-eyed stout-bodied
little jumping spiders (Attidæ) (fig. 99), which leap on their prey.
[Illustration: FIG. 98.--A crab-spider (Thomisidæ). (From Jenkins and
Kellogg.)]
[Illustration: FIG. 99.--A jumping spider (Attidæ). (From Jenkins and
Kellogg.)]
The sedentary or web-weaving spiders are of various kinds. They may be
grouped according to their spinning habits into cobweb weavers
(Therididæ), small slim-legged spiders which make the familiar
unsymmetrical cobwebs of houses and outbuildings; funnel-web weavers
(Agalenidæ), larger long-legged spiders of meadow and field which spin
a flat or concave horizontal web in the grass with a silken tube
leading down to the ground; the curled-thread weavers (Dictynidæ),
which use in addition to the usual lines peculiar broad lines made of
waved or curled threads in their irregular webs made in fence-corners
and on plants; and finally orb-weavers (Epeiridæ) (fig. 100), the host
of variously colored and patterned stout-bodied garden-spiders which
spin the beautiful symmetrical circular webs familiar to all (fig.
101). If a complete uninjured orb web be examined it will be found to
consist of a small central hub either open or closed, from which run
radii to the outer edges of the web. Around the hub is an open or free
zone, and farther out a spiral zone, so called because a line running
in close spiral turns fills in the space between the radii. This is
the real prey-catching part of the snare, and the silken line here is
sticky, while the radii and some other parts of the web are made of
silk that is not sticky. The web is supported by strong
foundation-lines, attached to leaves, stems, or whatever is firm in
the neighborhood of the web. The spider either rests on the web,
usually in the centre, or lies concealed in a nest or tent near at
hand from which a special path-line runs to the centre of the web. The
building of one of these orb webs is a great work, and is done with
extraordinary nicety of manipulation by the use of feet and
spinnerets. For account of web-making, etc., see McCook's "American
Spiders and their Spinning Work."
[Illustration: FIG. 100.--_Argiope_ sp., a large orb-weaver
(Epeiridæ). (From Jenkins and Kellogg.)]
[Illustration: FIG. 101.--Spider and its web in a rose-bush.
(Photograph from life by Cherry Kearton; from "Wild Life at Home," by
permission of Cassell & Co.).]
The habits and instincts of spiders in connection with the care of the
young, the building of webs and nests, ballooning by means of silken
lines, the active stalking and catching of prey, etc., are very
interesting and offer a good field for independent observation and
study by the student.
[Illustration: FIG. 102.--The triangle spider, _Hyptiotes_ sp.
(California), with its web; the spider rests on the taut guy-line,
with a loop of the line held between its fore and hind legs; when an
insect gets into the web the spider loosens the hold of its hind feet
on the guy-line, thus allowing the web to spring forward sharply and
further entangle the prey. (From Jenkins and Kellogg.)]
FOOTNOTES:
[9] There are in many forms a few internal projections from the
exterior cuticle which act as internal skeletal pieces.
[10] The labrum differs from the other mouth-parts in not being
composed of a pair of body appendages; it is simply a fold or flap of
the skin of the head.
[11] A Text-book of Zoology, Parker & Haswell, 1897.
[12] The Cambridge Natural History, vol. V, 1895, vol. VI, 1899.
[13] A Manual for the Study of Insects, J. H. and A. B. Comstock, 1897.
[14] It has been shown by experiment that the winged individuals,
which are able to leave the old food-plant and scatter over new
plants, do not appear until the food-supply begins to run short. At
the insectary of Cornell University ninety-four successive generations
of wingless individuals were bred, by taking care to provide a
constantly abundant supply of food. This experiment was continued for
more than four years.
CHAPTER XXII
MOLLUSCA: THE MOLLUSCS
THE FRESH-WATER MUSSEL (_Unio_ sp.)
=Structure= (fig. 103).--TECHNICAL NOTE.--The fresh-water or river
mussel lives commonly in the streams and lakes or ponds in the
United States. It frequents muddy or sandy bottoms. Specimens can
often be secured with a long-handled rake from the shore or picked
up in shallow streams with the hand. If possible to keep the animals
alive until ready for use, some of their habits may be observed.
Place them in a tub or trough with water and mud; when they have
settled themselves put some powdered carmine, starch, or similar
substance in the water near them, and note the water-currents.
Living mussels which have been placed in a dish with mud several
inches deep and covered with water will be seen to travel in a
definite direction. The end which is in front is the head end. Note
the process of thrusting out and retracting the fleshy _foot_ which
extends between the two _valves_ of the _shell_. Note that the two
valves are held together along the upper, or dorsal, surface by a
horny structure, the _hinge-ligament_. Note near the hinge-line a
prominence (_umbo_) in each valve from which extends a series of
concentric lines of growth. The umbo is the oldest part of the valve.
Note at the lower edge of the valves a soft membrane with a fringe
along its free border. This is the edge of the _mantle-lobes_, flaps
of the body-wall which cover the body and which aid in the functions
of respiration and nutrition.
TECHNICAL NOTE.--Specimens which are to be dissected should be
killed by dropping them for a few seconds into warm water, when
the muscles will relax enough so that a chip may be thrust between
the valves. If specimens are to be kept for some time before
dissecting they should be preserved in alcohol or 4% formalin. In
a dead specimen carefully remove the left valve. This is
accomplished by slipping in a thin knife-blade close to the inner
edge of the left valve and carefully cutting the two large
adductor muscles which bind the valves together. The dissection
should be made under water.
Before the removal of the valve, as just described, notice a portion
of the mantle adhering to the inner face of the valve, along a line of
attachment indicated by a crease. This is the _pallial line_. After
the left valve has been removed, the mantle being carefully separated
from it, note the large conical projections from the valves, the
_hinge teeth_, which fit into each other. Note the large muscle
impression just in front of the hinge-teeth; this is the point of
attachment of the _anterior adductor muscle_, while just behind and
adjoining it is the impression of the _anterior retractor muscle_.
Note posterior to the adductor and below the retractor a small
impression which affords attachment for the _protractor muscles_ of
the foot. At the other end of the valve, note the large impression of
the _posterior adductor muscle_ with the impression of the small
_posterior retractor muscle_ just above it.
TECHNICAL NOTE.--Lift back the left mantle-lobe, thus exposing the
body parts underneath.
Note the projecting muscular foot, the movements of which are governed
by the retractor and protractor muscles attached to the impressions
just mentioned. Note a pair of flattened plate-like structures
composed of thin, ribbed, membranous folds. These are the _gills_.
Note just beneath the anterior adductor muscle a small opening leading
into the soft _visceral mass_ of the body. This is the _mouth_. Note
near the mouth two pairs of plate-like structures much smaller than
the gills. These are the _labial palpi_, and it is by their action
that food-particles which have been brought in with the water are
conveyed to the mouth. Note at the posterior part of each mantle-lobe
a fringed portion which, together with a corresponding part on the
other side, forms the _inhalant siphon_. The cilia of the fringes
carry water and food-particles into the space enclosed by the
mantle-lobes; this space is the _mantle-cavity_. After the food has
been taken out and the water has passed through the finely striated
gills it is collected in a common cavity which extends above the two
sets of gills on each side. This space is called the _supra-branchial
cavity_. This cavity is continuous posteriorly with a space between
the right and left mantle-lobes, which is connected with the exterior
by an opening above the inhalant siphon called the _exhalant siphon_.
The function of the gills is partly to produce currents of water
carrying the food to the mouth, and partly respiratory. The mantle is
an important organ of respiration.
Make a drawing showing the organs described.
TECHNICAL NOTE.--Carefully cut away the mantle and gills from the
left side, and also the labial palpi, being careful not to disturb
the visceral mass.
Note two openings along the line where the gills and foot come together.
The uppermost is the opening of the ureter giving exit to the excretion
from the kidneys; the lower is the opening of the duct from the
reproductive organs and is called the _genital aperture_. The products
from both of these organs are carried out through the exhalant siphon.
Note that the mouth leads by a short tube (_œsophagus_ or _gullet_)
into a large cavity, the _stomach_, which is surrounded by a greenish
mass, the _digestive gland_.
TECHNICAL NOTE.--Carefully cut the delicate covering of the dorsal
portion of the visceral mass and expose a cavity.
The cavity thus exposed is the _pericardium_. Note within the
pericardium a long tube extending through it. This is a portion of the
alimentary canal, the _rectum_, which opens posteriorly through the
_anus_ into the _supra-branchial chamber_. Note a muscular sac about
the rectum midway of its course through the pericardium. This is the
unpaired _ventricle_ of the _heart_. Attached to each side of the
ventricle are thin-walled sacs, the _right_ and _left auricles_, which
are entered by fine blood-vessels, the _efferent branchial veins_,
from the right and left gills. The blood brought through these
blood-vessels from the gills flows into the auricles and from them
into the unpaired muscular ventricle, from which it is forced
anteriorly and posteriorly through two main arteries, the _anterior_
and _posterior aortas_, to all parts of the body. After bathing the
body-tissues the blood is collected into a median longitudinal vein
beneath the pericardium called the _vena cava_. From the vena cava the
blood passes through the kidneys and gills to be returned at last to
the heart. The mantle acts as an organ for the aeration of the blood,
and the blood it receives or at least part of it passes directly back
to the heart without passing through the kidneys and gills.
Note the delicate membranous dark-colored sac on the floor of the
pericardium, the _kidneys_ or _nephridia_. These are paired structures
which appear as two U-shaped tubes lying side by side. Each consists
of a lower portion with thick folded walls, the kidney proper, and an
upper thin-walled portion, the _ureter_. The kidneys open internally
through a pair of _reno-pericardial_ openings into the pericardium,
while the ureters communicate with the mantle-cavity by an opening on
the side of the body beneath the gills as already mentioned. The
kidneys are profusely supplied with fine blood-vessels and carry off
the waste matter from the blood.
Beneath the posterior adductor muscles note a small white
spider-shaped body, the more or less united _visceral ganglia_ of the
nervous system. Posteriorly these ganglia give off nerves to the
mantle and gills, while anteriorly there proceed two nerves, the
_cerebro-visceral connectives_, running forward, one on either side of
the foot close to the visceral mass, to the _cerebro-pleural ganglia_,
paired ganglia lying near the mouth. A delicate commissure running
over the gullet connects these ganglia.
TECHNICAL NOTE.--Cut away the skin and outer muscular layer from
the left side of the foot.
Note the large stomach-cavity, surrounded by the digestive gland. Trace
the convolutions of the alimentary canal through the foot to the anal
exit. Note in the anterior portion of the foot a fused pair of ganglia
similar to the visceral ganglia. These are the _pedal ganglia_, which
are connected by a pair of delicate commissures, the _cerebro-pedal
connectives_, with the cerebro-pleural ganglia. Note the glandular
tissue which fills the cavity of the foot and surrounds the loops of the
alimentary canal. This is the reproductive organ, which has its exit
beneath the gills on each side of the foot. The sexes of the mussel are
separate, but the reproductive organs are very similar.
=Life-history and habits.=--The eggs (ova) of the female pass first
into the supra-branchial chamber, whence, after being fertilized, they
drop into the outer pair of gill-chambers. These outer gills serve as
brood-pouches, and here it is that the embryonic stages are passed
through. The embryo when ready to issue has a soft body enclosed in
two triangular valves. At this stage it is called a _glochidium_. The
glochidium on being discharged through the exhalant siphon of the
parent falls to the bottom, where it remains for a time, when it
attaches itself to some fish by the lower hook-like projections of
the valves and leads a truly parasitic life for two months, after
which it undergoes a metamorphosis and falls to the bottom again,
there to begin an independent existence. Mussels often congregate in
favorite mud or sand banks. Their food consists primarily of small
organisms, both plants and animals, which are taken from the water
entering the mantle-cavity. Mussels move about slowly over the muddy
bottom of the stream by means of the muscular foot.
OTHER MOLLUSCS.
The branch Mollusca includes the fresh-water mussels, the clams,
oysters, snails, and slugs, the cuttlefishes, and all that host of
animals we call "shells" or shell-fish, which we know familiarly only
by the shell which they make, live in, and leave at death to tell the
tale of their existence. Not all the molluscs, however, form shells,
that is, external shells which serve as houses. The familiar slugs do
not, nor do a number of ocean forms called nudibranchs, which are
somewhat like the land-slugs, only much prettier and more attractive.
All the cuttlefishes and octopi are also without the hard calcareous
shell. But most of the molluscs are shell-bearing animals. The shell
may be bivalved, as in the mussel and clam, or univalved, that is,
composed of a single piece which may be spirally twisted, as with the
snail, or otherwise curiously shaped. The variety in the form, colors,
and markings of the shells indicates the great diversity among
molluscs. Molluscs live on land, in fresh water and in the ocean. No
depths of the ocean abysses are too great for the octopi, no coast but
has its many shells, hardly a pond or stream is without its mussels
and pond-snails, and in all regions the land-snails and slugs abound.
=Body form and structure.=--The molluscs are not to be mistaken for
any other of the lower animals; they have a structure peculiarly their
own. In them the body is not articulated or segmented as with the
worms and arthropods, nor radiate as in the echinoderms, nor
plant-like as with the sponges and polyps. (Where the typical
molluscan body is well developed it is composed of four principal
parts: a head, with the mouth, feelers, eyes, and other organs of
special sense; a trunk containing the internal organs; a foot which is
a thick muscular mass not at all foot- or leg-like in shape, but which
is the organ of locomotion by means of which the mollusc crawls; and a
mantle which is a fold of the skin enclosing most of the body and
which produces the shell. Such a typical molluscan body is possessed
by most of the snails. But in most of the other molluscs one or more
of these four body-regions are so fused with some other region as to
be indistinguishable. In the mussels and clams the head is not at all
set off from the rest of the body, the cuttlefishes and octopi have no
foot, the slugs have no shell. In the case of some of the molluscs
without external shell there are inside the body the rudiments or
vestiges of a shell.
With regard to the internal organs we note the constant presence of
three pairs of ganglia, viz., the brain, lying above the pharynx,
which sends nerves to the feelers, eyes, and auditory organs; the
pedal ganglion, which sends nerves to the foot, and the visceral
ganglion, which sends nerves to the viscera. This is a condition of
the nervous system characteristic of all molluscs. The heart is a
well-developed pulsating sac in the upper part of the body composed of
either two or three chambers, and there is a well-defined closed
system of arteries and veins, specially complete in the cuttlefishes
and octopi. This highly developed condition of the circulatory system
also distinguishes the molluscs from the other invertebrates.
=Development.=--Reproduction among the molluscs is always sexual.
Multiplication by budding or by the parthenogenetic production of eggs
is not known to occur. The eggs are usually laid in a mass held together
by a gelatinous substance. In most species the young mollusc on hatching
from the egg does not resemble its parent, but is a free-swimming larva
called a _veliger_. It is provided with cilia for organs of locomotion.
It must undergo a radical change in order to reach the adult stage. Thus
metamorphosis occurs in this branch as well as among the Arthropods and
Echinoderms. In the development of some molluscs, however, there is
little or no metamorphosis, the young being hatched in a condition much
resembling, except in size, the parent.
Some of the special characteristics of structure, life-history, and
habits of the molluscs will be noted in our consideration of the
various kinds.
=Classification.=--The branch Mollusca is divided into five classes,
three of which include the more familiar kinds. These three classes
are the Pelecypoda, including the mussels, cockles, clams, scallops,
oysters, etc., molluscs with a shell composed of two pieces, one on
each side of the body and hinged together; the Gastropoda, including
the snails, slugs, periwinkles, whelks, and a host of other univalved
shell-fish, that is, molluscs which have a shell composed of a single
piece; and the Cephalopoda, including the squids, cuttlefishes,
octopi, and the pearly nautilus.
=Clams, scallops, and oysters (Pelecypoda).=--TECHNICAL
NOTE.--Shells of scallops, oysters, and sea-mussels should be had
for examination; also specimens of _Teredo_ or _Pholas_ in alcohol
or formalin, and pieces of pile bored by _Teredo_. Make drawings
of various bivalve shells, and of _Teredo_.
[Illustration: FIG. 103.--Dissection of fresh-water mussel, _Unio_ sp.]
The fresh-water mussel which we have studied is an example of the
bivalve molluscs. The members of this class show a range in size
from the little fresh-water _Cyclas_ about 1 cm. long to the giant
clam of the Indian and Pacific islands "which is sometimes 60 cm. (2
feet) in length and 500 pounds in weight." They show also some variety
in the form and appearance of the shell, but not anything like the
degree of variety shown by the shells of the Gastropods.
The edible clams are of several different species. The hard-shell clam
(_Venus mercenaria_), or "quohog" as it is often called, is found
along the Atlantic coast from Texas to Cape Cod. It is "common on
sandy shores, living chiefly on the sandy and muddy plots, just beyond
low-water mark.... It also inhabits estuaries, where it most abounds.
It burrows a short distance below the surface, but is frequently found
crawling at the surface with the shell partly exposed." The shells of
this edible clam are white. The soft-shell clam (_Mya arenaria_), "the
clam _par excellence_, which figures so largely in the celebrated New
England clam-bake, is found in all the northern seas of the world....
All along the coasts of the eastern States, every sandy shore, every
mud flat, is full of them, and from every village and hamlet the
clam-digger goes forth at low tide to dig these esculent bivalves. The
clams live in deep burrows in the firm mud or sand, the shells
sometimes being a foot or fifteen inches beneath the surface. When the
flats are covered with water his clamship extends his long siphons up
through the burrow to the surface of the sand, and through one of
these tubes the water and its myriads of animalcules is drawn down
into the shell, furnishing the gills with oxygen and the mouth with
food, and then the water charged with carbonic acid and fæcal refuse
is forced out of the other siphon. When the tide ebbs the siphons are
closed and partly withdrawn." Ocean clams and mussels have furnished
food for man for ages, and along coasts are found here and there
great mounds made of heaps of clam-shells which have become covered
over with soil and vegetation. Such mounds are the old feasting-places
of the early coast inhabitants, and the archæologist often finds in
these "kitchen-middens," as they are called, various relics of the
early natives of the continent.
[Illustration: FIG. 104.--A group of marine Pacific Coast molluscs; in
upper left-hand corner, _Purpura saxicola_; next to the right,
_Littorina scutulata_; farthest to right, limpets, _Acmara spectrum_;
left-hand lower corner, _Mytilus californianus_; in right-hand lower
corner the black shells just above the large clam-shell, _Chlorostomum
funebrale_. (From living specimens in a tide pool in the Bay of
Monterey, California.)]
Even more widely known that the clams are the oysters (_Ostrea
virginiana_), also members of this class of molluscs. The oyster is
carefully cultivated by man in many countries. It has its two shells
or two shell-halves dissimilar, one valve being hollowed out to
receive the body, while the other is nearly flat. The oyster is
attached to the sea-bottom by the outside of the hollowed-out valve.
When first hatched the young oyster swims freely by means of its
cilia; after a few days it attaches itself to some solid object and
grows truly oyster-like. Much care has to be taken in cultivating
oysters to furnish proper conditions for growth and development. The
young oysters when first attached are called "spat"; when a little
older this "spat," now called "seed," may be transplanted to new beds,
which are stocked in this way. In fact some beds have constantly to be
thus restocked, the young oysters produced on them not finding good
places to attach themselves, and so swimming away. Sometimes pieces of
slate, pottery, etc., are strewed about the oyster-beds to serve as
"collectors," that is, as places for the attachment of the young
oysters. The extent of the acreage of the American oyster-beds is
larger than that of any other country. "The Baltimore oyster-beds on
the Chesapeake River and its tributaries cover 3,000 acres, and
produce an annual crop of 25,000,000 bushels."
[Illustration: FIG. 105.--_Dactylus_ sp., a mollusc, excavating granite.
(Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)]
[Illustration: FIG. 106.--_Pholas_ sp., a mollusc, burrowing in
sandstone. (Photograph by C. H. Snow; permission of Amer. Soc. Civil
Engineers.)]
The "pearl-oyster" is not a true oyster, that is, not a member of the
family to which the edible oysters belong, but it is a member of the
same class, that is, it is a bivalve mollusc. Pearls are obtained from
a number of different "pearl-oysters," but the finest pearls and
mother-of-pearl come from the tropical species _Meleagrina
margaritifera_. This pearl-oyster "has an extensive distribution,
being found in Madagascar, the Persian Gulf, Ceylon, Australia,
Philippine Islands, South Sea Islands, Panama, West Indies, etc."
Mother-of-pearl is simply the inner lining of the shell, which is
composed of numerous thin layers of carbonate of lime so arranged that
the edges of the successive layers produce many fine striæ very close
together. The beautiful iridescence of this inner shell-lining is
caused by the complicated diffraction and reflection (interference
effects) of the light by the fine striæ and the translucent superposed
thin plates of shell material. Pearls are simply isolated deposits of
shell material usually around some particle of foreign substance which
has found lodging in the mantle-cavity. Sometimes small objects are
purposely introduced into the shell in order to stimulate the
formation of pearls. The pearl-fishers go out in boats and dive to the
bottom, filling baskets with pearl-oysters. These are piled up in a
bin and left to die and decompose. "When the flesh is pretty
thoroughly disintegrated, it is washed away with water, great care
being taken that none of the pearls loose in the flesh are lost. When
the washing is concluded the shells themselves are examined for pearls
which may be attached to the interior of the valves." The principal
pearl-fishery is that on the coast of Ceylon; pearl-fishing has been
carried on here for over 2000 years.
[Illustration: FIG. 107.--_Martesia xylophaga_, a Pholad, in Panama
mahogany. (Photograph by C. H. Snow; permission of Amer. Soc. Civil
Engineers.)]
The ship-worm (_Teredo_) is an interesting member of this class of
bivalve molluscs, because of its unusual habits, and strangely
modified body form. The teredo is long and worm-like in general
appearance, with a small bivalve shell at one end and two elongated
siphons at the other. The young teredo is a free-swimming ciliated
embryo like the young of the other bivalve molluscs, but it soon
settles on a piece of submerged wood, usually the pile of a wharf, or
the bottom of a ship, and burrows into this wood. As it grows it
enlarges and deepens its tube-like burrow, and lines it with a
calcareous deposit. The burrow may be a foot long or longer, and when
thousands of teredos attack a pile or the bottom of a ship, the wood
soon becomes riddled with holes. These boring molluscs do great
damage to wharves and ships. In Holland where they were first
discovered they caused such injuries to the piles and other submerged
wood which supported the dikes and sea-walls that they seriously
threatened the safety of the country.
[Illustration: FIG. 108.--The giant yellow slug of California,
_Ariolimax californica_. This slug reaches a length when outstretched
of 12 inches. (From living specimen.)]
=Snails, slugs, nudibranchs and "sea-shells"
(Gastropoda).=--TECHNICAL NOTE.--Pond-snails can be readily found
clinging to submerged stems, leaves, or pieces of wood in almost any
pond. Collect some and carry alive, in a jar of water, to the
schoolroom. Observe the habits of these live snails in the school
aquarium. Note the movements, the coming to the surface to breathe,
the eating (by scraping the surface of the leaves with the "radula"
or tongue; provide fresh bits of cabbage or lettuce-leaves), the use
of the feelers. Make drawings illustrating these habits. Examine the
shell; note that it is univalved, that is, composed of one piece. Do
the whorls of all the shells turn the same way? Make a drawing of
the shell, naming such parts as the apex, spire (all the whorls
taken together), the aperture, the columella (the axis of the
spire), the lip (outer edge of the aperture), the lines of growth
(parallel to the tip), the suture (the spiral groove on the
outside). Examine the snail; note the character of the foot; note
the protrusible tentacles or feelers, the eyes (dark spots at bases
of the tentacles), the mouth, the respiratory opening (on right side
of body in the edge of the mantle which protrudes beneath the lip
when the snail's body is extended), the radula or ribbon-like tongue
with fine teeth. Compare with the body of the mussel.
Slugs may be found during the day concealed under boards or
elsewhere; they are nocturnal in habit. If specimens can be
obtained, compare with the pond-snails, noting the absence of a
shell, and the fleshy mantle on the dorsal surface near the head;
note the presence of two pairs of tentacles (the eyes being at the
tips of the second or hinder pair), and the respiratory pore. Note
the streak of mucus left by the slugs in crawling about.
Some sea-shells can be got from private collections of "curios" to
illustrate the variety of form of the univalve shells.
Perhaps one-half of all the known species of molluscs are snails and
slugs (fig. 108). Snails are either aquatic or terrestrial in habit,
but in either case they (the true pulmonate snails) breathe not by
means of gills, as do most of the other molluscs, but by means of a
so-called "lung." This lung is a sac with an external opening on the
right side of the body and with its inner surface richly furnished
with fine blood-vessels. The exchange of gases between the blood and
the outer air takes place through the thin walls of the blood-vessels.
Most snails which live in the water, as the pond-snails and the
river-snails, have to come occasionally to the surface to breathe.
These fresh-water and land-molluscs which possess a lung-sac instead
of gills constitute the order Pulmonata. The pulmonate pond- and
land-snails and slugs are vegetable feeders and where they occur in
large numbers do much injury to vegetation. While the common
pond-snails have but one pair of feelers, at the base of which are
found the eyes, most of the land-snails and slugs have two pairs of
"horns," the eyes being on the tips of the second pair. The lung-sac,
besides serving as a breathing organ, also enables the snail to rise
or sink according as the animal varies the size of the sac and
consequently the amount of air in it. All the Pulmonata are
hermaphroditic, each individual producing both sperm- and egg-cells.
The eggs of the pond-snail "are laid in gelatinous transparent
capsules, half an inch to an inch in length, flattened and linear or
oblong in outline. After a few snails have been kept a short time in a
small vessel of water with their appropriate food, these egg-capsules
may be looked for on the bottom and sides of the vessel or closely
adherent to the stems or leaves of plants placed in the water. They
are so transparent as to be easily overlooked." Young snails may be
reared from these eggs.
There are other snails common in ponds, also called, like the
pulmonate forms, pond-snails, which have gills and no lung-sac. These
pond-snails belong to a different order of molluscs, and live on the
bottom of the pond, crawling about in the soft mud and feeding on
animal instead of vegetable food.
The shells of the various kinds of snails vary much. In many of the
land-snails the spiral is not spire-shaped or conical, but is flat. In
some the whorls of the spiral run from left to right (dextral) when
the shell is looked at with apex held toward one, while in others the
whorls run from right to left (sinistral).
[Illustration: FIG. 109.--Three Pacific Coast nudibranchs; _Doris
tuberculata_ (in lower left-hand corner), _Echinodoris_ sp. (upper
one), and _Triopha modesta_ (at right). (From living specimens in a
tide-pool on the Bay of Monterey, California.)]
Of the hosts of marine Gastropods we can notice only a few kinds. The
nudibranchs (fig. 109) are a group of beautiful forms in which the
shell is wholly wanting and the mantle is usually absent. The gills
are thus exposed and are usually in the shape of delicate freely
projecting tufts arranged in rows along the back. The body is often
strikingly and variedly colored. These soft, naked "sea-slugs" live
near the shore, creeping about among the rocks and seaweeds. About a
thousand species of nudibranchs are known.
Among the shell-forming marine Gastropods there is great variety in
the size and shape and coloring of the shells. Many are beautifully
colored and patterned; others are oddly and fantastically shaped. The
cowries, or porcelain shells, familiar in collections of ocean
curiosities, have a large body whorl and a very short flat spire, and
the brightly colored shell looks as if enamelled. Some of the coast
tribes of Africa once used, and perhaps still use to some extent,
cowries as money. The limpets (fig. 104) are among the most abundant
of the seashore molluscs, their low, broadly conical shells being
plentifully scattered over the rocks between tide-lines. The
"oyster-drills" are Gastropods with odd spiny shells which do much
harm in oyster-beds by settling down on the oysters, boring holes
through the shells and eating the soft parts within. The
helmet-shells, from which shell cameos are cut, are composed of layers
of shell material of different colors. Among the specially beautiful
shells are the cone-shells, the olive-shells, the ivory-shells, etc.
=Squids, cuttlefishes, and octopi (Cephalopoda).=--TECHNICAL
NOTE.--Small squids preserved in alcohol or formalin can be had of
all dealers in biological supplies (see p. 453), and specimens
should be examined.
The squids (fig. 110), cuttlefishes, octopi or "devil-fishes," and the
three living species of _Nautilus_ constituting the class Cephalopoda
are very different from the other molluscs in appearance, and are in
fact different in important structural characters. They can move
swiftly, have strangely modified organs of prehension, strong biting
mouth-parts, and eyes of very complex organization. They are the most
highly organized molluscan forms, and their predaceous habits and the
great size to which some of them attain have given them distinction
among the fierce and dangerous creatures of the sea. They are all
strictly marine in habitat, and are all carnivorous. Most of them have
no shell, or where the shell is present it is internal in all but a very
few forms. The tentacle-like arms or feet surrounding the mouth which
occur in all the Cephalopods are provided with sucking organs or
suckers, in some cases with a horny toothed rim. These long, powerful,
grasping, tentacular feet, with the suckers and five hooks, are very
effective means of securing prey, and the pair of strong, sharp, cutting
mandibles or beaks are equally effective in tearing to pieces. The eyes
of the Cephalopods are almost as highly developed as those of the
vertebrates. They are unusually large and staring, and add much to the
terrifying appearance of the "devil-fishes." Cephalopods have the power
of quickly changing color, because of the presence in the skin of many
pigment-cells which can expand so as nearly to touch each other, thus
producing a uniform tint over the whole body, or which can contract so
as to destroy this uniformity of color. There are several sets of these
color-carrying cells or chromatophores, each set of a color different
from the others. The purpose of this change of color is protective, the
animal being thereby able to make its color so harmonize with that of
its immediate surroundings as to become indistinguishable.
There are two principal groups of Cephalopods, viz., the Decapods and
the Octopods. The Decapods, as their name indicates, have ten feet or
arms surrounding the mouth, and in them the body is usually elongate,
containing a horny "pen" or calcareous "bone." This group includes the
cuttlefishes or sepias, from which are obtained sepia ink and the
cuttlefish bone used to feed canary birds. The ink is a secretion
which the cuttlefish discharges when attacked to create a cloud in the
water and thus escape unperceived. The squids (_Loligo_) commonly used
as bait by fishermen belong to the Decapoda. The two extra feet or
arms which the Decapods have in addition to the eight possessed by the
Octopods, differ from the others in being longer and slenderer and
having suckers only on the distal extremities which are expanded into
"clubs" (fig. 110).
[Illustration: FIG. 110.--The giant squid, _Ommatostrephes californica_.
(From specimen with body (exclusive of tentacles) four feet long, thrown
by waves on shore of the Bay of Monterey, California.)]
The Octopods have a short, sac-like, sub-spherical body and neither
external nor internal shell. To this group belong the famous
devil-fishes (_Octopus_), whose strange and terrifying appearance
combined with their frequently great size has furnished the basis for
many a weird tale of the sea. _Octopi_ have been killed having
tentacles more than 30 feet in length. The largest members of the
class, however, are probably the giant squids (belonging to the
Decapoda) specimens of which have been captured with a body-length of
twenty feet, and arms thirty-five feet long.
The beautiful paper sailor or argonaut (_Argonauta argo_), which
secretes a thin shell (not homologous with the shell of the other
molluscs) to protect her eggs, is a member of the Octopod group. In
fine weather the argonauts sail in fleets on the surface of the ocean.
The pearly nautilus (_Nautilus pompilius_) is a Cephalopod with four
gills instead of two, as with the Decapoda and Octopoda, and is the
only existing member of what was in the earlier times of the earth's
history a large group of animals. The nautili live in rather shallow
water usually creeping over the bottom feeding on small marine
animals. They make a many-chambered spiral shell with its inner
surface lined with beautiful pearly nacre.
CHAPTER XXIII
BRANCH CHORDATA: THE VERTEBRATES,
ASCIDIANS, ETC.
The branch Chordata includes all the backboned animals or vertebrates,
comprising the fishes, salamanders, frogs and toads, lizards,
crocodiles, turtles and snakes, birds, and all the quadrupeds or
mammals, and includes also a few small unfamiliar ocean animals which
do not look at all like the backboned animals, but which agree with
them in possessing a peculiar structure called the notochord. This
notochord consists of a series or cord of cells extending
longitudinally through the body from head to tail, above the
alimentary canal and below the spinal nerve-cord. In all the
vertebrates excepting a few low forms, the notochord while present in
the young, is replaced in the adult by a segmented bony or
cartilaginous axis, the spinal or vertebral column. But in the
ascidians or sea-squirts (called also tunicates) it persists
throughout life. In addition to this characteristic notochord, nearly
all the Chordata are marked by the presence, either in embryonic or
larval stages only, or else persisting throughout life, of a number of
slits or clefts in the walls of the pharynx which serve for breathing,
and which are called gill-slits.
=Structure of the vertebrates.=--As the backboned or vertebrate
animals make up almost the whole of the branch Chordata, and as the
few other chordates are animals the special structures of which we
shall not undertake to study in this book, we may note here some of
the other more obvious structural characteristics of the true
vertebrates. The possession of a backbone or bony (sometimes
cartilaginous) spinal column is the characteristic by which we
distinguish them from the invertebrate or backboneless animals.
Furthermore, all of the vertebrates possess an internal skeleton which
is in most cases composed of bone, and is firm and strong. In some of
the lower fishes, as the sharks and sturgeons, the skeleton is made up
of cartilage, tough but not hard. The vertebrate skeleton consists
typically of an axial portion comprising the spinal column and head,
and of two pairs of appendages or limbs, variously developed as fins,
wings, legs and arms. In some vertebrates these limbs are represented
by mere rudiments, and in the lowest fish-like forms, the lancelets
and lampreys, there is not the slightest trace of limbs. A part of the
central nervous system, the spinal cord, runs longitudinally through
the body on the dorsal side of the alimentary canal; the circulatory
system is closed, the blood being always confined in the heart and in
vessels called arteries, veins, and capillaries, and the blood is red
in color owing to the presence of numerous red corpuscles or
blood-cells. The nervous system is highly developed, with a large
brain in all the typical forms, and with complex and usually highly
efficient special sense-organs. Respiration is carried on by means of
external gills, or by internal lungs which communicate with the
outside through the mouth and nostrils. To the lungs and gills the
blood is brought to be "purified," i.e., to give up its carbonic-acid
gas and to take up oxygen.
=Classification.=--The Chordata are variously divided by zoologists
into eight or ten classes, of which (in the eight-class system) the
five classes[15] Pisces (fishes), Batrachia (batrachians), Reptilia
(reptiles), Aves (birds), and Mammalia (mammals), belong to the true
vertebrates. These classes will be considered in the five following
chapters.
The remaining three classes include a number of strange marine forms
which until recent years were considered as worms, but which are now
known to be the nearest living allies of the earliest or primitive
vertebrates. The relationship of these forms to early types is
manifest, not in the appearance or structure of the adult stage, but
only during embryonic or larval stages.
[Illustration: FIG. 111.--An ascidian or sea-squirt from the coast of
California. (After Jordan and Kellogg.)]
=The ascidians.=--The sea-squirts, or Ascidians, common on the seashore,
compose one class of these primitive chordate animals. They possess a
simple, sac-like body (fig. 111), fastened to the rocks by one end, the
other being provided with two openings, one for the ingress and the
other for the exit of water, a strong current of which flows constantly
through the body. By means of this current the ascidian obtains food.
Usually sea-squirts live together in large colonies, and in some cases a
number of individuals enclose themselves in a common gelatinous mass,
forming what is called a compound ascidian.
The ascidian when born is a tiny, free-swimming, tadpole-like creature
with a slender finned tail. It swims about freely for only a few
hours, however, soon attaching itself to a rock, and in its further
development becoming degenerate. It loses its tail and with it the
short notochord possessed by the larva; the eye and the auditory organ
are lost, and the nervous system and alimentary canal become much
reduced and simplified. Sea-squirts in their adult stage are very
simple degenerate animals, with low functional development, yet their
embryonic and larval conditions show a considerable degree of
structural specialization, and the presence of the notochord in these
early stages reveals their affinity with the backboned animals.
FOOTNOTE:
[15] The animals included by some zoologists in the single class
Pisces, are held by other zoologists to constitute three distinct
classes, thus making a subdivision of the branch into ten classes.
CHAPTER XXIV
BRANCH CHORDATA (_Continued_): CLASS PISCES
(THE FISHES)
THE GOLDEN SUNFISH OR PUMPKIN SEED (_Apomotis_ sp.)
TECHNICAL NOTE.--The species of sunfish named, or some closely
related species, can be obtained in any brook or stream in the
United States. _Gibbosus_ lives in all streams north of Dubuque,
Chicago, Pittsburg, and along the eastern coast north of
Charleston. Closely allied species live in all the other parts of
the country except in the higher Rocky Mountains west of Bismarck,
Pueblo, and Santa Fe. One species is found in the streams of
California, but none occurs in Washington or Oregon. In the few
places where a sunfish cannot be had, any species of bass or perch
may be used. Sunfish live in ponds and sluggish streams in deep
holes under a log or at the foot of a stump. They take eagerly a
hook baited with a worm, or they may be caught in nets. When
sunfish cannot be kept fresh for study in class, specimens may be
preserved in alcohol or 4% formalin. But if possible to keep some
alive for a time in a jar or tub with plenty of fresh water, the
colors of the living fish, together with its manner of swimming
and mode of breathing, can be observed.
=External structure=[16] (fig. 112).--Examine the general
configuration and make-up of the body. Note the deep, laterally
flattened _trunk_ and paddle-like _tail_. The _head_ is closely fitted
to the trunk without any neck. Note that the body is thickly covered
with firm, hard _scales_, arranged like the shingles on a roof. Remove
one of these scales and examine it under a hand lens. What sort of an
edge has it? Such a scale is said to be _ctenoid_.
The body of the sunfish terminates behind in the _caudal fin_, a
series of cartilaginous rays connected by thin skin and attached to a
bony plate at the end of the backbone. Along the median dorsal line
will be noted another fin composed anteriorly of spines and
posteriorly of soft rays jointed and branched. This is the _dorsal
fin_. How many spines has it? Anterior to the caudal fin on the
ventral surface is a median unpaired _anal fin_. How many spines has
it? Anterior to the anal fin are the _ventral fins_, while on the
sides of the body back of the head in a line with the mouth are found
the _pectoral fins_. The ventral fins, attached to a rudimentary
pelvis, correspond to the hind legs of the other vertebrates. The
pectoral fins, attached to the shoulder girdle, correspond to the
arms. In front of the anal fin note a small pit-like opening, the
opening from the kidneys and reproductive organs, and just anterior to
this a large aperture, the _anus_. At the anterior end of the head
note the broad _mouth_, surrounded by a complicated system of bones.
Note the large _eyes_ surrounded by a series of small bones, the
_orbital chain_. Just anterior to the eyes are two pairs of openings,
one pair of each side opening into a closed sac. What are these
openings? Note the presence of various bones on the side of the head,
each covered with a thin layer of skin. These are _membrane bones_,
characteristic of fishes. Are there any external ears in the fish?
Examine the inside of the mouth. Is there a _tongue_? If so, of what
character? Are there _teeth_? If so, where are they situated?
Note along each side extending to the base of the tail a line of
modified scales, on each scale a little mucous tube, the whole series
constituting the _lateral line_. These scales are intimately
associated with a large nerve (the _vagus_), and probably serve an
important part, not yet clearly understood, in the life of the fish.
Lift up the flap in front of one of the pectoral fins. This is the
_opercular flap_ which covers the gills that lie beneath. Bend this
forward and find four _gill-arches_, each with its double fringe of
_gills_. Note the _gill-rakers_, short and blunt, on the first
gill-arch. Note also on the under side of the flaps turned back,
delicate red gill-like structures covered by a membrane. These are the
_false gills_ or _pseudo-branchiæ_, larger in most fishes than in the
sunfish. The gills in the fish subserve the same function as the gills
of the crayfish, that of purifying the blood by eliminating
carbonic-acid gas from it and taking up oxygen from the air mixed with
or dissolved in the water. Organs subserving the same purposes in
different kinds of animals as, for example, the gills in fish and in
crayfish, are called _analogous structures_. But there is an important
morphological difference between the fish's gills and the gills of the
crayfish. In the latter animal they are outgrowths of the basal segments
of the walking-legs; in the fish they are outgrowths from the alimentary
canal. The internal gills of the young toad (tadpole) arise in the same
way as those of a fish. Structures which are identical in their origin,
like the gills of tadpole and fish, are called _homologous structures_.
Make a drawing of the sunfish from a lateral aspect, showing the
external parts named.
=Internal Structure.=--TECHNICAL NOTE.--Insert one point of the
scissors a little to one side of the anus and cut dorsally on the
left side of the body to the backbone. Now cut anteriorly from the
anus along the ventral wall to where the jaws unite, and cut, also
anteriorly, along the dorsal wall until the left side of the body
can be removed. Bend the opercular flap backward over the eye and
pin the entire fish, uncut side down, to the bottom of the
dissecting-pan, covering it with water.
The above operation will have severed the large powerful _muscles_
forming the body-wall and extending along the sides. Note a membranous
sac completely filling a large dorsal cavity. This is the
_swim-bladder_, a float filled with air which tends to give the fish
the same weight as the water it displaces. It arises as a diverticulum
from the alimentary canal, but soon becomes permanently shut off from
it. Beneath the swim-bladder is a large cavity filled with various
organs, collectively known as the _viscera_. In vertebrate animals the
cavity which contains the viscera is generally called the _peritoneal
cavity_. It is lined by the _peritoneum_, a delicate membrane, part of
which is deflected as the _mesentery_ over the alimentary canal and
the other organs, thus suspending them all from the dorsal wall. Note
in the anterior end of the peritoneal cavity a large bi-lobed gland,
the _liver_, red in fresh, yellowish in alcoholic specimens. Its
function, like that of the liver of the toad, is to store up nutriment
for the blood and to secrete a digestive fluid called _bile_. Behind
the liver note a long, convoluted tube. What is this tube? Unfold this
tube, separating it from its enveloping membrane, the mesentery.
Thrust a probe down the throat and note that it passes into a
thick-walled sac, the _stomach_. The mouth and gill-slits open into
the front part of the alimentary canal called the _pharynx_, which
leads by a short tube, the _œsophagus_, into the stomach. Note the
large, thickened portion of the alimentary canal leading from the
stomach. This is the _pylorus_, and to its walls are attached a number
of finger-like projections, the _pyloric cæca_. The pyloric cæca
secrete a fluid which is poured into the alimentary canal and which
assists in the process of digestion somewhat as does the secretion
from the pancreas of the toad. From the pylorus, passing backwards in
one or two loops, is the _small intestine_. Trace this to its exit.
Lying within the mesentery near the posterior end of the body-cavity
note a small red glandular mass, the _spleen_.
At the anterior end of the body in front of the liver and between the
sets of gills note the small _pericardial cavity_ within which is
contained the _heart_. The pericardial cavity is separated from the
peritoneal cavity by a thick muscular wall against which the liver
abuts. The heart consists of four parts. The posterior part is a
thin-walled reservoir, the _sinus venosus_, into which blood enters
through the _jugular vein_ from the head and through the _cardinal vein_
from the kidney. From the sinus venosus it passes forward into a large
chamber, the _auricle_. Next it flows into the _ventricle_, where, by
the contraction of the walls, rhythmical pulsations force it into the
_conus arteriosus_, thence into the _ventral aorta_, and lastly into the
gills, where it is purified. After passing through the capillaries in
the fine gill-filaments it is again collected, now pure, by paired
arteries from each pair of gills, which arteries unite to form the
_dorsal aorta_ extending backward just below the backbone to the end of
the tail. From the dorsal aorta a pair of arteries, the _subclavian_,
are given off to the pectoral fins. At this point two other arteries
branch off ventrally, the first being the _cardiac artery_, which
distributes blood to the stomach and pyloric cæca. The second divides
into several long _mesenteric arteries_ supplying blood to all parts of
the intestine and spleen. In the caudal region blood is taken up through
the _caudal vein_ and carried forward to the kidneys. These strain out
the impurities arising from waste of tissues, after which the blood is
carried back to the sinus venosus through the cardinal vein. From the
intestine it is gathered into the large _portal vein_ as in the toad.
The portal vein carries blood to the liver, where nutriment may be
stored up, and from thence it flows back to the sinus venosus through a
very short thin-walled vessel, the _hepatic sinus_.
The _kidneys_, more or less united in one mass, lie in the posterior
part of the body-cavity along the dorsal wall. Note running from each
side of the kidney a _ureter_ which unites with its fellow and opens
into a small _urinary bladder_ which discharges through a small
opening immediately back of the anus.
The reproductive organs lie below the swim-bladder near the posterior
end of the body-cavity. If the fish are caught in the spring, the
greater part of the body-cavity of the female is found to be filled with
small eggs. When mature, these eggs are deposited by the mother fish in
the gravel of the stream-bed where they are fertilized by the
sperm-cells poured over them by the male and left floating in the water.
The nervous system of fishes is best studied in a specimen treated
with nitric acid. Carefully remove the roof of the skull, thereby
exposing the brain. Most anteriorly make out, as in the toad, the
paired _olfactory lobes_. These are attached by long stalks to the
_cerebrum_ or _forebrain_, which is followed by two large hollow
lobes, the _midbrain_ or _optic lobes_. Behind the midbrain is the
_cerebellum_. Following the cerebellum is the elongate _medulla
oblongata_, which tapers backward into the _spinal cord_. How far
backward does the spinal cord extend? On each side of the brain-case
about opposite the cerebellum are located the _auditory organs_, each
consisting of three _semicircular canals_ which lie in different
planes, and of the _vestibule_. These parts are filled with liquid,
and suspended in the liquid in the vestibule are small calcareous
bodies called _otoliths_ or _ear-stones_. Running out beneath from the
midbrain are the _optic nerves_, which cross, the left one connected
with the right eye, the right one with the left eye. From each side of
the medulla oblongata there is given off a large nerve, the _vagus_,
which sends branches to the lateral line organs on either side, and
extends backward to the stomach and viscera.
For further study of the nervous system see Parker's "Zootomy," pp.
122-130.
Make a drawing of the nervous system as worked out.
TECHNICAL NOTE.--To make a good skeleton immerse a fresh or
preserved specimen for some time in a hot soap solution. When the
muscles have commenced to soften remove the body from the
solution, pick the flesh away, and leave to dry.
Note that the main axis of the _skeleton_ is composed of _vertebræ_
placed end to end. How many vertebræ are there? What vertebræ bear
_ribs_? The ribless ones beyond the body-cavity are called _caudal
vertebræ_. Note the _interspinal bones_ which support the fins, with
large muscles on either side to control their action. Note that the
group of bones supporting the pectoral fin is attached to the back of
the brain-case and makes up the _shoulder girdle_. The ventral fins
are attached to a rudimentary _pelvic girdle_, attached in front to
the shoulder girdle, as the shoulder girdle is in turn attached to the
skull. It will be seen that the sunfish has no neck and we may say,
also, no back. Its skeleton consists only of a tail attached to the
skull. The brain-case is made up of a number of bones closely joined
together. From it is suspended the lower jaw, which comprises a number
of bones but loosely attached to each other. Overlying these is the
system of membrane bones already mentioned, including the opercle or
gill-cover.
For a detailed study of the fish-skeleton see Parker's "Zootomy," pp.
86-101, or Parker and Haswell's "Zoology," vol. II. pp. 183-195.
=Life-history and habits.=--The sunfish or "pumpkin-seed" lives in
quiet corners of the brooks and rivers, preferably under a log or at
the root of an old stump. It is a beautiful fish, shining "like a
coin fresh from the mint." Its body is mottled golden, orange and
blue, with metallic lustre, darker above, pale or yellowish below. Its
fins are of the same color. The tip of its opercle is prolonged like
an ear and jet black in color, with a dash of bright scarlet along its
lower edge. Nearly all the thirty species of sunfish found in the
United States have this black ear, but some have it long, some short,
and in some it is trimmed with yellow or blue instead of scarlet.
The sunfish lays its eggs in the spring in a rude nest it scoops in the
gravel, over which it stands guard with its bright fins spread, looking
as big and dangerous as possible. When thus employed it takes the hook
savagely, perhaps regarding the worm as a dangerous enemy. The young
fishes soon hatch, looking very much like their parents, although more
transparent and not so brightly colored. They grow rapidly, feeding on
insects and other small creatures, and reach their growth in two or
three years. They do not wander far and never willingly migrate.
Students should verify this account on the different species. A more
exact study of the nests of the different species and the fishes'
defence of them would be a valuable addition to our knowledge. The most
striking traits of the habits of this fish are its vivacity and courage;
it reveals its great muscular strength when captured. The sexes are
similar in appearance and both defend the nest alike.
OTHER FISHES.
[Illustration: FIG. 112.--Dissection of the sunfish, _Apomotis_ sp.]
Fishes constitute the largest class of vertebrate animals and are to
be found everywhere in ponds, streams, or ocean. About 15,000 species
of fish are known, of which 3,000 live in North America. The largest
of all fishes is the basking shark (_Cetorhinus_), which reaches a
length of thirty-six feet. The smallest is the dwarf goby
(_Mistichthys_), less than half an inch long, found in Luzon, one of
the Philippine Islands. Between these extremes is every variety in
size, form, and relative proportions. The body, for example, may be
greatly elongated and almost cylindrical as in the eels; or long and
flattened from side to side as in the ribbon-fishes; or the head may
be very large, wider and higher than the rest of the body as in the
anglers, or may have a great beak as in the sword-fish.
=Body form and structure.=--When we consider the fish as a whole, we
find first a body formed for progression in the water, the typical fish
being pointed at each end (the shorter point in front), and having the
sides flattened, the back and belly rather narrow, and the motive power
located in the fin on the tail. From this typical form diverge all
conceivable variations, adaptations to every sort of fish life.
Most fishes have the body covered with scales, although many have the
skin naked or covered with small scales so hidden in the skin as to be
hardly visible. The scales are small horny or bony plates which fit
into small pockets or folds of the skin, and are usually arranged
shingle-fashion, overlapping each other. They are of various shapes,
mostly classified as of three kinds, namely, squarish enamelled scales
called ganoid, roundish smooth-edged called cycloid, and roundish
tooth-edged called ctenoid.
The skeleton of the fish is relatively complex. Its bones are
comparatively soft, having little lime in them, indeed in many cases
they are mere cartilage. The vertebral column is made of twenty-four
vertebræ in the typical fishes, the number in the others being variously
increased, or sometimes diminished. These vertebræ are of two classes,
abdominal or body, and caudal or tail vertebræ. The former have a neural
arch which encloses the spinal cord and from which projects a spine.
Below, the processes spread apart, surrounding the kidneys and partly
enclosing the air-bladder. To these processes ribs are loosely attached.
The caudal vertebræ have no ribs and leave no room below for viscera.
Their lower arch (hæmal), similar to the dorsal (neural) arch, surrounds
a blood-vessel. The fins of a fish are composed of bony rods or rays
joined by membrane. Some of these rays may be unbranched and unjointed,
being then known as spines, and usually occupy the front part of the
fin. Other rays are made up of little joints and are usually branched
toward their tip. Such ones are called soft rays. Soft rays make up the
greatest part of most fins. The vertical fins are on the middle line of
the body. These are the dorsal above, anal below, and caudal forming the
end of the tail. The paired pectoral and ventral fins are ranged one on
each side corresponding to the arms and legs of higher animals. The
pectoral fin or arm is fastened to a series of bones called the shoulder
girdle. These bones do not correspond to those in the shoulder girdle of
the higher animals, and the various parts in the two structures are
differently named. The uppermost bone of the shoulder girdle is usually
attached to the skull. To the lowermost is attached the rudimentary
pelvis, which supports the hinder limb or ventral fin. Usually the
pelvis is farther back and loose in the flesh, but sometimes it is
placed far forward, being occasionally attached at the chin.
The head contains the various bones of the cranium, usually closely
wedged together and not easily distinguished. The jaws are each made
of several pieces; the lower one is suspended from the skull by a
chain of three flat bones. The jaws may bear any one of a great
variety of forms of teeth or no teeth at all, and any of the bones of
the mouth-cavity and throat may have teeth as well. On the outside of
the head are numerous bones called membrane bones, because they are
made up of ossified membrane. The most important of these is the
opercle or gill-cover. Within are the tongue with the five gill-arches
attached to it below and to the floor of the skull above, the last
arch being usually modified to form the pharyngeal jaw.
The stomach may be a blind sac with entrance and exit close together,
or it may have the form of a tube or siphon. At its end are often
found the large glandular tubes called pyloric cæca which secrete a
digestive fluid; and to its right side is attached the red spleen. The
liver is large, having usually, but not always, a gall-bladder; it
pours its secretion into the upper intestine. In fishes which feed on
plants the intestine is long, but it is short in those which eat
flesh, because flesh is digested in the stomach, not in the
intestines. The kidney is usually a long slender forked gland showing
little variation. The egg-glands differ greatly in different sorts of
fishes, the size and number of eggs varying equally. The air-bladder
is a lung which has lost both lung structure and respiratory function,
being simply a sac filled with gas secreted from the blood, and lying
in the upper part of the abdominal cavity. It is subject to many
variations. In the gar pike, bow-fin and the lung-fishes of the
tropics, the air-bladder is a true lung used for breathing and
connected by a sort of glottis with the œsophagus. In others it is
rudimentary or even wholly wanting, while in still others its function
as an air-sac is especially pronounced, and in many it is joined
through the modified bones of the neck to the organ of hearing.
The blood of the fish is purified by circulation through its gills.
These are a series of slender filaments attached to bony arches. Among
them the blood flows in and out, coming in contact with the water
which the fish takes in through its mouth and which passes across the
gills to be expelled through the gill-openings. The blood is received
from the body into the first chamber of the heart, a muscular sac
called the auricle. From here it passes into the ventricle, a chamber
with thicker walls, the contraction of which sends it to the gills,
thence without return to the heart it passes over the body. The
circulation of blood in fishes is slow, and the blood, which receives
relatively little oxygen, is cold, being but little warmer than the
water in which the individual fish lives.
Inside the cranium or brain-case is the brain, small and composed of
ganglia which are smooth at the surface and contain little gray
matter. At the posterior end of the brain is the thickened end of the
spinal cord, called the medulla oblongata. Next overlapping this is
the cerebellum, always single. Before this lie the largest pair of
ganglia, the optic lobes or midbrain, round, smooth, and hollow. From
the under side of these, nerves run to the eyes with or without a
chiasma or crossing. In front of the optic lobes and smaller than them
is the cerebrum or forebrain, usually of two ganglia but sometimes (in
the sharks) united into one. In front of these are the small olfactory
lobes which send nerves to the nostrils.
The sense organs are well developed. The sense of touch has in some
fishes special organs for its better effectiveness. For instance
certain fin-rays in some fishes, or, as in the catfish, slender,
fleshy, whip-like processes on the head, are developed as feelers or
special tactile organs. Other fishes, the sucker and loach for
example, have specially sensitive lips and noses with which they
explore their surroundings. The sense of taste does not seem to be
well developed in this group. Taste-papillæ are often present in small
numbers on the tongue or on the palate. The sense of smell is good.
The olfactory organs, one on each side of the head, are hollow
sac-like depressions, closed at the rear. In most cases each sac has
two openings or nostrils. The sense of hearing is not very keen. The
ears are fluid-filled sacs buried in the skull, and without external
or (except in a few cases) internal opening. Fishes are far more
sensitive to sudden jars or sudden movements than to any sound. They
possess what is generally believed to be a special sense organ not
found in other animals. This is the lateral line which extends along
the sides of the body and which consists of a series of modified
scales (each one with a mucous channel) richly supplied with nerves.
The eyes are usually large and conspicuous. They differ mainly from
the eyes of other vertebrates in their myopic spherical crystalline
lens, made necessary by the density of the medium in which fishes
live. There are usually no eyelids, the skin of the body being
continuous but transparent over the eyes. Being near-sighted, fishes
do not discriminate readily among forms, their special senses fitting
them in general to distinguish motions of their enemies or prey rather
than to ascertain exactly the nature of particular things.
The colors of fishes are in general appearance protective. Thus most
individuals are white on the belly, mimicking the color of the sky to
the enemy which pursues them from below. Seen from above most of them
are greenish, like the water, or brownish gray and mottled, like the
bottom. Those that live on sand are sand-colored, those on lava black,
and those among rose-red sea-weeds bright red. In many cases,
especially among kinds that are protected by their activity, brilliant
colors and showy markings are developed. This is especially true among
fishes of the coral reefs, though species scarcely less brilliant are
found among the darters of our American brooks.
Among fresh-water fishes bright colors, crimson, scarlet, blue, creamy
white, are developed in the breeding season, the then vigorous males
being the most highly colored. Many of the feeble minnows even become
very brilliant in the nuptial season of May and June. Color in fishes is
formed by minute oil-sacs on the scales, and it often changes quickly
with changes in the nervous condition of the individuals.
=Development and life-history.=--The breeding habits of fishes are
extremely varied. Most fishes do not pair, but in some cases pairing
takes place as among higher animals. Ordinarily fishes lay their eggs
on the bottom in shallow water, either in brooks, lakes, or in the
sea. The eggs of fishes are commonly called spawn, and egg-laying is
referred to as spawning. The spawn of some fishes is esteemed a
special food delicacy. Spring is the usual time of spawning, though
some fishes spawn in summer and some even in winter; generally they
move from their usual haunts for the purpose. The eggs of the
different species vary much in size, ranging from an inch and a half
in diameter (barn-door skate) down to the tiniest dots, like those of
the herring. The number of eggs laid also varies greatly. The trout
lays from 500 to 1,000, the salmon about 10,000, the herring 30,000 to
40,000, and some species of river fish 500,000, while certain
flounders, sturgeons, and others each lay several millions of eggs.
The adults rarely pay any attention to the eggs, which are hatched
directly by the heat of the sun or by heat absorbed from the water.
The length of incubation varies much. When the young fish leaves the
egg-shell it carries, in the case of most species, a part of the yolk
still hanging to its body. Its eyes are very large, and its fins are
represented by thin strips of membrane. It usually undergoes no great
changes in development from the first, resembling the adult except in
size. But some of the ocean fishes show a metamorphosis almost as
striking as that of insects or toads or frogs.
Some fishes build nests. Sticklebacks build elaborate nests in the
brooks and defend them with spirit. Sunfishes do the same, but the
nests are clumsier and not so well cared for.
The salmon is the type of fishes which run up from the sea to lay
their eggs in fresh water. The king salmon of the Columbia River, for
example, leaves the sea in the high waters of March and ascends
without feeding for over a thousand miles, depositing its spawn in
some small brook in the fall. After making this long journey to lay
the eggs, the salmon become much exhausted, battered and worn, and are
often attacked by parasitic fungi. They soon die, probably none of
them ever surviving to lay eggs a second time.
=Classification.=--A fish is an aquatic vertebrate, fitted to breathe
the air contained in water, and never developing fingers and toes.
Accepting this broad general definition we find at once that there are
very great differences among fishes. Some differ more from others than
the ordinary forms differ from rabbits or birds. So although we have
entitled this chapter as if all fishes belonged to the class Pisces,
we cannot arrange them satisfactorily in less than three classes.
=The lancelets (Leptocardii).=--The lowest class of fish-like animals
is that of the lancelets, the Leptocardii. These little creatures,
translucent, buried in the sand, of the size and form of a small
toothpick, are fishes reduced to their lowest terms. They have the
form, life, and ways of a fish, but no differentiated skull, brain,
heart, or eyes. Moreover they have no limbs, no jaws, no teeth, no
scales. The few parts they do have are arranged as in a fish, and they
show something in common with the fish embryo. Lacking a distinct
head, the lancelets are put by some zoologists in a group called the
Acrania, as opposed to the Craniata, which includes all the other
vertebrates. Lancelets have been found in the North Atlantic and
Mediterranean, on the west coast of North America, on the east coast
of South America and on the coasts of Japan, Australia, New Zealand,
the East Indies and Malayan Islands. The best-known members of the
group belong to the genus _Amphioxus_. There are but one to two other
genera in the class.
[Illustration: FIG. 113.--A lamprey, _Petromyzon marinus_. (After
Goode.)]
=The lampreys and hag-fishes (Cyclostomata).=--The next class of
fish-like animals is that of the lampreys (fig. 113) and hag-fishes,
the Cyclostomata. The lampreys and hags are easily distinguished from
the true fishes by their sucking mouth without jaws, their single
median nostril, their eel-like shape and lack of lateral appendages or
paired fins. The hag-fishes (_Myxine_), which are marine, attach
themselves by means of a sucker-like mouth to living fishes (the cod
particularly), gradually scraping and eating their way into the
abdominal cavity of the fish. These hags or "borers" "approach most
nearly to the condition of an internal parasite of any vertebrate."
The lampreys, or lamprey-eels as they are often called because of
their superficial resemblance to true eels, are both marine and
fresh-water in their habitat, and most of them attach themselves to
live fishes and suck their blood. They also feed on crustacea,
insects, and worms. The brook-lamprey, _Lampetra wilderi_, is never
parasitic. It reaches its full size in larval life and transforms
simply for spawning. The sea- and lake-lampreys ascend small
fresh-water streams when ready to lay their eggs, few living to
return. Sometimes small piles of stones are made for nests. The young
undergo a considerable metamorphosis in their development. The largest
sea-lampreys reach a length of three feet. The common brook-lampreys
are from eight to twelve inches long only.
=The true fishes (Pisces).=--All the other fish-like animals are
grouped in the class Pisces. They are characterized, when compared
with the lower fish-like forms just referred to, by the presence of
jaws, shoulder girdle, and pelvic girdle. The class includes both the
cartilaginous and bony fishes, and is divided into three sub-classes,
namely, the Elasmobranchii, including the sharks, rays, skates,
torpedoes, etc., the Holocephali, including the chimæras (a few
strange-bodied forms), and the Teleostomi, including all the other
fishes, as the trout, catfishes, darters, bass, herring, cod,
mackerel, sturgeons, etc., etc.
=The sharks, skates, etc. (Elasmobranchii).=--The sharks and skates
are characterized by the possession of a skeleton composed of
cartilage and not bone, as in the bony fishes; they have no operculum;
their teeth are distinct, often large and highly specialized, and
their eggs are few and very large. There are two principal groups
among Elasmobranchii, viz., the sharks, which usually have an elongate
body, and always have the gill-openings on the sides, and the rays or
skates, which have a broad flattened body with the gill-openings
always on the under side. All the members of both groups are marine.
The sharks are active, fierce, usually large fishes, which live in the
surface-waters of the ocean and make war on other marine animals, all
of the species except half a dozen being fish-eaters. The shark's
mouth is on the under side of the usually conical head, and the
animal often turns over on its back in order to seize its prey. The
largest American sharks, and the largest of all fishes, are the great
basking-sharks (_Cetorhinus_), which reach a length of nearly forty
feet. They get their name from their habit of gathering in numbers and
floating motionless on the surface. They feed chiefly on fishes.
The hammer-headed sharks (_Sphyrna_) are odd sharks which have the
head mallet or kidney shaped, twice as wide as long, the eyes being
situated on the ends of the lateral expansions of the head. The
man-eating or great white sharks (_Carcharodon_) are nearly as large
as the basking-sharks, and are extremely voracious. They will follow
ships for long distances for the refuse thrown overboard. They do not
hesitate to attack man. Among the more familiar smaller sharks are the
dog-fishes and sand-sharks of our Atlantic coast.
The rays and skates are also carnivorous, but are with few exceptions
sluggish, lying at the bottom of shallow shore-waters. They feed on
crabs, molluscs, and bottom-fishes. The small common skates,
"tobacco-boxes" (_Raja erinacea_) (fig. 114), about twenty inches long,
and the larger "barn-door skates" (_R. lævis_), are numerous along the
Atlantic coast from Virginia northward. Especially interesting members
of this group, because of the peculiar character of the injuries
produced by them, are the sting-rays and torpedoes or electric-rays. The
sting-rays (_Dasyatis_) have spines near the base of the tail which
cause very painful wounds. The torpedoes (_Narcine_) have two large
electrical organs, one on each side of the body just behind the head,
with which they can give a strong electric shock. "The discharge from a
large individual is sufficient to temporarily disable a man, and were
these animals at all numerous they would prove dangerous to bathers."
Very different from the typical rays in external appearance are the
saw-fishes (_Pristis pectinatis_) which belong to this group. The body
is elongate and shark-like, and has a long saw-like snout. This saw,
which in large individuals may reach a length of six feet and a breadth
of twelve inches, makes its owner formidable among the small sardines
and herring-like fishes on which it feeds. The saw-fishes live in
tropical rivers, descending to the sea.
[Illustration: FIG. 114.--The common skate, _Raja erinacea_. (From
Kingsley.)]
=The bony fishes (Teleostomi).=--The bony or true fishes are
distinguished from the lampreys and sharks and rays by having in
general the skeleton bony, not cartilaginous, the skull provided with
membrane bones, and the eggs small and many. In this group are
included all the fishes of our fresh-water lakes, ponds, and streams
as well as most of the marine forms. Fish life, being spent under
water, is not familiar to most of us, and beginning students are
rarely helped enough in getting acquainted with the different kinds
and the interesting habits of fishes. But they offer a field of study
which is really of unusual interest and profit. We can refer in the
following paragraphs to but few of the numerous common and readily
found kinds, and to these but briefly.
Closely related to the sunfish, studied as example of the bony fishes,
are the various kinds of bass, as the "crappie" (_Pomoxis annularis_),
the calico bass (_P. separoides_), the rock-bass (_Ambloplites
rupestris_) and the large-mouthed and small-mouthed black bass
(_Micropterus salmoides_ and _M. dolomieu_ respectively). All the
members of this sunfish and bass family are carnivorous fishes
especially characteristic of the Mississippi valley.
Another family of many species especially common in the clear, swift,
and strong Eastern rivers is that of the darters and perches. The
darters are little slender-bodied fishes which lie motionless on the
bottom, moving like a flash when disturbed and slipping under stones
out of sight of their enemies. Some are most brilliantly colored,
surpassing in this respect all other fresh-water fishes.
Unlike the sunfishes and darters are the catfishes, composing a great
family, the Siluridæ. The catfish (_Ameiurus_) gets its name from the
long feelers about its mouth; from these feelers also come its other
names of horned pout, or bull-head. It has no scales, but its spines are
sharp and often barbed or jagged and capable of making a severe wound.
Remotely allied to the catfish are the suckers, minnows, and chubs,
with smooth scales, soft fins and soft bodies and the flesh full of
small bones. These little fish are very numerous in species, some
kinds swarming in all fresh water in America, Europe, and Asia. They
usually swim in the open water, the prey of every carnivorous fish,
making up by their fecundity and their insignificance for their lack
of defensive armature. In some species the male is adorned in the
spring with bright pigment, red, black, blue, or milk-white. In some
cases, too, it has bony warts or horns on its head or body. Such forms
are known to the boys as horned dace.
Most interesting to the angler are the fishes of the salmon and trout
(fig. 115) family, because they are gamy, beautiful, excellent as food
and above all perhaps because they live in the swiftest and clearest
waters in the most charming forests. The salmon live in the ocean most
of their life, but ascend the rivers from the sea to deposit their
eggs. The king salmon (_Oncorhynchus tschawytscha_) of the Columbia
goes up the great river more than a thousand miles, taking the whole
summer for it, and never feeding while in fresh water. Besides the
different kinds of salmon, the black-spotted or true trout, the charr
or red-spotted trout of various species, the whitefish (_Coregonus_),
the grayling (_Thymallus signifer_) and the famous ayu of Japan belong
to this family.
[Illustration: FIG. 115.--The rainbow-trout, _Salmo iridens_. (From
specimen.)]
In the sea are multitudes of fish forms arranged in many families. The
myriad species of eels agree in having no ventral fins and in having
the long flexible body of the snake. Most of them live in the sea, but
the single genus (_Anguilla_) or true eel which ascends the rivers is
exceedingly abundant and widely distributed. Most eels are extremely
voracious, but some of them have mouths that would barely admit a
pin-head. The codfish (_Gadus callarias_) is a creature of little
beauty but of great usefulness, swarming in all arctic and subarctic
seas. The herring (_Clupea harengus_), soft and weak in body, are more
numerous in individuals than any other fishes. The flounders (fig.
116) of many kinds lie flat on the sea-bottom. They have the head so
twisted that the two eyes occur both together on the uppermost side.
The members of the great mackerel tribe swim in the open sea, often in
great schools. Largest and swiftest of these is the sword-fish
(_Xiphias gladius_), in which the whole upper jaw is grown together to
form a long bony sword, a weapon of offence that can pierce the wooden
bottom of a boat.
[Illustration: FIG. 116.--The winter flounder, _Pseudopleuronectes
americanus_. (After Goode.)]
Many of the ocean fishes are of strange form and appearance. The
sea-horses (_Hippocampus_ sp.) (fig. 117) are odd fishes covered with
a bony shell and with the head having the physiognomy of that of a
horse. They are little fishes rarely a foot long, and cling by their
curved tails to floating seaweed. The pipefish (_Syngnathus fuscum_)
is a sea-horse straightened out. The porcupine-fishes and swellfishes
(_Tetraodontidæ_) have the power of filling the stomach with air which
they gulp from the surface. They then escape from their pursuers by
floating as a round spiny ball on the surface. The flying-fishes
(_Exocœtus_) leap out of the water and sail for long distances through
the air, like grasshoppers. They cannot flap their long pectoral fins
and do not truly fly; nevertheless they move swiftly through the air
and thus escape their pursuers. In its structure a flying-fish differs
little from a pike or other ordinary fish.
[Illustration: FIG. 117.--A sea-horse, _Hippocampus heptagonus_.
(After Goode.)]
For an account of the fishes of North America see Jordan's "Manual of
Vertebrates," eighth edition, pp. 5-173, and Jordan and Evermann's
"Fishes of North and Middle America," where the 3,127 species known
from our continent are described in detail with illustrative figures.
=Habits and adaptations.=--The chief part of a fish's life is devoted
to eating, and as most fishes feed on other fishes, all are equally
considerably occupied in providing for their own escape.
In general the provisions for seizing prey are confined to sharp teeth
and the strong muscles which propel the caudal fin. But in some cases
special contrivances appear. In one large group known collectively as
the "anglers" the first spine of the dorsal fin hangs over the mouth.
It has at its tip a fleshy appendage which serves as a bait. Little
fishes nibble at this, the mouth opens, and they are gone. In the deep
seas, many fishes are provided with phosphorescent spots or lanterns
which light up the dark waters, and enable them to see their prey. In
storms these lantern-fishes sometimes lose their bearings and are
thrown upward to the surface.
In general the more predatory in its habits any fish is the sharper
its teeth, and the broader its mouth. Among brook-fishes the pickerel
has the largest mouth and the sharpest teeth. It has been called a
"mere machine for the assimilation of other organisms." The trout has
a large mouth and sharp teeth. It is a swift, voracious, and predatory
fish, feeding even on its own kind. The sunfish is less greedy and its
mouth and teeth are smaller, though it too eats other fish.
As means of escape, most fishes depend on their speed in swimming. But
some hide among rocks and weeds, disguising themselves by a change in
color to match their surroundings. Others, like the flounders and
skates, lie flat on the bottom. Still others retreat to the shallows
or the depths or the rock-pools or to any place safer than the open
sea. Some are protected by spines which they erect when attacked. Some
erect these spines only after they have been swallowed, tearing the
stomach of their enemy and killing it, but too late to save
themselves. Again in some species the spines are armed with poison
which benumbs the enemy. Sometimes an electric battery about the head
or on the sides gives the biting fish a severe shock and drives him
away. Such batteries are found in the electric rays or torpedo, in the
electric eel of Paraguay, the electric catfish of the Nile, the
electric stargazer and other fishes.
Some fishes are protected by their poor and bitter flesh. Some have
bony coats of mail and sometimes the coat of mail is covered with
thorns, as in the porcupine-fish. This fish and various of its
relatives have the habit of filling the stomach with air when
disturbed, then floating belly upward, the thorny back only within
reach of its enemies.
[Illustration: FIG. 118.--The remora, or cling fish, _Remoropsis
brachyptera_. Note sucker on top of head. (After Goode.)]
Many species (cling fishes) attach themselves to the rocks by a fleshy
sucking-disk. Some (_Remora_) (fig. 118) cling to larger fishes by a
strange sucking-disk on the head, a transformed dorsal fin, being thus
shielded from the attacks of fish smaller than their protectors. Some
small fishes seek the shelter of the floating jellyfishes, lurking
among their poisoned tentacles. Others creep into the masses of
floating gulf-weed. Some creep into the shell of clams and snails. In
the open channel of a sponge, the mouth of a tunicate and in similar
cavities of various animals, little fishes may be found. A few fishes
(hag-fishes) are parasitic on others, boring their way into the body
and devouring the muscles with their rasp-like teeth.
Some fishes are provided with peculiar modifications of the gills
which enable them to breathe for a time out of water. Such fish have
the pectoral fins modified for a rather poor kind of locomotion on
land, thus enabling them to move from pond to pond or from stream to
stream. In cold climates the fishes must either migrate to warmer
latitudes in winter, as some do, or withstand variously the cold,
often freezing weather. Some fish can be frozen solid, and yet thaw
out and resume active living. Some lie at the bottoms of deep pools
through the colder periods, while many others, such as the minnows,
chubs, and other kinds common in small streams, bury themselves in the
mud, and lie dormant or asleep through the whole winter. On the other
hand in countries where the long intense rainless summers dry up the
pools, some fishes have the habit of burying themselves in the mud,
which, with slime from the body, forms about them a sort of tight
cement ball in which they lie dormant until the rains come. "Thus a
lung-fish (called _Protopterus_), found in Asia and Africa, so
completely slimes a ball of mud around it that it may live for more
than one season, perhaps many; it has been dug up and sent to England,
still enclosed in its round mud-case, and when it was placed in warm
water it awoke as well as ever."
=Food-fishes and fish-hatcheries.=--Most fishes are suitable for food,
though not all. Some are too small to be worth catching or too bony to
be worth eating. Some of the larger ones, especially the sharks, are
tough and rank. A few are bitter and in the tropics a number of
species feed on poisonous coelenterates about the coral reefs,
becoming themselves poisonous in turn. But a fish is rarely poisonous
or unwholesome unless it takes poisonous food. Where fishes of a kind
specially used for food gather in great numbers at certain seasons of
the year, fishing is carried on extensively and with an elaborate
equipment. Such fisheries, some of which have been long known, are
scattered all over the world. Along the shores of the Mediterranean
Sea, and on the coasts of Norway, France, the British Isles and Japan
are numerous great fishing-places. But "nowhere are there found such
large fisheries as those along the northern Atlantic coasts of our own
continent, extending from Massachusetts to Labrador. Especially on the
banks of Newfoundland are codfish, herring, and mackerel caught."
Among our fresh-water fisheries the great salmon fisheries of the
Penobscot and Columbia rivers and of the Karluk and other rivers of
Alaska are the best known. The whitefish of our Great Lakes is also
one of the important food-fishes of the world.
In many places fishes are raised in so-called hatcheries, not usually
for immediate consumption but for the purpose of stocking ponds and
streams either in the neighborhood of the hatchery or in distant
waters which the special species cultivated has not been able
naturally to reach. The eggs of some fishes are large and
non-adherent, two features which greatly favor artificial impregnation
and hatching. In the hatcheries the eggs are put first into warm
water, where development begins; they are then removed into cool
water, which arrests development without injury, making shipment
possible. The eggs of salmon and trout in particular can be sent long
distances to suitable streams or ponds. The eggs of the shad have been
thus carried from the East to the streams of California and trout have
been distributed to many streams in our country which by themselves
they could never have reached.
The salmon is a conspicuous example of those fishes which can be
artificially propagated. The eggs of the salmon are large, firm, and
separate from each other. If the female fish be caught when the eggs
are ripe and her body be pressed over a pan of water the eggs will
flow out into the water. By a similar process the milt or male
sperm-cells can be procured and poured over the eggs to fertilize
them. The young after hatching are kept for a few days or weeks in
artificial pools, till the yolk-sacs are absorbed and they can take
care of themselves. They are then turned into the stream, where they
drift tail foremost with the current and pass downward to the sea. All
trout may be treated in similar fashion, but there are many
food-fishes which cannot be handled in this way. In some the eggs are
small or soft, or viscid and adhering in bunches. In others the
life-habits make artificial fertilization impossible. Such species are
artificially reared only by catching the young and taking them from
one stream to another. To this type belong the black bass, the
sunfish, the catfish and other familiar forms.
FOOTNOTE:
[16] The author wishes to call the attention of teacher and student to
the plan (referred to in the Preface, page v) adopted in writing the
directions for the dissections. The sequence of the references to the
various organs depends on the actual course of the dissection, and not
upon the association of organs in systems. And the directions are so
much condensed that they are hardly more than a means of orienting the
student, leaving him to work out independently, or by the aid of more
detailed accounts (sometimes specifically referred to), the details of
the dissection.
CHAPTER XXV
BRANCH CHORDATA (_Continued_). CLASS BATRACHIA:
THE BATRACHIANS
The structure, life-history, and habits of the garden-toad (_Bufo
lentiginosus_) have already been studied (see Chapter II and Chapter
XII).
OTHER BATRACHIANS.
The class Batrachia includes the animals familiarly known as cœcilians,
sirens, mud-puppies, salamanders, toads, and frogs. Although differing
plainly from fishes in appearance and habits, the batrachians are really
closely related to them, resembling them in all but a few essential
characters. Among the distinctive characters of batrachians may be noted
the absence of fins supported by fin-rays, the presence usually of
well-developed legs for walking or leaping, and the absence or reduction
of certain bones of the head connected with the gills and lower jaw and
which are well developed in the fishes. The batrachians stand in
somewhat intermediate position between the fishes and the reptiles,
showing some of the characters of both. They are, like fishes and
reptiles, cold-blooded. In their adult condition some are terrestrial
and some aquatic as to habitat, but all have an aquatic larval life. The
water-inhabiting young breathe at first by means of gills, later lungs
begin to develop, and for a time both gills and lungs are used in
respiration. Finally in the adult condition in almost all of the forms
the gills are wholly lost and breathing is done by the lungs and skin
solely. Correlated with the change of habits from larval to adult stage
there is usually a well-marked metamorphosis in post-embryonic
development. This metamorphosis is specially striking among the frogs
and toads. None of the aquatic forms is marine, salt water always
killing eggs, larvæ or adults. Batrachians are found all over the world,
although there are few in the extreme North. They are most abundant in
warm and tropical lands.
[Illustration: FIG. 119.--The tiger salamander. (From Jenkins and
Kellogg.)]
=Body form and organization.=--The body varies from a long and
slender, truly snake-like form as in the tropical cœcilians through
the usual salamander (fig. 119) shape, where it is more robust but
still elongate and tailed, to the heavy, squat, tailless condition of
the toads. Legs, with five digits, are usually present, and are used
for swimming, walking, or leaping. The legs are longest and best
developed in the short tailless frog and toad forms which are mostly
terrestrial, and are short and weak in the tailed salamander forms,
many of which are aquatic. The skin is almost always naked, showing a
marked difference from the scaled condition of reptiles and most of
the fishes, and its cells secrete a slimy, sticky, usually whitish
fluid, which in some cases is irritating, or even poisonous. The skin
is sometimes thrown up into folds or ridges, and in some species is
elevated to form a kind of fin on the tail or back. This unpaired fin
differs from the dorsal fin (and other fins) of fishes in not being
supported by rayed processes of the skeleton. There are in some
batrachians traces of an exoskeleton in the presence of scale-like
structures in the skin or in the horny nails on the digits, but these
cases are rare. The skin contains pigment-cells and many of the
batrachians are brilliantly colored and patterned; some of the pigment
is carried by special contractile or expansile cells, the
chromatophores (see account of chromatophores of the Cephalopoda, p.
256), so that the animal can change its tint and markings more or less
rapidly. All the batrachians possess external gills in their aquatic
larval stage, and in a few forms, as the sirens and mud-puppies, gills
are retained all through life. These gills are branched folds of the
skin abundantly supplied with blood-vessels.
In the organization of the batrachian body the usual vertebrate
characters appear, the body-organs being arranged with reference to a
supporting and protecting internal bony skeleton. The head is plainly
set off from the rest of the body and bears the mouth and the organs of
hearing and sight. Certain so-called lateral sense organs, the function
of which is not exactly known, occur arranged in three lines on each
side of the body of some of the forms. Both pairs of limbs are present
and functional in almost all of the species. In the cœcilians the limbs
are wholly wanting; in the sirens only the fore legs are present.
=Structure.=--The most obvious skeletal differences among batrachians
are those due to variations in external form. While there are as many
as 100 vertebræ in some of the elongate long-tailed salamanders (even
250 in the strange snake-like cœcilians), there are but 10 (the last
or tenth being the rod-shaped bone called the urostyle) in the short,
tailless frogs and toads. To any of the vertebræ except the first (the
single cervical vertebra) and the last, ribs may be attached and the
cœcilians have about as many pairs of ribs as vertebræ. In the frogs
and toads, however, the ribs are lost. In any case they are never
fastened by their lower ends to the breast-bone.
The alimentary canal is usually not much longer than the body and is
plainly divided into mouth, pharynx, œsophagus, small intestine, large
intestine or rectum, and anal opening. The teeth when present occur on
both the jaws and the palate. They are small, sharp, point backward
and are fused to the bones. They are wholly wanting in the toad and in
some other allied forms. The tongue may be wanting, or may be
immovably fixed to the floor of the mouth, or as in the frogs,
fastened at its front end but free behind, so that the hinder end can
be protruded far from the mouth for the purpose of catching insects.
The organs of respiration are gills, external and internal, lungs,
trachea or windpipe, and the skin. In the earliest larval stages all
batrachians have gills; later, in most cases, the gills become reduced
and disappear, while at the same time lungs are developing. In some
salamanders the lungs never develop, but the animals, in their adult
stage, breathe wholly by means of the skin. In a few cases, as in the
siren and mud-puppies, gills are retained through the whole life,
although lungs are also present in the adult stage. The lungs are two
in number, a right and a left lung, and are simple sacs with the walls
more or less folded or thrown into ridges and richly supplied with
blood-vessels. The front end of the lungs opens directly into the
pharynx or, in the more elongate batrachians, is connected with it by
a tubular trachea or windpipe. In the frogs and toads there are vocal
cords stretched across the short windpipe; the vibration of these
cords produces the croaking.
The heart is always three-chambered, consisting of the right and left
auricles and a single ventricle. The circulation of the more
generalized salamanders like the mud-puppies is essentially like that
of a fish. In the frogs and toads there is a distinct advance beyond
this condition. The red corpuscles of the blood are oval in shape and
are the largest found among any of the vertebrates.
In the nervous system the small size of the hindbrain or cerebellum is
noticeable. The sense organs are fairly well developed. The skin of the
whole body is provided with tactile nerve-endings. There are special
taste organs on the lining membrane of the tongue and mouth-cavity. The
eyes have no lids in some of the lower forms; most of the frogs and
toads have an upper lid but no under one, although a thin membrane,
called the nictitating membrane, arises from the lower margin of the eye
and can be drawn up over it. The ears have no external parts, other than
the thin tympanic membranes. The nostrils of frogs and toads can be
closed by the contraction of certain special muscles.
=Life-history and habits.=--The sexes are distinct, and in most cases
the young hatch from eggs. A few of the salamanders give birth to free
young. The eggs are usually in strings or chains enclosed in a clear
gelatinous substance; these chains of eggs are either simply dropped
into the water or are fastened to water-plants. The young, called
tadpoles (fig. 120), in their earlier larval stages are extremely
fish-like in character, long-bodied, tailed, swimming freely about by
means of the fin-like flattened tail, and breathing by means of
external gills. Nor do they show any sign of legs. As the tadpoles
grow and develop the legs begin to appear, the hind legs first in the
frogs and toads, the fore legs first in the salamanders; lungs
develop and the gills disappear (except in the cases of the few forms
which retain gills through life). The tail shortens and finally
disappears in the frogs and toads; with the salamanders the tail-fin
only is lost. At the same time the change from water to land is made.
Further growth is very slow; frogs are not really adult, that is,
capable of producing young, until they are five years old, and they
may continue to increase in size until they are ten years old.
[Illustration: FIG. 120.--Tadpoles. (Photograph from life by Cherry
Kearton; permission of Cassel & Co.)]
The food of the adult batrachians is almost exclusively small animals,
particularly insects and worms. Crustaceans, snails, and young fish
are also eaten. The tadpoles also eat vegetable matter. Almost all
batrachians are nocturnal in habit, remaining concealed by day. In the
zones in which cold winters occur they hibernate or pass the winter in
a torpid condition, or state of "suspended animation," or, as it is
said, they sleep through the winter. Frogs burrow into the mud at the
bottom of ponds at the approach of winter and come forth early in the
spring to lay their eggs. Most batrachians are very tenacious of life,
being able to withstand long periods of fasting and serious
mutilation, and most of them can regenerate certain lost parts, such
as the tail or legs.
=Classification.=--The living Batrachia are divided into three orders,
viz., the Urodela, including the sirens, mud-puppies, salamanders, and
newts, batrachians which retain the tail throughout life, having
generally two pairs of limbs of approximately equal size, and
sometimes possessing gills or gill-slits in the adult condition; the
Anura, or frogs and toads, with no tail in the adult condition, with
short and broad trunk, with hind limbs greatly exceeding the fore
limbs in size, and never with gills or gill-slits in the adult stage;
and the Gymnophiona, or cæcilians, snake-like batrachians having
neither limbs nor tail, with a dermal exoskeleton and without gills or
gill-slits in the adult.
=Mud-puppies, salamanders, etc. (Urodela).=--TECHNICAL NOTE.--If
possible obtain specimens of mud-eels (_Siren_), common in the
South, or mud-puppies (_Necturus_), common in the central North,
as examples of batrachians with gills persisting in the adult
stage. One or more species of _Amblystoma_ may be found in almost
any part of the country, and larvæ of large size may be found with
the external gills. For an example of the general long-tailed or
Urodelous type of batrachian any salamander or newt occurring in
the vicinity of the school may be used. The little green triton or
eft (_Diemictylus viridiscens_) of the eastern States, or its
larger brown-backed congener of the Pacific coast (_D. torosus_)
is common in water, while another eft, the little red-backed
salamander, (_Plethodon_) is common in the woods under logs and
stones. The external characters of the body should be compared
with those of the toad. The skeleton should be prepared by
macerating away the flesh (for directions, see p. 452), and the
presence of the many caudal vertebræ and the ribs, the equality in
size of the legs, and other points should be noted. Compare with
skeleton of toad. Make drawings. It will be well, also, to dissect
out and examine the various internal organs of the salamander,
comparing them with the same organs in the toad. The salamander,
indeed, is in many ways better than the toad as an example of the
class. Its body is less adaptively modified and shows the
essentially fish-like character of the batrachian structure.
The batrachians which retain external gills in the adult stage are the
members of two families of which the American representatives are known
as mud-eels (_Siren_) and mud-puppies or water-dogs (_Necturus_). The
mud-eels, which are found "in the ditches in the swamps of the southern
States from South Carolina to the Rio Grande of Texas and up the
Mississippi as high as Alton, Illinois," are blackish in color, have no
hind legs and are long and slender, with the tail shorter than the rest
of the body. They reach a length of nearly three feet. The mud-puppies,
found in the Great Lakes and in the rivers of the upper Mississippi
valley, are brown with colored spots, and are about two feet long when
full grown. They have both fore and hind legs.
A few salamanders, while not possessing external gills when adult, have
a spiracle or small circular opening in the side of the neck which leads
into the throat. The best-known American salamander of this kind is the
large heavy-bodied blackish water-dog or "hellbender" (_Cryptobranchus_)
of the Ohio River. It is about two feet long, and is "a very
unprepossessing but harmless creature." It has a conspicuous
longitudinal fold of skin along each side of the body. The largest known
batrachian, the giant salamander of Japan (_Megalobatrachus_), reaching
a length of three feet, is related to the water-dog.
Of all the salamanders the most interesting are the blunt-nosed
salamanders (_Amblystoma_). A dozen or more species of _Amblystoma_
occur in North America, of which _tigrinum_, a dark-brown species with
many irregular yellow blotches sometimes arranged in cross-bands, is
the most widespread. The larvæ of some _Amblystoma_ retain their gills
until they have reached a large size, and in one or two species the
usual metamorphosis is very long delayed and the salamanders produce
young while in the larval condition, that is, while retaining the
gills and a compressed fin-like tail. In the case of a certain Mexican
species (_A. maculatum_) it is believed that the final metamorphosis
never occurs. The Mexicans call these gilled larval _Amblystoma_
axolotls, and use them for food. For a long time naturalists supposed
the _Amblystoma_ larvæ which produce young to be the adults of a
species of salamanders which retained their gills through life, like
the sirens and mud-puppies, and classified them in a distinct genus.
[Illustration: FIG. 121.--The Western brown eft, or salamander,
_Diemyctylus torosus_. (From living specimen.)]
Of the various common salamanders or newts some are found in streams,
ponds, and ditches, and some under logs and stones in the woods. The
aquatic forms have the tail compressed (flattened from side to side),
while the land forms have the tail cylindrical, tapering to a point.
Most of the land-salamanders produce their young alive, while the
water forms lay eggs which are usually attached to a submerged
plant-stem. The salamanders are, almost without exception, found only
in the northern hemisphere.
=Frogs and toads (Anura).=--There are about a dozen species of frogs
in the United States. The largest of these, and indeed the largest of
all the frogs, is the well-known bullfrog (_Rana catesbiana_), which
reaches a length (head to posterior end of body) of eight inches. It
is found in ponds and sluggish streams all over eastern United States
and in the Mississippi valley. It is greenish in color with the head
usually bright pale green. Its croaking is very deep and sonorous. The
pickerel-frog (_R. palustris_), which is bright brown on the back with
two rows of large oblong square blotches of dark brown on the back, is
found in the mountains of eastern United States. The little pale
reddish-brown wood-frog (_R. sylvatica_) with arms and legs barred
above is common in damp woods and is "an almost silent frog." The
peculiar and infrequently seen frogs known as the "spade-foots"
(_Scaphiopus_) are subterranean in habit and usually live in dry
fields or even on arid plains and deserts. They pass through their
development and metamorphosis very rapidly, appearing immediately
after a rain and laying their eggs in temporary pools. At this time of
egg-laying they utter extraordinarily loud and strange cries. Some
frogs in other parts of the world live in trees, and the eggs of one
species are deposited on the leaves of trees, leaves which overhang
the water being selected so that the issuing young may drop into it.
The true tree-frogs or tree-toads (Hylidæ) constitute a family
especially well represented in tropical America. They have little disk-
or pad-like swellings on the tips of their toes to enable them to hold
firmly to the branches of the trees in which they live. Some, like the
swamp tree-frog and the cricket-frog, are not arboreal in habit,
remaining almost always on the ground. The common tree-frog of the
eastern States (_Hyla versicolor_) is green, gray, or brown above with
irregular dark blotches, and yellow below. It croaks or trills,
especially at evening and in damp weather. Pickering's tree-frog (_Hyla
pickeringii_) makes the "first note of spring" in the eastern States.
This tree-frog is the one most frequently heard in the autumn too, but
"its voice is less vivacious than in the spring and its lonely pipe in
dry woodlands is always associated with goldenrods and asters and
falling leaves." The tree-frogs of North America lay their eggs in the
water on some fixed object as an aquatic plant, in smaller packets than
those of the true frogs, and not in strings as do the toads.
The toads (Bufonidæ) differ from the true frogs in having no teeth and
in not having, as the frogs do, a cartilaginous process uniting the
shoulder-bones of the two sides of the body. The absence of this
uniting process makes the thoracic region capable of great expansion.
There are only a few species of toads in North America, but one of
these species, the common American toad (_Bufo lentiginosus_), is very
abundant and widespread. It appears also in two or three varieties,
the common toad of the southern States differing in several
particulars from that of the northern. The toad is a familiar
inhabitant of gardens, and does much good by feeding on noxious
insects. It is most active at twilight. Its eggs are laid in a single
line in the centre of a long slender gelatinous string or rope, which
is nearly always tangled and wound round some water-plant or stick
near the shore on the bottom of a pond. The eggs are jet black and
when freshly laid are nearly spherical. At the time of egg-laying the
toads croak or call, making a sort of whistling sound and at the same
time pronouncing deep in the throat "bu-rr-r-r-r." The toad does not
open its mouth when croaking, but expands a large sac or resonator in
its throat. The toad-tadpoles are blacker than those of frogs or
salamanders, and undergo their metamorphosis while of smaller size
than those of frogs. When they leave the water they travel for long
distances, hopping along so vigorously that in a few days they may be
as far as a mile from the pond where they were hatched. They conceal
themselves by day, but will appear after a warm shower; this sudden
appearance of many small toads sometimes gives rise to the false
notion that they have fallen with the rain.
=Cœcilians (Gymnophiona).=--The third order of batrachians, the
cœcilians, includes about twenty species of slender worm- or
snake-like limbless forms which are confined to the tropics. Some of
them are wholly blind and the others have only rudimentary eyes. In
them the skin is folded at regular intervals so that the body appears
to be rigid or segmented, and in some species there are small
concealed horny scales in the skin.
CHAPTER XXVI
BRANCH CHORDATA (_Continued_). CLASS REPTILIA:
THE SNAKES, LIZARDS, TURTLES,
CROCODILES, ETC.
THE GARTER SNAKE (_Thamnophis_ sp.)
TECHNICAL NOTE.--Garter snakes may be found almost anywhere during
the spring and summer months. If possible each student should have a
specimen, but in case it is difficult to get enough snakes two
students can use a single specimen. If garter snakes are rare, take
any other snake. Snakes will live a long time without feeding and
specimens should be kept alive until ready to use. Kill with
chloroform as directed for the toad (p. 5). After completing the
study of the external characters place each specimen in a
dissecting-pan and with a pair of scissors cut through the scales on
the ventral side, passing backwards from the eighteenth to the
fortieth. Pin back the edges of the cut and thus expose the heart.
Through its lower end, the ventricle, insert a large canula; inject
with a fairly large syringe the glue mass which is described on p.
452. This injection will fill the entire arterial system. To inject
the venous system make another cut through the ventral scales,
cutting forward from the anal scale through about forty of them.
Note the injected mass in some of the vessels already filled. Take
one of the large vessels still containing blood and pass two
ligatures beneath it. Get ready a small canula and cut a slit in the
vessel, elevating the head so that the blood will run out as much as
possible. Now wash the blood off, insert the canula in the slit and
tie one ligature about the vessel containing the canula; have the
other ready to tie after the vein has been injected. Use a new color
for the venous system. Leave specimen in cold water for a time until
the injection is hard. Then continue the cut from the anal plate
forward to the lower jaw and pin out the edges of the cut on both
sides in the dissecting-pan.
=Structure= (fig. 122).--Note that the snake is covered with horny
_scales_ somewhat as the fish is. How do these scales differ from those
of the fish? In snakes the scales are not bony, but are true skin
structures. Note the modification of the scales on the head, back, and
ventral surface. Those on the dorsal surface often have minute ridges,
the _keels_. How do the ventral scales differ from the dorsal ones and
others? By a system of muscles these ventral scales are rhythmically
moved and as their posterior edges are pushed back against some
resisting object the body glides forward. On the head note the pair of
_eyes_. Are there eyelids? In front of each eye note an opening. What
are these openings? Thrust a bristle into the opening and see where it
enters the mouth-cavity through the _internal nares_. Does the snake
have external ears? Observe the very long _jaws_ and note that they are
loosely hinged. Examine the inside of the _mouth_. Are there _teeth_? If
so where are they situated, and how arranged? Note that all of the teeth
point backwards. Food is not chewed. When some object of prey, a frog,
or mouse, for example, is seized, the teeth hold it fast to the roof of
the mouth and by a backward and forward movement of the lower jaws it is
gradually drawn into the large œsophagus. What is the character and
situation of the _tongue_? Just behind the tongue note the narrow slit,
_glottis_, opening into the _windpipe_, or _trachea_. Back of the
trachea opens the _œsophagus_.
When the snake is laid open the elongate _heart_ will be conspicuous
in the anterior third of the body. Insert a blowpipe or quill into the
glottis just back of the tongue, and inflate the _lung_, which is a
long, thin-walled bag extending from the region of the heart
posteriorly for two-thirds of the length of the body. There is but one
developed lung, the right; note at the anterior end of the lung a
small mass of tissue, the atrophied left lung. Running forward from
the lung is a long tube composed of incomplete cartilaginous rings,
connected by membrane, the _trachea_. Note the long straight
_alimentary canal_. Distinguish the _œsophagus_, _stomach_,
_intestine_, _rectum_ and the _anus_.
In the region of the lung is an elongated dark-red glandular mass, the
_liver_. The secretion from the liver passes down through the long
_hepatic duct_ to the oval-shaped green _gall-bladder_ and into the
intestine.
TECHNICAL NOTE.--The bile-duct may be injected through the
gall-bladder with some colored injecting mass.
Note that the duct running off from the gall-bladder to the intestine
passes through a pink glandular organ, the _pancreas_. At the anterior
end of the pancreas is a dark-red nodular structure, the _spleen_. The
alimentary canal, the liver and the spleen are all suspended from the
dorsal wall of the body-cavity by a delicate sheet of tissue. What is
this? This condition we have also noted in the toad and fish.
Toward the posterior end of the body cavity are two long, dark-red
glands, the _kidneys_, which are the principal excretory organs of the
body. Through a long, slender tube (the _ureter_) each of the kidneys
passes off its wastes. Where do the ureters open?
Anterior to the kidneys are the reproductive organs. The eggs,
produced by the female snake, after being fertilized, pass backward
through the egg-tubes. During the breeding season these tubes are much
distended. This is due to the presence of the developing eggs, for the
young snakes are hatched in the egg-tubes.
A successful injection as directed in the first technical note will
have filled both arterial and venous systems. How does the general
shape of the snake's _heart_ compare with that of the toad? The heart
consists of two _ventricles_, incompletely separated, and _two
auricles_. In the snake the _conus arteriosus_ is very much shortened
and is not visible. Note two large vessels arising from the median
portion of the ventricle. The one on the left side is the _left aortic
artery_ or _left aortic arch_, while the right gives off two branches.
Where does the anterior one of these run? The main branch, or _right
aortic arch_, passes back to meet its fellow, the left aortic artery,
forming with it the _dorsal aorta_, which runs posteriorly to the end
of the tail. Note the various branches given off by the dorsal aorta
and trace some of them. Arising from the ventricles beneath the two
aortic arches is the _pulmonary artery_, which goes to the lung. There
the blood is purified, after which it is taken up by the _pulmonary
vein_ and carried back to the left auricle, whence it passes into the
ventricle to be mixed with the impure blood from the right auricle.
From the arteries the blood flows to all parts of the body through
fine _capillaries_, bathing the tissues, giving off oxygen and taking
up the carbonic acid gas. From these capillaries it passes into veins
and so back to the heart; from the anterior end of the body through
the _jugular veins_ and from the posterior portion of the body through
the _postcaval vein_. Flowing forward from the tail in the _caudal
vein_, the blood enters the capillaries of the kidneys, where the
waste matter is taken from it. This part of the circulatory system is
known as the _renal-portal_ circulation. From the kidneys the blood
flows through the postcaval vein anteriorly to the heart.
The blood which passes out from the dorsal aorta to all parts of the
alimentary canal is again collected into veins which unite to form the
_mesenteric vein_. This vein runs to the liver, where it breaks up
into capillaries. Thence the blood is carried into the postcaval vein,
which leads directly to the heart. This part of the circulatory system
which collects blood from the alimentary canal and carries it to the
liver is called the _hepatic-portal_ system.
Just in front of the heart will be noted a nodular structure, the
_thyroid gland_, while a little in advance of the thyroid may be seen
a long glandular mass, the _thymus gland_. The functions of these
glands are not certainly understood.
Remove the alimentary canal and muscles from a part of the body and
note that the _axial skeleton_, like that of the other vertebrates
studied, consists of a series of _vertebræ_ placed end to end. Are
there _arms_ or _legs_? Are _shoulder_ and _pelvic girdles_ present?
How many of the vertebræ bear _ribs_? The ribs connect at their lower
ends with the ventral scales. Note the great number of the vertebræ
and ribs as compared with those of the toad or fish. What are those
vertebræ called which bear no appendages or ribs? Examine carefully
the elongated _skull_ of the snake, especially the modified jaws. A
detailed study of the skeleton may be made by referring to the account
of the skeleton of the lizard in Parker's "Zootomy," pp. 130 _et seq._
The nervous system may be worked out in a specimen which has been
immersed in 20 per cent nitric acid. The description of the nervous
system of the toad (see pp. 12-13) will suffice for a guide to the
study of the nervous system of the snake. The special sense organs, as
eyes and ears, should be examined and compared with those of the fish
and toad.
=Life-history and habits.=--The garter snakes are more or less aquatic
in habit and are good swimmers. They are often found far from water,
but in greatest abundance where the cat-tails and rushes grow
thickest. They feed on frogs, salamanders, and field-mice, which they
swallow whole. All the garter snakes are ovoviviparous, i.e., hatch
eggs within the body-cavity. The eggs, often as many as eighteen or
twenty, are enclosed within widened portions of the oviducts during
embryonic existence; when the young are born they are able to shift
for themselves. During cold weather the garter snake hibernates,
hiding then in some gopher-hole, or, in the warmer climates, under
some log or stone, there to lie dormant until the warm days of spring
come, when it resumes activity.
The garter snake sheds its skin at least once a year, sometimes
oftener. This process may be observed in snakes kept in confinement.
For some time before molting the animal remains torpid, the eyes
become milky, and the skin loses its lustre. After a few days it
conceals itself, the skin about the lips and snout pulls away and the
animal slips out of its entire skin. The snake not only sheds the skin
of the body but also the covering of the eyes. Snakes have no eyelids,
as we have already noted, that which represents the eyelid being a
transparent membrane which covers the eyeball.
No species of the garter snake group is poisonous. Sometimes a garter
snake may appear to be vicious, but its teeth are very short and at
best it can only make a small scratch scarcely piercing the skin.
OTHER REPTILES.
The class Reptilia includes the lizards, snakes, tortoises, turtles,
crocodiles, and alligators. Although popularly associated in the common
mind with the batrachians, the reptiles are really more nearly related
to the birds than to the salamanders and frogs. In general shape they
more nearly resemble the batrachians, but in the structural condition of
the internal body organs they are more like the birds. They are
cold-blooded, and breathe exclusively by means of lungs, the forms which
live in water coming to the surface to breathe. They are covered with
horny scales or plates, which with the entire absence of gills after
hatching readily distinguish them from all the batrachians. While most
reptiles live on land, some inhabit fresh water and some the ocean. As
the young have the same habitat and general habits as the adult, there
is no such metamorphosis in their life-history as is shown by the
batrachians. The reptiles are widespread geographically, occurring,
however, in greatest abundance in tropical regions, and being wholly
absent from the Arctic zone. They are not capable of such migrations as
are accomplished by the birds and many mammals, but withstand severely
hot or cold seasons by passing into a state of suspended animation or
seasonal sleep or torpor.
[Illustration: FIG. 123.--A lizard in the grass. (Photograph from life
by Cherry Kearton; permission of Cassell & Co.)]
=Body form and organization.=--The chief variations in body form among
the reptiles are manifest when a turtle, lizard, and snake are compared.
In the turtles, the body is short, flattened, and heavy, and provided
always with four limbs, each terminating in a five-toed foot; in the
lizards the body is more elongate and with usually four legs, but
sometimes with two only, or even none at all; while in the snakes the
long, slender, cylindrical body is legless or at most has mere rudiments
of the hinder limbs. With the reptiles locomotion is as often effected
by the bending or serpentine movements of the trunk as by the use of
legs. Among lizards and snakes the body is covered with horny epidermal
scales or plates, while among the turtles and crocodiles there may be,
in addition to the epidermal plates, a real deposit of bone in the skin
whereby the effectiveness of the armor is increased. The epidermal
covering of snakes and lizards is periodically molted, or, as we say,
the skin is shed. The bright colors and patterns of snakes and of many
lizards are due to the presence and arrangement of pigment-cells in the
skin. Among some reptiles, notably the chameleons, the colors and
markings can be quickly and radically changed by an automatic change in
the tension of the skin.
=Structure.=--In reptiles, as in batrachians, the chief variations in
the body skeleton are correlated with differences in external body
form. In the short compact body of the turtles and tortoises the
number of vertebræ is much smaller than in the snakes. Some turtles
have only 34 vertebræ; certain snakes as many as 400. The reptilian
skull, in the number and disposition of its parts and in the manner of
its attachment to the spinal column, resembles that of the birds,
although the cranial bones remain separate, not fusing as in the
birds. In the snake the two halves of the lower jaw are not fused in
front but are united by elastic ligaments, which condition, together
with the extremely mobile articulation of the base of the jaws, allows
the snakes to open their mouths so as to take in bodies of great size.
All of the reptiles, except the turtles, are provided with small teeth
which serve, generally, for seizing or holding prey and not for
mastication. The poisonous snakes have one or more long, sharp, and
grooved or hollow fangs (fig. 131). In the legless reptiles both
shoulder and pelvic girdles may be wholly lacking; in the limbed forms
both girdles are more or less well developed.
[Illustration: FIG. 122.--Dissection of the garter snake, _Thamnophis_
sp.]
The tongue of many reptiles, notably the snakes, is bifid or forked,
and is an extremely mobile and sensitive organ. The œsophagus is long
and in the snakes can be stretched very wide so as to permit the
swallowing of large animals whole. Reptiles breathe solely by lungs,
of which there is a pair. They are simple and sac-like, the left lung
being often much smaller than the other. In turtles and crocodiles the
lungs are divided internally by septa into a number of chambers.
Because of the rigidity of the carapace or "box" of turtles the air
cannot be taken in the ordinary way by the use of the ribs and
rib-muscles, but has to be swallowed. The reptilian heart consists of
two distinct auricles and of two ventricles, which in most reptiles
are only incompletely divided, the division into right and left
ventricles being complete only among the crocodiles and alligators,
the most highly organized of living reptiles.
The organs of the nervous system reach a considerable degree of
development in the animals of this class. The brain in size and
complexity is plainly superior to the batrachian brain and resembles
quite closely that of birds. Of the organs of special sense those of
touch are limited to special papillæ in the skin of certain snakes and
many lizards. Taste seems to be little developed, but olfactory organs
of considerable complexity are present in most forms, and consist of a
pair of nostrils with olfactory papillæ on their inner surfaces. The
ears vary much in degree of organization, crocodiles and alligators
being the only reptiles with a well-defined outer ear. This consists
of a dermal flap covering a tympanum. Eyes are always present and are
highly developed. They resemble the eyes of birds in many particulars.
All reptiles, excepting the snakes and a few lizards, have movable
eyelids, including a nictitating membrane like that of the birds. With
the snakes the eye is protected by the outer skin, which remains
intact over it, but is transparent and thickened to form a lens just
over the inner eye. Turtles and lizards have a ring of bony plates
surrounding the eyes similar to that of the birds. In addition to the
usual eyes there is in many lizards a remarkable eye-like organ, the
so-called pineal eye, which is situated in the roof of the cranium,
and is believed to be the vestige of a true third eye, which in
ancient reptiles was probably a well-developed organ.
=Life-history and habits.=--Most reptiles lay eggs from which the young
hatch after a longer or shorter period of incubation. Usually the eggs
are simply dropped on the ground in suitable places (although certain
turtles dig holes in which to deposit them), where they are incubated by
the general warmth of the air and ground. However, some of the giant
snakes, the pythons for instance, hold the eggs in the folds of the
body. In the case of some snakes and lizards the eggs are retained in
the body of the mother until the young hatch; such reptiles are said to
be ovoviviparous, because the young, although born alive, are in reality
enclosed in an egg until the moment of birth. Among reptiles the newly
hatched young resemble the parents in most respects except in size, yet
striking differences in coloration and pattern are not rare. But there
is in this class no metamorphosis such as characterizes the
post-embryonic development of the batrachians.
The food of reptiles consists almost exclusively of animal substance,
although some species, notably the green turtles and certain
land-tortoises, are vegetable-feeders. The animal-feeders are mostly
predaceous, the smaller species catching worms and insects, while the
larger forms capture fishes, frogs, birds, and their eggs, small
mammals, and other reptiles.
=Classification.=--The living Reptilia are divided into four orders, of
which one includes only a single genus, _Hatteria_, a peculiar lizard
found in New Zealand. The other three are the Squamata, which includes
the lizards and snakes,[17] distinguished by the scaly covering of the
body, the Chelonia, which includes the tortoises and turtles,
distinguished by the shell of bony plates which encloses the body, and
the Crocodilia, which includes the crocodiles and alligators, whose
bodies are covered with rows of sculptured bony scutes.
=Tortoises and turtles (Chelonia).=--TECHNICAL NOTE.--Obtain
specimens of some pond- or land-turtle common in the vicinity of
the school. The red-bellied and yellow-bellied terrapins
(_Pseudemys_) or the painted or mud-turtles (_Chrysemys_) are
common over most of the United States. (_Pseudemys_ is found south
of the Ohio River and _Chrysemys_ north of it.) They may be raked
up from creek-bottoms or fished for with strong hook and line,
using meat as bait. They will live through the winter if kept in a
cool place, without food or special care of any kind. Observe
their swimming and diving, the retraction of head and limbs into
the shell, the use of the third eyelid (nictitating membrane), and
the swallowing of air.
Examine the external structure of a dead specimen (kill by
thrusting a bit of cotton soaked with chloroform or ether into
the windpipe; see opening just at base of tongue). Note shell
consisting of a dorsal plate, the carapace, and ventral plate,
the plastron, and the lateral uniting parts, the bridge. Note
legs, and head with horny beak but no teeth. Compare with snake.
The examination of the internal structure of the turtle can be
readily made by sawing through the bridge on either side and
removing the plastron. Note the ligaments which attach the
plastron to the shoulder and pelvic girdles. Note muscles
covering these bones. Note just behind the shoulder girdle the
heart (perhaps still pulsating) and the dark liver on each side
of it. Work out the alimentary canal, the trachea and lungs, and
other principal organs, comparing them with those of the snake.
The skeleton can be studied by dissecting and boiling and
brushing away the flesh which still adheres to the bones. The
comparison of the skeleton of the turtle with that of the snake
is very instructive; marked differences in the skeletons of the
two kinds of reptiles are obviously correlated with the
differences in habits and shape of body. Note in the skeleton of
the turtle especially the shoulder and pelvic girdles and limbs
(absent in the snake) and small number of vertebræ and ribs.
Among the common turtles and tortoises of the United States are
several species of soft-shelled turtles (Trionychidæ) with carapace
not completely ossified and both carapace and plastron covered by a
thick leathery skin which is flexible at the margins; the
snapping-turtle (_Chelydra serpentina_), common in streams and ponds,
with shell high in front and low behind and head and tail long and not
capable of being withdrawn into the shell; the red-bellied and
yellow-bellied terrapins (_Pseudemys_), red and yellow, with
greenish-brown and black markings, common on the ground in woods and
among rocks and also near water and sometimes in it; the pond- or
mud-turtle (_Chrysemys_), also brightly colored and usually confined
to ponds and pond-shores; and the box-tortoise (_Cistudo carolina_),
common in woods and upland pastures and readily recognizable by its
ability to enclose itself completely in its shell by the closing down
of the lids of the plastron. All of these fresh-water and land-turtles
except the soft-shelled turtles belong to one family, the Emydidæ, but
have somewhat diverse habits. Most of them are carnivorous, but few
catch any very active prey. While some are strictly aquatic, others
are as strictly terrestrial, never entering the water. The eggs of all
are oblong and are deposited in hollows, sometimes covered in sand.
The newly hatched young are usually circular in shape, and vary in
color and pattern from the parents.
The "diamond-back terrapin" (_Malaclemmys palustris_), used for food,
is a salt-water form "inhabiting the marshes along the Atlantic coast
from Massachusetts to Texas. About Charleston [and Baltimore] they are
very abundant and are captured in large numbers for market, especially
at the breeding season, when the females are full of eggs. Further
north they are dug from the salt mud early in their hibernation and
are greatly esteemed, being fat and savory."
[Illustration: FIG. 124.--The giant land-tortoise of the Galapagos
Islands, _Testudo_ sp. These tortoises reach a length of four feet.
(Photograph from life by Geo. Coleman.)]
Strongly contrasting with the usually small land- and fresh-water
turtles are the great sea-turtles, such as the leather-back, the
loggerhead and the green turtles. Some of these animals reach a length
of six feet and more and a weight of nine hundred pounds, and have the
feet compressed and fin-shaped for swimming. They live in the open
ocean, coming on land only to lay their eggs, which are buried in the
sand of ocean islands. These egg-laying visits are almost always made at
night, and the turtles are then often caught by "turtlers." The flesh of
most of the sea-turtles is used for food, and from the shell of certain
species, notably the "hawk-bill" (_Eretmochelys imbricata_) the
beautiful "tortoise-shell" used for making combs and other articles is
obtained. The common green turtle (_Chelonia mydas_) of the Atlantic
coast is the species most prized for food. It is a vegetarian, feeding
on the roots of _Zostera_, the plant known in New England as eel-grass,
though farther south it is called turtle-grass. When grazing the
turtles eat only the roots, the tops thus rising to the surface, where
they indicate to the turtler the animal's whereabouts. The turtler,
armed with a strong steel barb attached to a rope and loosely fitted to
the end of a pole, carefully rows up to the unsuspecting animal, and
with a strong thrust plunges the barb through its shell, withdraws the
pole, and, grasping the rope, now firmly attached to the turtle's back,
lifts the animal to the surface. Here, with assistance, he turns it into
the boat, where it is rendered helpless by being thrown on its back and
by having its flippers tied. These turtles are also caught on their
breeding-grounds, being found on the sand at night by the turtler,
turned over on their backs, and left thus securely caught until
assistance comes to help get them into the boats.
=Snakes and lizards (Squamata).=--TECHNICAL NOTE.--A snake has
already been dissected and studied. It will be instructive to
compare the external structures, at least, of a lizard with that of
the snake. Specimens of some species of the common swift
(_Sceloporus_) are obtainable almost anywhere in the United States.
The "pine-lizards" of the east belong to this genus. Lizards may be
sought for in woods, along fences, and especially on warm rocks.
[Illustration: FIG. 125.--The blue-tailed skink, _Eumeces
skeltonianus_. (From living specimen.)]
The group of lizards is a very large one, about 1,500 species being
known, but it is represented in the United States by comparatively few
species. Lizards are especially abundant in the tropics of South
America. The strange and fantastic appearance presented by some of
them has made certain species the object of much interest and often
fear on the part of the natives of tropical lands. In those regions
are current extraordinary stories and beliefs regarding the habits and
attributes of certain lizards like the basilisk and chameleon. Lizards
are all more or less elongate and some are truly snake-like in form.
The legs, though usually present and functional, are in many cases
much reduced, and in some forms, as the glass-snake, either one or
both pairs are so rudimentary as to have no external projection
whatever. Although lizards are often regarded as being poisonous, only
one genus, _Heloderma_, the Gila Monster, is really so. All others are
perfectly harmless as far as poison is concerned, and most of them are
unusually timid. They vary in size from a few inches to six feet in
length. Most of them are terrestrial, some arboreal, and some aquatic.
[Illustration: FIG. 126.--The Gila monster, _Heloderma horridum_, the
only poisonous lizard. (Photograph from life by J. O. Snyder.)]
Among the lizards of this country the swifts and ground-lizards are
familiar everywhere. In certain regions the glass-snake or joint-snake
(_Opheosaurus ventralis_) is common. This animal, popularly considered
to be a snake, has no external limbs, and its tail is so brittle, the
vertebræ composing it being very fragile, that part of it may break off
at the slightest blow. In time a new tail is regenerated. It lives in
the central and northern part of the United States, and burrows in dry
places. In the western part of the country horned toads (_Phrynosoma_)
are common, about ten different species being known. These are lizards
with shortened and depressed body and well-developed legs. The body is
covered with protective spiny protuberances, and in individual color
and pattern resembles closely the soil, rocks, and cactus among which
the particular horned toad lives. All the species of _Phrynosoma_ are
viviparous, seven or eight young being born alive at a time.
In New Mexico, Arizona, and northern Mexico the only existing poisonous
lizards, the Gila Monster (_Heloderma_) (fig. 126) is found. This is a
heavy, deep-black, orange-mottled lizard about sixteen inches long.
There is much variance of belief among people regarding the Gila
Monster, but recent experiments have proved the poisonous nature of the
animal. The poison which is secreted by glands in the lower jaw flows
along the grooved teeth into the wound. A beautiful and interesting
little lizard found in the South is the green chameleon (_Anolis
principalis_). Its body is about three inches long with a slender tail
of five or six inches. The normal color of the chameleon is grass-green,
but it may "assume almost instantly shades varying from a beautiful
emerald to a dark and iridescent bronze color."
In the tropics many of the lizards reach great size and are of strange
shape and patterns. The flying dragons (_Draco_) have a sort of
parachute on each side of the body composed of a fold of skin
supported by five or six false posterior ribs. These lizards live in
the trees of the East Indies and "fly" or sail from tree to tree. They
are very beautifully colored. The iguanas (_Iguana_) of the tropics of
South America are commonly used for food. They live mostly in trees,
and reach a length of five or six feet. The monitor (_Varanus
niloticus_) is a great water-lizard that lives in the Nile, and feeds
on crocodiles' eggs, of which it destroys great numbers. It is the
principal enemy of the crocodile. When full grown it reaches a length
of six feet or even more.
About 1,000 living species of snakes are known. Usually they have the
body regularly cylindrical, and without distinct division into body
regions. Legs are wanting, locomotion being effected by the help of
the scales and the ribs. No snake can move forward on a perfectly
smooth surface and no snake can leap. In some forms, such as the
pythons, external rudiments of the hind limbs are present, but do not
aid in locomotion. The mouth is large and distensible so that prey of
considerably greater size than the normal diameter of the snake's body
is frequently swallowed whole. The sense of taste is very little if at
all developed, as the food is swallowed without mastication. The
tongue, which is protrusible and usually red or blue-black, serves as
a special organ of touch. Hearing is poor, the ears being very little
developed. The sense of sight is also probably not at all keen. Snakes
rely chiefly on the sense of smell for finding their prey and their
mates. The colors of snakes are often brilliant, and in many cases
serve to produce an effective protective resemblance by harmonizing
with the usual surroundings of the animal. The food of snakes consists
almost exclusively of other animals, which are caught alive. Some of
the poisonous snakes kill their prey before swallowing it, as do some
of the constrictors. While most snakes live on the ground, some are
semi-arboreal and others spend part or all of their time in water.
Cold-region snakes spend the winter in a state of suspended animation;
in the tropics, on the contrary, the hottest part of the year is spent
by some species in a similar "sleep."
[Illustration: FIG. 127.--A garter snake, _Thamnophis parietalis_.
(Photograph from life by J. O. Snyder.)]
There are so many common snakes in the United States that only a few
of the more familiar forms can be mentioned. The non-poisonous species
of America belong to the family Colubridæ, while all but one of the
poisonous species belong to the family Crotalidæ, characterized by the
presence of a pair of erectile poison-fangs on the upper jaw. Among
the commonest of the Colubridæ are the garter snakes (_Thamnophis_)
(fig. 127), always striped and not more than three feet long. The most
widespread species is _Thamnophis sirtalis_, rather dully colored with
three series of small dark spots along each side. The common
water-snake (_Natrix sipedon_) is brownish with back and sides each
with a series of about 80 large square dark blotches alternating with
each other. It feeds on fishes and frogs, and although "unpleasant and
ill-tempered" is harmless. One of the prettiest and most gentle of
snakes is the familiar little greensnake (_Cyclophis æstivus_), common
in the East and South in moist meadows and in bushes near the water.
It feeds on insects and can be easily kept alive in confinement. A
familiar larger snake is the blacksnake or blue racer (_Bascaniom
constrictor_), "lustrous pitch black, general color greenish below and
with white throat." It is "often found in the neighborhood of water,
and is particularly partial to thickets of alders, where it can hunt
for toads, mice, and birds, and being an excellent climber it is often
seen among the branches of small trees and bushes, hunting for young
birds in the nest." The chain-snake (_Lampropeltis getulus_) of the
southeast and the king-snake (also a _Lampropeltis_) (fig. 128) of the
central States are beautiful lustrous black-and-yellow spotted snakes
which feed not only on lizards, salamanders, small birds and mice but
also on other snakes. The king-snake should be protected in regions
infested by "rattlers." The spreading adder or blowing viper
(_Heterodon platirhinos_), a common snake in the eastern States,
brownish or reddish with dark dorsal and lateral blotches, depresses
and expands the head when angry, hissing and threatening. Despite the
popular belief in its poisonous nature this ugly reptile is quite
harmless. It specially infests dry sandy places.
[Illustration: FIG. 128.--A king-snake, _Lampropeltis boylii_.
(Photograph from life by J. O. Snyder.)]
[Illustration: FIG. 129.--The gopher-snake, _Pituophis bellona_.
(Photograph from life by J. O. Snyder.)]
With the exception of the coral or beadsnake (_Elaps fulvius_), a
rather small jet-black snake with seventeen broad yellow-bordered
crimson rings, found in the southern States, the only poisonous snakes
of the United States are the rattlesnakes and their immediate
relatives, the copperhead and water-moccasin. These snakes all have a
large triangular head, and the posterior tip of the body is, in the
rattlesnakes, provided with a "rattle" composed of a series of partly
overlapping thin horny capsules or cones of shape as shown in figure
130. These horny pieces are simply the somewhat modified successively
formed epidermal coverings of the tip of the body, which instead of
being entirely molted as the rest of the skin is, are, because of
their peculiar shape, loosely attached to one another, and by the
basal one to the body of the snake. The number of rattles does not
correspond to the snake's years for several reasons, partly because
more than one rattle can be added to the tail in a year, and
especially because rattles are easily and often broken off. As many as
thirty rattles have been found on one snake. There are two species of
ground-rattlesnakes or massasaugas (_Sistrurus_) in the United States
and ten species of the true rattlesnakes (_Crotalus_). The centre of
distribution of the rattlesnakes is the dry tablelands of the
southwest in New Mexico, Arizona, and Texas. But there are few
localities in the United States outside the high mountains in which
"rattlers" do not occur or did not occur before they were exterminated
by man. The copperhead (_Agkistrodon contortix_) is light chestnut in
color, with inverted Y-shaped darker blotches on the sides, and
seldom exceeds three feet in length. It occurs in the eastern and
middle United States from Pennsylvania and Nebraska southward. It is a
vicious and dangerous snake, striking without warning. The
water-moccasin (_Agkistrodon piscivorous_) is dark chestnut-brown with
darker markings. The head is purplish black above. It is found along
the Atlantic and Gulf coasts from North Carolina to Mexico, extending
also some distance up the Mississippi valley. It is distinctively a
water-snake, being found in damp swampy places or actually in water.
It reaches a length of over four feet and is a very venomous snake,
striking on the slightest provocation. The common harmless water-snake
is often called water-moccasin in the southern States, being popularly
confounded with this most dangerous of our serpents. The poison of all
of these snakes is a rather yellowish, transparent, sticky fluid
secreted by glands in the head, from which it flows through the hollow
maxillary fangs. The character and position of the fangs are shown in
figure 131. Remedial measures for the bite of poisonous snakes are,
first, to stop, if possible, the flow of blood from the wound to the
heart, by compressing the veins between the wound and heart, then to
suck (if the lips are unbroken) the poison from the wound, next to
introduce by hypodermic injection permanganate of potash, bichloride
of mercury or chromic acid into the wound, and finally perhaps to take
some strong stimulant as brandy or whiskey.
[Illustration: FIG. 130.--The rattles of the rattlesnake; the lower
figure shows a longitudinal section of the rattle.]
[Illustration: FIG. 131.--Dissection of head of rattlesnake; _f_,
poison-fangs; _p_, poison-sac.]
Of the kinds of snakes not found in this country perhaps the most
interesting are the gigantic boa constrictors, anacondas, and pythons.
Pythons are found in India, the islands of the Malay archipelago, and
Australia, while the boas and anacondas live in the tropics of America.
The largest pythons reach a length of thirty feet and some of the boas
are nearly as large. These snakes feed on small mammals such as fawns,
kids, water-rats, etc., and birds. The prey is swallowed whole, being
first encircled and crushed to death in folds of the body. After a meal
the python or boa lies in a sort of torpor for some time. A famous snake
is the deadly cobra-da-capello of India. These snakes are so abundant
and the bite is so nearly certainly fatal that thousands of persons are
killed each year in India by it. Other extremely poisonous snakes are
the vipers (_Vipera cerastes_), which live in the hot deserts of
northern Africa. Over each eye there is a scaly spine or horn, from
which the name horned viper is derived. The most poisonous snake of
South Africa is the large and ugly puff-adder, which puffs itself up
when irritated. An interesting group of snakes is that of the Hydrophidæ
or sea-snakes, which swim on the surface of the ocean by means of their
flattened and oar-like tails. These forms live in the tropical portions
of the Indian and Pacific oceans, ranging as far north as the Gulf of
California, and spend their whole life in the water, "out of which they
appear to be blind and soon die." They are extremely venomous, but are
all of small size, rarely two feet long.
=Crocodiles and alligators (Crocodilia).=--The crocodiles and
alligators are reptiles familiar by name and appearance, though seen
in nature only by inhabitants or visitors in tropical and semitropical
lands. In the United States there are two species of these great
reptiles, the American crocodile (_Crocodilus americanus_), living in
the West Indies and South America and occasionally found in Florida,
and the American alligator (_Alligator mississippiensis_), common in
the morasses and stagnant pools of the southern States. The alligator
differs from the crocodiles in having a broader snout. It is rarely
more than twelve feet long. The best-known crocodile is the Nile
crocodile, which is not limited to the Nile, but is found throughout
Africa. In the Ganges of India is found another member of this group
of reptiles called the gavial. It is among the largest of the order,
reaching a length of twenty feet. The crocodiles, alligators, and
gavials comprise not more than a score of species altogether, but
because of their wide distribution, great size, and carnivorous habits
they are among the most conspicuous of the larger living animals. They
live mostly in the water, going on land to sun themselves or to lay
their eggs. They move very quickly and swiftly in water but are
awkward on land. Fish, aquatic mammals and other animals which
occasionally visit the water are their prey. The gavial and Nile
crocodile are both known to attack and devour human beings, and these
species annually cause a considerable loss of life. But few such
fatalities, however, are accredited to the American alligator.
FOOTNOTE:
[17] By many zoologists the lizards and snakes are held to form two
distinct orders, Lacertilia and Ophidia.
CHAPTER XXVII
BRANCH CHORDATA (_Continued_). CLASS AVES:
THE BIRDS
THE ENGLISH SPARROW (_Passer domesticus_)
TECHNICAL NOTE.--The English sparrow may be found now in cities and
villages all over the United States. It has become a veritable pest,
and the killing of the few needed for the laboratory may be looked
on as desirable rather than deplorable, as is the killing of birds
in almost all other cases. The males have a black throat, with the
other head-markings strong and contrasting (black, brown, and
white), while the females have a uniform grayish and brownish
coloration on the head.
Specimens are best taken alive, as shooting usually injures them
for dissection. One can rely on the ingenuity of the boys of the
class to procure a sufficient number of specimens. Observations on
the habits of the birds should be made by the pupils as they go to
and from school. For dissection use fresh specimens if possible. If
desirable a pigeon or dove may be used in place of the sparrow.
=External structure.=--Note in the sparrow the same general arrangement
of body parts as in the toad, the body being divided into _head_, _upper
limbs_, _trunk_, and _lower limbs_. In the toad, however, all of the
limbs are fitted for walking and jumping, whereas in the sparrow the
anterior pair of appendages, the _wings_, are modified to be organs of
flight, and the posterior limbs are specially adapted for perching. Note
that the sparrow is covered with _feathers_, some long, some short, in
some places thick and in others thin, but all fitting together to form a
complete covering for the body. Note also that the anterior end of the
head is prolonged into a hard bony structure, the _bill_, covered with
horny substance. This horny substance together with the feathers and
horny covering of the feet are modified portions of the skin. Note the
long _quill-feathers_ attached to the posterior edge of the wing. By
these the bird sustains its flight. Other long quill-feathers are
attached to the posterior end of the body, forming the _tail_. By a
system of muscles connected with these feathers they act together,
serving as a rudder during flight and as a balancing contrivance when
perching. Note just above the bill two openings protected by tufts of
feathers. What are these openings? How are they connected with the
_mouth_? Note the large _eyes_, and at the inner angle of each the
delicate _nictitating membrane_ which can be drawn over the ball. Does
the bird have external _ears_? Lift the feathers just above the tail
(the upper tail-coverts) and note a small median gland, the _oil-gland_,
from which the bird derives the oil with which it oils its feathers.
Beneath the tail note the opening from the alimentary canal and from the
kidneys and reproductive organs. This is called the _cloacal opening_.
Examine in detail some of the feathers. In one of the quill-feathers
note the central _stem_ or _shaft_ composed of two parts, a basal
hollow _quill_, which bears no web and by which the feather is
inserted in the skin, and a longer, terminal, four-sided portion, the
_rachis_, which bears on either side a _web_ or _vane_. Each vane is
composed of many narrow linear plates, the _barbs_, from which rise
(like miniature vanes) many _barbules_. Each barbule bears many fine
_barbicels_ and _hamuli_ or _hooklets_. The barbs of the feather are
interlocked. How is this effected? The feathers which overlie the
whole body and bear the color pattern are called _contour-feathers_.
How do they differ from or correspond with the quill-feathers in
structure? Soft feathers called _down-feathers_ or _plumules_, cover
the body more or less completely, being, however, mostly hidden by
the contour-feathers; the barbs of these are sometimes not borne on a
rachis, but arise as a tuft from the end of the quill. Certain other
feathers which have an extremely slender stem and usually no vane,
except a small terminal tuft of barbs, are called _thread-feathers_,
or _filoplumules_. They are rather long, but are mostly hidden by the
contour-feathers. In certain birds they stand out conspicuously, as
the _vibrissæ_ about the nostrils.
In the determination of birds by the use of a classificatory "key" (see
p. 359) it is necessary to be familiar with the names applied to the
various external regions of the body and plumage, and with the terms
used to denote the special varying conditions of these parts. By
reference to figure 133 the names of the regions or parts most commonly
referred to may be learned. A full account of all of the external
characters with definitions of the various terms used in referring to
them may be found in Coues's "Key to North American Birds."
TECHNICAL NOTE.--Pull the feathers from the body, being careful
not to tear the skin.
In the fish and toad, already studied, the head is closely joined to
the trunk. How is it with the bird? Observe that the _knee_ of the
sparrow is covered by feathers and that it is the _ankle_ which
extends down as the bare unfeathered part to the _digits_. How many
digits have the feet of the bird? How are they arranged?
=Internal structure= (fig. 132).--TECHNICAL NOTE.--With a pair of
scissors cut just beneath the skin anteriorly from the cloacal
opening to the angle of the lower jaw. Pin the sparrow on its back
by the wings, feet, and bill. Push back the skin from both sides
and pin out.
[Illustration: FIG. 133.--Diagrammatic outline of bird's body with
names of external parts and regions.]
Note the large powerful _pectoral muscles_. Note a hard median
projection of bone, the _sternum_, which is a large keel-shaped bone
with lateral expansions to which are attached the _ribs_. Where are
the largest and most powerful muscles of the toad located? Where are
they in the fish? In the bird the most powerful muscles are these
pectoral muscles, which move the wings in flight.
TECHNICAL NOTE.--Cut the pectoral muscles from the left side of
the sternum, push back and pin to one side. With a strong pair of
scissors cut through the ribs on the left side of the sternum and
through the overlying bones. Lift the whole sternum, with the
right pectoral muscle attached, to the left side of the pan and
pin it down. Cut through the membrane which covers the viscera and
cover the dissection with water.
In this operation note the V-shaped _wishbone_ in front of the
sternum. It is composed of the two _clavicles_ with their inner ends
fused. Note the stout _coracoid_ bones extending from the anterior end
of the sternum to the shoulder.
Note near the middle of the body the _heart_ with the large
blood-vessels proceeding from it. Behind the heart lies the large
reddish-brown _liver_, and on the left side below the liver is the
large _gizzard_ or _muscular stomach_. Note the _viscera_ folded over
themselves in the body-cavity. Push them temporarily aside and note in
the dorsal region under the heart large pinkish spongy sacs, the
_lungs_. These are attached by short tubes, the _bronchi_, to the long
cartilaginous _trachea_. At the union of the bronchi with the trachea
is a small expansion with cartilaginous walls, within which are
stretched small bands of muscles. This organ is the _syrinx_, the
song- or voice-apparatus of the bird. It should be cut open and
carefully examined. Trace the trachea forward to its anterior end. It
opens by a _glottis_ into the _larynx_, a slightly swollen chamber
with cartilaginous walls. Note the U-shaped _hyoid bone_ surrounding
the front of the glottis. Through a blowpipe or quill inserted into
the glottis blow air into the trachea and observe the inflation of the
lungs and of certain large _air-sacs_ in the abdomen, which
communicate with them.
Beneath the trachea note the long _œsophagus_. Inflate the œsophagus
with a blowpipe and note how distensible is its lower end near the
breast. This distensible portion is called the _crop_. If the
alimentary canal be drawn out straight the œsophagus will be found to
run as an almost straight tube down the left side of the body to the
gizzard. This latter organ has very thick muscular walls and in it the
food is ground up among the small bits of gravel it contains.
Extending from the gizzard near the entrance of the œsophagus note the
long _pyloric loop_ of the intestine called _duodenum_. Within this
loop is a long pinkish gland, the _pancreas_, which empties by a duct
into the duodenum. Into the duodenum also the overlying liver empties
its secretion of bile from the median-placed _gall-bladder_. From the
duodenum the _small intestine_ or _ileum_ extends with many
convolutions to its exit through the cloacal aperture. On the
intestine near the cloacal opening note a pair of glandular
structures, the _cæca_. The short part of intestine between the cæca
and cloaca is called the _rectum_. On the left side of the body
beneath the gizzard note a dark glandular structure, the _spleen_.
Make a drawing of the dissection as so far worked out.
TECHNICAL NOTE.--Remove the alimentary canal, cutting it free
posteriorly at the cæca and anteriorly just above the muscular
gizzard. Cut open the gizzard and note its structure. The contained
sand and gravel grains are picked up by the bird as it eats.
On either side of the throat note the well-defined _thyroid gland_; in
young sparrows will be noted on each side of the neck a mass of tissue,
the remains of the _thymus gland_, which disappears in the adult.
Cut transversely through the lower end of the heart and note that the
ventricles are wholly distinct, whereas in the toad and snake they are
incompletely separated. In the bird there is a complete double
circulation. Its blood is not mixed, the pure with the impure, as in
the toad and snake. Blood passing through the _right auricle_ and
_ventricle_ goes to the lungs; on its return to the heart purified, it
enters the _left auricle_ and _left ventricle_ thence to pass out over
the body through the arteries.
Note the large _aorta_ given off from the left ventricle. Note the two
large branches, the _innominate arteries_, given off by it near its
origin. Each innominate divides into three smaller arteries, a
_carotid_, _branchial_, and _pectoral_. The aorta itself turns toward
the back and continues posteriorly through the body as the _dorsal
aorta_. To the right auricle come three large veins, the _right_ and
_left præcavæ_ and the _postcava_. Each præcava is formed by three
veins, the _jugular_ from the head, the _branchial_ from the wing, and
the _pectoral_ from the pectoral muscles. The postcava comes from the
liver. From the right ventricle go the short _right_ and _left pulmonary
arteries_ to the lungs, and from the lungs the blood is brought to the
left auricle through the _right_ and _left pulmonary veins_.
TECHNICAL NOTE.--For a detailed study of the circulation of the
bird the teacher should inject the blood system of some larger
bird, as a pigeon or fowl, for a class-demonstration. (For a
guide, use Parker's "Zootomy," p. 209, or Martin and Moale's "How
to Dissect a Bird," pp. 135-140 and pp. 148, 149.)
In the posterior dorsal region of the body-cavity will be found large
three-lobed organs fitting into the spaces between the bones of the
back on either side. These are the _kidneys_, and from their outer
margins on each side a _ureter_ runs posteriorly into the cloaca.
Overlying the anterior ends of the kidneys are the reproductive
organs. In the male these glands consist of firm, whitish, glandular
bodies. From each runs a long convoluted _vas deferens_, which enters
the cloaca. This tube corresponds to the _egg-duct_ of the female. In
the female the _right egg-gland_ and _egg-duct_ or _oviduct_ are
wanting. The _left egg-gland_ appears as a glandular mass; during the
breeding season yellow _ova_ or _eggs_ in various stages of
development project from its surface. The oviduct opens by a
funnel-shaped mouth near the _egg-gland_ and runs thence to the
cloaca. The eggs pass from the egg-gland into the body-cavity, where
they are caught in the upper end of the oviduct and carried down and
out through the cloacal opening. It is in the oviduct that the egg
derives its accessory covering, which consists of a white or
albuminous portion, together with several enveloping membranes and the
hard shell enclosing all.
Remove the top of the skull and note the large _brain_. What portions
of the brain make up the greater part of it? Note the differences
between this brain and that of the toad. Trace the principal _cranial
nerves_. Work out the _spinal cord_ and principal _spinal nerves_. For
an account of the nervous system of the sparrow see Martin and Moale's
"How to Dissect a Bird," pp. 150-163.
TECHNICAL NOTE.--For a study of the skeleton of the sparrow a
specimen should be cleaned by boiling in a soap-solution (see p.
452).
[Illustration: FIG. 132.--Dissection of the English sparrow, _Passer
domesticus_.]
In the sparrow's _skeleton_ note the compactness of the _skull_ and
the fusion of its bones. Observe the long _cervical vertebræ_ which
support the skull, also the _thoracic vertebræ_ bearing the ribs and
sternum. How many of each of these kinds of vertebræ are there? The
vertebræ posterior to the thorax are more or less fused together to
form the _sacrum_, which, with the _pelvic girdle_, supports the
_leg-bones_. The bones of the tail consist of a number of very small
vertebræ, some of which are fused together. Note the correspondence
between the bones of the leg and those of the wing. What are the names
of each of the bones of each limb, and what are the corresponding
bones in the two limbs? The wings and legs being modified for
different uses, their various bones have assumed different relations
to each other and to the body, for they are bent at directly opposite
angles and the attachment of muscles is different. Compare the
skeleton of the bird with that of the toad. (For a detailed account of
the skeleton of the bird see Parker's "Zootomy," pp. 182-209, or
Martin and Moale's "How to Dissect a Bird," pp. 102-125.)
=Life-history and habits.=--The English sparrow was first introduced
into the United States in 1850, and since that time has rapidly
populated most of the cities and towns of the country. On account of
its extreme adaptability to surroundings, its omnivorous food-habits
and its fecundity it survives where other birds would die out. It also
crowds out and has caused the disappearance or death of other birds
more attractive and more useful. The sparrow annually rears five or
six broods of young, laying from six to ten eggs at each sitting. Had
it no enemies a single pair of sparrows would multiply to a most
astonishing number. The sparrow has, however, a number of enemies,
most common among them perhaps being the "small boy," but birds and
mammals play the chief part in the destruction. The smaller hawks prey
upon them, and rats and mice destroy great numbers of their young and
of their eggs whenever the nests can be reached. The sparrow is
omnivorous and when driven to it is a loathsome scavenger, though at
other times its tastes are for dainty fruits. Its senses of perception
are of the keenest; it can determine friend or foe at long range. The
nesting habits are simple, the nests being roughly made of any sort of
twigs and stems mixed with hair and feathers and placed in cornices or
trees. A maple-tree in a small Missouri town contained at one time
thirty-seven of these nests.
OTHER BIRDS.
Birds are readily and unmistakably distinguishable from all other kinds
of animals by their feathers. They are further distinguished from the
reptiles on one hand by their possession of a complete double
circulation and by their warm blood (normally of a temperature of from
100-112° F.), and from the mammals on the other by the absence of
milk-glands. There are about 10,000 known species of living birds; they
occur in all countries, being most numerous and varied in the tropics.
Birds are exceptionally available animals for the special attention of
beginning students, because of their abundance and conspicuousness and
the readiness with which their varied and interesting habits may be
observed. The bright colors and characteristic manners which make the
identification of the different kinds easy, the songs and flight, and
the feeding, nesting and general domestic habits of birds are all
excellent subjects for personal field-studies by the students. We shall
therefore devote more attention to the birds than to the other classes
of vertebrates, just as we selected the insects among the invertebrates
for special consideration.
=Body form and structure.=--The general body form and external
appearance of a bird are too familiar to need description. The
covering of feathers, the modification of the fore limbs into wings,
and the toothless, beaked mouth are characteristic and distinguishing
external features. The feathers, although covering the whole of the
surface of the body, are not uniformly distributed, but are grouped in
tracts called _pterylæ_, separated by bare or downy spaces called
_apteria_. They are of several kinds, the short soft plumules or
down-feathers, the large stiffer contour-feathers, whose ends form the
outermost covering of the body, the quill-feathers of the wings and
tail, and the fine bristles or vibrissæ about the eyes and nostrils
called thread-feathers. The fore limbs are modified to serve as wings,
which are well developed in almost all birds. However, the strange
Kiwi or Apteryx of New Zealand with hair-like feathers is almost
wingless, and the penguins have the wings so reduced as to be
incapable of flight, but serving as flippers to aid in swimming
underneath the water. The ostriches and cassowaries also have only
rudimentary wings and are not able to fly. Legs are present and
functional in all birds, varying in relative length, shape of feet,
etc., to suit the special perching, running, wading, or swimming
habits of the various kinds. Living birds are toothless, although
certain extinct forms, known through fossils, had large teeth set in
sockets on both jaws. The place of teeth is taken, as far as may be,
by the bill or beak formed of the two jaws, projecting forward and
tapering more or less abruptly to a point. In most birds the jaws or
mandibles are covered by a horny sheath. In some water and shore forms
the mandibular covering is soft and leathery. The range in size of
birds is indicated by comparing a humming-bird with an ostrich.
Many of the bones of birds are hollow and contain air. The air-spaces
in them connect with air-sacs in the body, which connect in turn with
the lungs. Thus a bird's body contains a large amount of air, a
condition helpful of course in flight. The breast-bone is usually
provided with a marked ridge or keel for the attachment of the large
and powerful muscles that move the wings, but in those birds like the
ostriches, which do not fly and have only rudimentary wings, this keel
is greatly reduced or wholly wanting. The fore limbs or wings are
terminated by three "fingers" only; the legs have usually four,
although a few birds have only three toes and the ostriches but two.
As birds have no teeth with which to masticate their food, a special
region of the alimentary canal, the gizzard, is provided with strong
muscles and a hard and rough inner surface by means of which the food
is crushed. Seed-eating birds have the gizzard especially well
developed, and some birds take small stones into the gizzard to assist
in the grinding. The lungs of birds are more complex than those of
batrachians and reptiles, being divided into small spaces by numerous
membranous partitions. They are not lobed as in mammals, and do not
lie free in the body-cavity, but are fixed to the inner dorsal region
of the body. Connected with the lungs are the air-sacs already
referred to, which are in turn connected with the air-spaces in the
hollow bones. By this arrangement the bird can fill with air not only
its lungs but all the special air-sacs and spaces and thus greatly
lower its specific gravity. The vocal utterances of birds are produced
by the vocal cords of the syrinx or lower larynx, situated at the
lower end of the trachea just where it divides into the two bronchial
tubes, the tracheal rings being here modified so as to produce a
voice-box containing two vocal cords controlled by five or six pairs
of muscles. The air passing through the voice-box strikes against the
vocal cords, the tension of which can be varied by the muscles. In
mammals the voice-organ is at the upper or throat end of the trachea.
The heart of birds is composed of four distinct chambers, the septum
between the two ventricles, incomplete in the Reptilia, being in this
group complete. There is thus no mixing of arterial and venous blood in
the heart. The systemic blood-circulation being completely separated
from the pulmonic, the circulation is said to be double. The circulation
of birds is active and intense; they have the hottest blood and the
quickest pulse of all animals. In them the brain is compact and large,
and more highly developed than in batrachians and reptiles, but the
cerebrum has no convolutions as in the mammals. Of the special senses
the organs of touch and taste are apparently not keen; those of smell,
hearing, and sight are well developed. The optic lobes of the brain are
of great size, relatively, compared with those of other vertebrate
brains, and there is no doubt that the sight of birds is keen and
effective. The power of accommodation or of quickly changing the focus
of the eye is highly perfected. The structure of the ear is
comparatively simple, there being ordinarily no external ear, other than
a simple opening. The organs of the inner ear, however, are well
developed, and birds undoubtedly have excellent hearing. The nostrils
open upon the beak, and the nasal chambers are not at all complex, the
smelling surface being not very extensive. It is probable that the sense
of smell is not, as a rule, especially keen.
=Development and life-history.=--All birds are hatched from eggs,
which undergo a longer or shorter period of incubation outside the
body of the mother, and which are, in most cases, laid in a nest and
incubated by the parents. The eggs are fertilized within the body of
the female, the mating time of most birds being in the spring or early
summer. Some kinds, the English sparrow, for example, rear numerous
broods each year, but most species have only one or at most two. The
eggs vary greatly in size and color-markings, and in number from one,
as with many of the Arctic ocean birds, to six or ten, as with most of
the familiar song-birds, or from ten to twenty, as with some of the
pheasants and grouse. The duration of incubation (outside the body)
varies from ten to thirty days among the more familiar birds, to
nearly fifty among the ostriches. The temperature necessary for
incubation is about 40° C. (100° F.). Among polygamous birds (species
in which a male mates with several or many females) the males take no
part in the incubation and little or none in the care of the hatched
young; among most monogamous birds, however, the male helps to build
the nest, takes his turn at sitting on the eggs, and is active in
bringing food for the young, and in defending them from enemies. The
young, when ready to hatch, break the egg-shell with the "egg-tooth,"
a horny pointed projection on the upper mandible, and emerge either
blind and almost naked, dependent upon the parents for food until able
to fly (altricial young), or with eyes open and with body covered with
down, and able in a few hours to feed themselves (precocial young).
[Illustration: FIG. 134.--The nest and eggs of the black phœbe,
_Sayornis nigricans_. (Photograph by J. O. Snyder.)]
More details regarding the eggs, nest, and young of birds will be
given later in this chapter.
=Classification.=--The class Aves is usually divided into numerous
orders, the number and limits of these as published in zoological
manuals varying according to the opinions of various zoologists. The
rank of an order in this group is far lower than in most other
classes. In other words, the orders are very much alike and are
recognized mainly for the convenience in breaking up the vast
assemblage of species. In North America practically all the
ornithologists have agreed upon a scheme of classification, which will
therefore be adopted in this book. According to this classification
the eight hundred (approximately) known species of North American
birds represent seventeen orders. Certain recognized orders, for
example, the ostriches, are not represented naturally in North America
at all. As birds can usually be readily identified, the species being
easily distinguished by general external appearance, and as there are
many excellent book-guides to their classification, the beginning
student can specially well begin with them his study of systematic
zoology, which concerns the identification and classification of
species. In a later paragraph are given therefore some suggestions for
field and laboratory work in the determination of local bird-faunæ. In
the following paragraphs each of the American orders is briefly
discussed, as is also the foreign order of ostriches.
=The ostriches, cassowaries, etc. (Ratitæ).=--The ostriches, familiar to
all from pictures and to some from live individuals in zoological
gardens and menageries, or stuffed specimens in museums, together with a
few other similar large species, are distinguished from all other birds
by having the breast-bone flat instead of keeled. There are about a
score of species of ostriches and ostrich-like birds all confined to the
southern hemisphere. In them the wings are so reduced that flight is
impossible, but the legs are long and strong, and they can run as
swiftly as a galloping horse. They are said to have a stride of over
twenty feet. They use their legs also as weapons, kicking viciously when
angered. The true ostriches (_Struthio camelus_) (fig. 135) live in
Africa. They are the largest living birds, reaching a height of nearly
seven feet and weighing as much as two hundred pounds. They are hunted
for their feathers, and are now kept in captivity and bred in South
Africa and California for the same purpose. About five million dollars'
worth of ostrich-feathers are used each year. The eggs, which are from
five to six inches long and nearly five inches thick, are laid in
shallow hollows scooped out in the sand of the desert. The male
undertakes most of the incubation, although when the sun is hot no
brooding is necessary. The young (fig. 136) hatch in from seven to
eight weeks, and can run about immediately.
[Illustration: FIG. 135.--Ostriches on ostrich farm at Pasadena,
California. (Photograph from life.)]
The rheas, found in South America, and the cassowaries of Australia
are the only other living ostrich-like birds. Their feathers are of
much less value than those of the true ostrich.
[Illustration: FIG. 136.--Young ostriches just from egg; on ostrich
farm at Pasadena, California. (Photograph from life.)]
=The loons, grebes, auks, etc. (Pygopodes).=--The loons, grebes, and
auks are aquatic birds, living in both ocean and fresh waters. Their
feet are webbed or lobed, and their legs set so far back that walking is
very difficult and awkward. But all the birds of this order are
excellent swimmers and divers. They are distinctively the diving birds.
They have short wings and almost no tail. The dab-chick or pied-billed
grebe (_Podilymbus podiceps_) is common in ponds over all the country.
Its eggs are laid in a floating nest of pond vegetation and are often
covered with decaying plants. The horned grebe (_Colymbus auritus_) is
common west of the Mississippi in lakes and ponds. The loon or great
northern diver (_Gavia imber_), found all over the United States in
winter, is the largest of this group, reaching a length (from bill to
tip of tail) of three feet. It is black above with many small white
spots, and with a patch of white streaks on each side of the neck and on
the throat; it is white on breast and belly. The female is duller, being
brownish instead of black.
[Illustration: FIG. 137.--Murres, _Uria troile californica_, on Walrus
Island, (Pribilof Group) Behring's Sea. Note the eggs scattered about
over the bare rocks. (Photograph from life by the Fur Seal
Commission.)]
The auks, guillemots, puffins, and murres (fig. 137) are ocean birds
which gather, in the breeding season, in countless numbers on the
bleak rocks and inaccessible cliffs of the northern oceans. Each
female lays a single egg (in some cases two or at most three) on the
bare rock or in a crevice or sort of burrow. These birds mostly fly
well, but are especially at home in the water, feeding exclusively on
animal substances found there. A famous species is the great auk
(_Alca impennis_), which has become extinct in historical times. The
last living specimen was seen in 1844.
=The gulls, terns, petrels, and albatrosses (Longipennes).=--The
Longipennes are water-birds, mostly maritime, with webbed feet and very
long and pointed wings. They are all strong flyers, and most of them are
beautiful birds. Their prevailing colors are white, slaty or lead-blue,
black, and, in the young, mottled brownish. They subsist chiefly on
fish, but any animal substance will be eagerly picked up from the water;
some of the gulls forage inland. Occasionally great flocks may be seen
following a plow near the shore and feeding on the grubs and worms
exposed in the freshly-turned soil. Some of the gulls, like the great
black-backed gull (_Larus marinus_), attain a length of two and one-half
feet. The terns (_Sterna_) are mostly smaller than the gulls, have a
bill not so heavy and not hooked, and have the tail forked.
The fulmars, shearwaters, petrels, and albatrosses are strictly
maritime. The albatrosses are very large, the largest being three feet
long with a spread of wing of seven feet. They are often found flying
easily over the open ocean at great distances from land. Like the auks
and puffins, the fulmars and shearwaters gather in extraordinary
numbers on rocky ocean islets or cliffs of the coast to breed.
=The cormorants, pelicans, etc. (Steganopodes).=--The Steganopodes are
water-birds with full-webbed feet, and prominent gular pouch, swimmers
rather than flyers like the Longipennes. The cormorants
(_Phalacrocorax_) inhabit rocky coasts and are green-eyed, large,
heavy, black birds with greenish-purple and violet iridescence; they
are among the most familiar of seashore birds. They feed chiefly on
fish and dive and swim under water with great ability. Cormorants are
rather gregarious, keeping together in small groups when fishing,
migrating often in great flocks, and in the breeding season gathering
in immense numbers on certain rocky cliffs or islets. They build their
nests of sticks and sea-weed; the eggs are three or four, and usually
bluish green with white, chalky covering substance.
The pelicans are large, long-winged, short-legged water-birds with
enormous bill and large gular sac which is used as a dip-net to catch
fish. There are three species in North America, the white pelican
(_Pelecanus erythrorhynchus_) occurring over most of the United
States, the brown pelican (_P. fuscus_) of the Gulf of Mexico, and the
California brown pelican (_P. californicus_) of the Pacific coast.
An interesting member of this order is the famous frigate or man-of-war
bird (_Fregata aquila_), with very long wings and tail and feet
extraordinarily small. The frigates have the greatest command of wing of
all the birds. They cannot dive and can scarcely swim or walk.
=The ducks, geese, and swans (Anseres).=--The familiar wild ducks, of
which there are forty species in North American fresh and salt waters;
the geese, of which there are sixteen species, and the three species
of wild swans constitute the order Anseres. The bill in these birds is
more or less flattened and is also lamellate, i.e. furnished along
each cutting-edge with a regular series of tooth-like processes; the
feet are webbed, and the body is heavy and flattened beneath. Of the
fresh-water or inland ducks, the more familiar are the mallard (_Anas
boschas_), a large duck with head (male) and upper neck rich glossy
green; the blue-winged teal (_Querquedula discors_) and green-winged
teal (_Nettion carolinense_); the shoveller (_Spatula clypeata_) with
spoon-shaped bill; the beautiful crested wood-duck (_Aix sponsa_); the
expert diver, the plump little ruddy duck (_Erismatura rubida_), and
others. Of the coastwise ducks, the canvas-back (_Aythya vallisneria_)
is famous because of its fine flavor, while among the strictly
maritime ducks the eiders (_Somateria_), which live in Arctic regions,
are well known for their fine down. Of the geese, the commonest is the
well-known Canada goose (_Branta canadensis_), while the pure-white
snow-goose (_Chen hyperborea_), with black wing-feathers and red bill,
is not unfamiliar. The wild swans (_Olor_) are the largest birds of
the order, and are less familiar than the ducks and geese.
=The ibises, herons, and bitterns (Herodiones).=--The tall, long-necked,
long-legged, wading birds, known as herons and ibises, compose a small
order, the Herodiones, of which but few representatives are at all
familiar. Perhaps the most abundant species is the green heron (_Ardea
virescens_) or "fly-up-the-creek," one of the smaller members of the
order. The crown, back, and wings are green, the neck purplish cinnamon,
and the throat and fore neck white-striped. This bird is commonly seen
perching on an overhanging limb, or flying slowly up or down some small
stream. The great blue heron (_Ardea herodias_) is common over the whole
country. It is four feet long and grayish blue, marked with black and
white. It may be seen standing alone in wet meadows or pastures, or
flying heavily, with head drawn back and long legs outstretched. It
breeds singly, but oftener in great heronries, in trees or bushes. Its
large bulky nests contain three to six dull, greenish-blue eggs about
two and one-half inches long. The white egrets of the Southern States
are shot for their plumes and have been locally exterminated in some
places. The night-herons (_Nycticorax_) differ from the other forms in
having both the neck and legs short. The bittern (_Botaurus
lentiginosus_), Indian hen, stake-driver, or thunder-pumper, as it is
variously called, is a familiar member of the order, found in marshes
and wet pastures, and known by its extraordinary call, sounding like the
"strokes of a mallet on a stake." In color it is brownish, freckled and
streaked with tawny whitish and blackish. Its nest is made on the
ground; its eggs, from three to five in number, are brownish drab and
about two inches long.
=The cranes, rails, and coots (Paludicolæ).=--The cranes, of which
three species are known in North America, are large birds with long
legs and neck, part of the head being naked or with hair-like
feathers. The rare whooping crane (_Grus americana_) is pure white
with black on the wings, and is fifty inches long from tip of bill to
tip of tail. The sand-hill crane (_G. mexicana_) is slaty gray or
brownish in color, never white, and although rare in the East is quite
common in the South and West. Cranes build nests on the ground, and
lay but two eggs, about four inches long, brownish drab in color with
large irregular spots of dull chocolate-brown.
The rails are smaller than the cranes, with short wings and very short
tail. They live in marshes and swamps, and in flying let the legs hang
down. Their legs are strong, and for escape they trust more to speed in
running than to flight. They are hunted for food. The most abundant rail
is the "Carolina crake" or "sora" (_Porzana carolina_), small and
olive-brown with numerous sharp white streaks and specks. Many of these
birds are shot each year during migration in the reedy swamps of the
Atlantic States. The American coot or mud-hen (_Fulica americana_), dark
slate-color with white bill, is one of the most familiar pond-birds over
all temperate North America. Its nest consists of a mass of broken reeds
resting on the water; the eggs number about a dozen, and are clay-color
with pin-head dots of dark brown.
=The snipes, sandpipers, plover, etc. (Limicolæ).=--The large order
Limicolæ, the shore-birds, includes the slender-legged,
slender-billed, round-headed, rather small wading birds of shores and
marshes familiar to us as snipes, plovers, sandpipers, curlews,
yellow-legs, sandpeeps, turnstones, etc. Most of them are game-birds,
such forms as the woodcock and Wilson's or English snipe being much
hunted. The food of these birds consists of worms and other small
animals, which are chiefly obtained by probing with the rather
flexible, sensitive, and usually long bill in the mud or sand. The
killdeer (_Ægialitis vocifera_), familiar to all in its range by its
peculiar call and handsome markings, the upland or field plover
(_Bartramia longicauda_), with its long legs and melodious quavering
whistle, the tall, yellow-shanked "telltale" or yellow-legs (_Totanus
melanoleucus_) of the marshes and wet pastures, are among the most
widespread and familiar species of the order. On the seashore the
dense flocks of white-winged, whisking sandpipers and the quickly
running groups of plump ring-necked plover are familiar sights. One
of the largest birds of this order is the long-billed curlew
(_Numenius longirostris_) of the upland pastures. The bill of the
curlew is long and curved downwards. The nests of these shore-birds
are made on the ground and are usually little more than shallow
depressions in which the few spotted eggs (four is a common number)
are laid. The young are precocial.
=The grouse, quail, pheasants, turkeys, etc. (Gallinæ).=--The Gallinæ
include most of the domestic fowls, as the hen, turkey, peacock,
guinea-fowls, and pheasants, and the grouse, quail, partridges, and
wild turkeys. The chief game-birds of most countries belong to this
order. They have the bill short, heavy, convex, and bony, adapted for
picking up and crushing seeds and grains which compose their principal
food. Their legs are strong and usually not long, and are often
feathered very low down. The Gallinæ are mostly terrestrial in habit
and are sometimes known as the Rasores or "scratchers." Among the more
familiar wild gallinaceous birds are the quail or "Bob white"
(_Colinus virginianus_), abundant in eastern and central United
States, the ruffed grouse (_Bonasa umbellus_) of the Eastern woods,
and the prairie-chicken (_Tympanuchus americanus_) of the Western
prairies. The sage-hen (_Centrocercus urophasianus_), the largest of
the American grouse, reaching a length of two and one-half feet, is an
interesting inhabitant of the sterile sagebrush plains of the West.
The ptarmigan (_Lagopus_) or snow-grouse, represented by several
species, are found either among the rocks and snow-banks above timber
line on high mountains, or in the Arctic regions. In summer their
plumage is brown and white; in winter they turn pure white to
harmonize with the uniform snow-covering. On the Pacific coast are
several species of quail, all differing much from those of the East.
These Western species have beautiful crests of a few or several long
plume-feathers, the body-plumage being also unusually beautiful. The
eggs of all the Gallinæ are numerous and are laid in a rude nest or
simply in a depression on the ground. In many of the species polygamy
is the rule. The young are precocial.
=The doves and pigeons (Columbæ).=--The doves and pigeons constitute a
small order, the Columbæ, closely related to the Gallinæ. A
distinguishing characteristic of the Columbæ lies in the bill, which
is covered at the base with a soft swollen membrane or cere in which
the nostrils open. The members of this order feed on fruits, seeds,
and grains. Our most familiar wild species is the mourning-dove or
turtle-dove (_Zenaidura macroura_) found abundantly all over the
country. It lays two eggs in a loose slight nest in a low tree or on
the ground. The beautiful wild or passenger pigeon (_Ectopistes
migratorius_) was once extremely abundant in this country, moving
about in tremendous flocks in the Eastern and Central States. But it
has been so relentlessly hunted that the species is apparently
becoming extinct. In the Rocky and Sierra Nevada mountains is a rather
large dove, the band-tailed pigeon (_Columba fasciata_), which
subsists chiefly on acorns. The domestic pigeon represented by
numerous varieties, pouters, carriers, ruff-necks, fan-tails, etc., is
the artificially selected descendant of the rock-dove (_Columba
livia_). The young of all pigeons are altricial.
[Illustration: FIG. 138.--Screech-owl, _Megascops asio_. (Photograph
by A. L. Princeton permission of Macmillan Co.)]
=The eagles, owls, and vultures (Raptores).=--The "birds of prey"
compose one of the larger orders, the members of which are readily
recognizable. In all the bill is heavy, powerful, and strongly hooked
at the tip. The feet are strong, with long, curved claws (small in the
vultures) and are fitted for seizing and holding living prey, such as
smaller birds, fish, reptiles, and mammals which constitute the
principal food of the true raptorial species. The vultures feed on
carrion. The turkey buzzard (_Cathartes aura_) is the most familiar
of the three species of carrion-feeding Raptores found in the United
States. The buzzard nests on the ground or in hollow stumps or logs,
and lays two white eggs (sometimes only one) blotched with brown and
purplish. The largest North American vulture is the California condor
(_Pseudogryphus californianus_), which attains a length of four and
one-half feet, with a spread of wing of nine and one-half feet. Of the
eagles, the most widespread and commonest is the bald eagle (_Haliætus
leucocephalus_). It is three feet long and when adult has the head and
neck white. The golden eagle (_Aquila chrysætos_) has the neck and
head tawny brown. Of the many species of hawks, the marsh harrier
(_Circus hudsonius_), abundant all over the country and readily known
by its white rump, is one of the most familiar. The name
"chicken-hawk" is given to two or three different species of large
broad-winged hawks of the genus _Buteo_. The stout little sparrow-hawk
(_Falco sparverius_), common over the whole country, is familiar and
readily recognizable by its pronounced bluish and black wings and
black-and-white banded chestnut tail. Altogether fifty species of
hawks and eagles are found in this country. Of the owls, the barn-owl
(_Strix pratincola_) with its long triangular face and handsome
mottled and spotted tawny coat is more or less familiar; the great
horned owl (_Bubo virginianus_), the snowy owl (_Nyctea nyctea_), and
the great gray owl (_Scotiaptex cinerea_) are the common large
species, while the red screech-owl (_Megascops asio_) (fig. 138), the
most abundant owl in the country, and the strange burrowing owl
(_Speotyto cunicularia_), which lives in the holes of prairie-dogs and
ground-squirrels in the West, are familiar smaller ones. Thirty-two
species of owls are recorded from North America.
=The parrots (Psittaci).=--The parrots, of which only one species is
native in the United States, constitute an interesting order of birds,
the Psittaci. They are abundant in tropical America. They have a very
thick strongly hooked bill, with a thick and fleshy tongue. The feet
have two toes pointing forward and two backward. The plumage is
usually brightly and gaudily colored. The natural voice is harsh and
discordant, but many of the species can imitate with surprising
cleverness the speech of man. Parrots are long-lived and usually
docile, and are much kept as pets. The single native species, the
Carolina paroquet (_Conurus carolinensis_), is about a foot in length,
is green, with yellow head and neck and orange-red face. Its range
once extended from the Gulf of Mexico north to the Great Lakes, but it
has been nearly exterminated in all the States but Florida.
=The cuckoos and kingfishers (Coccyges).=--The cuckoos and kingfishers
are regarded as constituting an order, Coccyges, a small group whose
members are without any definite bond of union. Only ten species of
North American birds belong to this order. The yellow-billed and
black-billed cuckoos (_Coccyzus_) or "rain-crows" are long-tailed,
slender, lustrous drab birds, which lay their eggs in the nests of
others. They are notable for their peculiar rolling call. On the plains
and hills of California and the southwest lives the road-runner or
chaparral cock (_Geococcyx californianus_), a strange bird belonging to
the cuckoo family. It is nearly two feet long, of which length the tail
makes half. These birds run so rapidly that a horse is little more than
able to keep up with them. They feed on fruits, various reptiles,
insects, etc. The one common kingfisher of this country, the belted
kingfisher (_Ceryle alcyon_), a thick-set, heavy-billed, ashy
blue-and-white bird, is familiar along streams. As it flies swiftly
along it gives its rattling cry. It nests in deep holes in the
stream-banks, and lays six or eight crystal-white spheroidal eggs.
=The woodpeckers (Pici).=--The familiar woodpeckers and sap-suckers
compose a well-defined order, Pici, which is represented in North
America by twenty-five species. The bill of the woodpecker is stout
and strong, usually straight, fitted for driving or boring into wood;
the tongue is long, sharp-pointed, and barbed, fitted for spearing
insects. The feet have two toes turned forward and two backward; the
tail-feathers are stiff and sharp-pointed and help support the bird as
it clings to the vertical side of a tree-trunk or branch (fig. 139).
The food of most woodpeckers consists chiefly of insects, usually
wood-boring larvæ (grubs). These birds do much good by destroying many
noxious insect pests of trees. A few species, the true sap-suckers,
probably feed on the sap of trees. Their nests are made in holes in
trees, and the eggs are pure white and rounded. The harsh and shrill
cries of the woodpeckers are familiar to all.
[Illustration: FIG. 139.--The yellow-hammer, _Colaptes auratus_.
(Photograph by W. E. Carlin; permission of G. O. Shields.)]
The largest and one of the most interesting woodpeckers is the
ivory-billed (_Campephilus principalis_), twenty inches long, glossy
blue-black, with a high head-crest which is scarlet in the male. This
bird lives in the heavily wooded swamps of the Southern States. Among
the more abundant and widespread, and hence better known, woodpeckers
are the yellow-hammers (fig. 139) or flickers (_Colaptes auratus_ in
the East, _C. cafer_ in the West), the red-headed woodpecker
(_Melanerpes erythrocephalus_), with its crimson head and neck and
pure-white "vest"; and the black-and-white downy (_Dryobates
pubescens_) and hairy (_D. villosus_) woodpeckers or "sap-suckers."
The California woodpecker (_M. formicivorus_), a near relative of the
red-headed woodpecker, has the curious habit of boring small holes in
the bark of oak- or pine-trees and sticking acorns into these holes.
Sometimes thousands of acorns are put into the bark of one tree, to
which the birds come occasionally to break open some acorns and feed
on the grubs inside.
=The whippoorwills, chimney-swifts and humming-birds
(Macrochires).=--All the birds of this order are remarkable for their
power of flight. They have long and pointed wings; their feet are
small and weak and used only for perching or clinging. All feed on
insects, which are caught on the wing by the short-beaked,
wide-mouthed swifts and whippoorwills and extracted from flower-cups
by the humming-birds with their long and slender bills. The
whippoorwill (_Antrostomus vociferus_) is common in the woods of the
East and is readily known by its call. Its two brown-blotched white
eggs are laid loose on the ground or on a log or stump. The night-hawk
(_Chordeiles virginianus_), common over the whole country, is seen at
twilight flying vigorously about in its search for insects. Its
nesting habits are like those of the whippoorwill. The sooty-brown
chimney-swifts (_Chætura pelagica_), popularly confused with the
swallows, are the common inhabitants of old chimneys, in which they
build their curious saucer-shaped open-work nests. Their eggs are pure
white and number four or five. Of the humming-birds but one species,
the ruby-throat (_Trochilus colubris_), is to be found in the Eastern
States, but in the western and especially southwestern parts of the
country several other species occur. In all seventeen species have
been found in the United States. The nests (fig. 140) of the hummers
are very dainty little cups lined with hair or wool or plant-down. The
ruby-throat lays two tiny pure-white eggs.
[Illustration: FIG. 140.--Nest and eggs of ruby-throat humming-bird,
_Trochilus colubris_, seen from above, in apple-tree. (Photograph by
E. G. Tabor; permission of Macmillan Co.)]
[Illustration: FIG. 141.--Horned larks, _Otocoris alpestris_, and
snowflakes, _Plectrophenax nivalis_. (Photograph from life by H. W.
Menke; permission of Macmillan Co.)]
=The perchers (Passeres).=--Nearly one-half of the birds of North
America belong to the great order Passeres, and of all the known birds
of the world more than half are included in it. The Passeres or perching
birds include the familiar song-birds and a great majority of the birds
of the garden, the forest, the roadside, and the field. The feet of
these birds always have four toes and are fitted for perching. The
syrinx or musical apparatus is, in most, well developed. The nesting and
other domestic habits are various, but the young are always hatched in a
helpless condition and have to be fed and otherwise cared for by the
parents for a longer or shorter time. The North American species of this
order are grouped into eighteen families, as the fly-catcher family
(Tyrannidæ), the crow family (Corvidæ), the sparrows and finches
(Fringillidæ), the swallows (Hirundinidæ), the warblers (Mniotiltidæ),
the wrens (Troglodytidæ), the thrushes, robins and bluebirds (Turdidæ),
etc. In this book nothing can be said of the various species which
belong to this order. However, as the passerine birds are those which
most immediately surround us and which, by their familiar songs and
nesting habits, most interest us, the out-door study of birds by
beginning students will be devoted chiefly to the members of this order,
and many species will soon be got acquainted with. The robin and
bluebird will introduce us to the shyer and less familiar song-thrushes;
the study of the kingbird or bee-martin will interest us in some of the
other fly-catchers; from the familiar chipping sparrow and tree-sparrow
we shall be led to look for their cousins the swamp-sparrows and
song-sparrows, and the larger grosbeaks and cross-bills, and so on
through the order.
=Determining and studying the birds of a locality.=--To identify the
various species of birds in the locality of the school it will be
necessary to have some book giving the descriptions of all or most of
the species of the region, with tables and keys for tracing out the
different forms. Such manuals or keys are numerous now; the study of
birds is one of the most popular lines of nature study, and a host of
bird books has been published in the last few years. The best general
manual is Coues's "Key to the Birds of North America," which includes
not only keys for tracing and descriptions of all the known species of
birds on this continent, but also accounts of the distribution, of the
nesting and eggs, and of the plumage of the young birds, besides a
thorough introduction to the anatomy and physiology of birds, and
directions for collecting and preserving them. Jordan's "Manual of
Vertebrates" gives keys and short descriptions of the birds found east
of the Missouri River; Chapman's "Handbook of the Birds of Eastern
North America" is excellent. To be able to use these manuals it is
necessary to have the bird's body in hand; and that means usually
death for the bird. Recently there have been published several
bird-keys which attempt to make it possible to determine species, the
commoner ones at any rate, without such close examination. The birds
in these books are usually grouped wholly artificially (without any
reference to their natural relationships) according to such salient
characteristics as color, markings, size, habit of perching, or
running, or flying, etc. These characteristics are such as can
presumably be made out in the living bird by aid of an opera-glass or
often with the unaided eye. Such books make no pretence to be
scientific manuals nor to include any but the more usual and strongly
marked species. They are usually limited to the birds of a restricted
region. Such books are readily obtainable. There are several popular
illustrated "bird-magazines" devoted to accounts of the life and
habits of birds. Of these "Bird-lore" is the organ of the Audubon
Society for the Protection of Birds.
[Illustration: FIG. 142.--Western chipping sparrow, _Spizella socialis
arizonae_. (Photograph from life by Eliz. and Jos. Grinnell.)]
In trying to become acquainted with the birds of a locality it must be
borne in mind that the bird-fauna of any region varies with the
season. Some birds live in a certain region all the year through;
these are called _residents_. Some spend only the summer or breeding
season in the locality, coming up from the South in spring and flying
back in autumn; these are _summer residents_. Some spend only the
winter in the locality, coming down from the severer North at the
beginning of winter and going back with the coming of spring; these
are _winter residents_. Some are to be found in the locality only in
spring and autumn as they are migrating north and south between their
tropical winter quarters and their northern summer or breeding home;
these are _migrants_. And finally an occasional representative of
certain bird species whose normal habitat does not include the given
locality at all will appear now and then blown aside from its regular
path of migration or otherwise astray; these are _visitants_. As to
the relative importance, numerically, of these various categories
among the birds which may be found in a certain region and thus form
its bird-fauna we may illustrate by reference to a definite region. Of
the 351 species of birds which have been found in the State of Kansas
(a region without distinct natural boundaries and fairly
representative of any Mississippi valley region of similar extent), 51
are all-year residents; 125 are summer residents, 36 are winter
residents, 104 are migrants, and 35 are rare visitants.
It must also be kept in mind in using bird-keys and descriptions to
determine species that the descriptions and keys refer to adult birds
and in ordinary plumage. Among numerous birds the young of the year,
old enough to fly and as large as the adults, still differ
considerably in plumage from the latter; males differ from females,
and finally both males and females may change their plumage (hence
color and markings) with the season. The seasonal changes of plumage
accomplished by molting may be marked or hardly noticeable. "All birds
get new suits at least once a year, changing in the fall. Some change
in the spring also, either partially or wholly, while others have as
many as three changes--perhaps, to a slight extent, a few more.... It
is claimed by some that now all new colors are acquired by molt, and
by others that in some instances (young hawks) an infusion or loss,
as the case may be, of pigment takes place as the feather forms, and
continues so long as it grows."
There is much lack and uncertainty of knowledge concerning the molting
and change of plumage by birds, and careful observations by
bird-students should be made on the subject.
In connection with learning the different kinds of birds in a
locality, together with their names, observations should be made, and
notes of them recorded, on their habits and on the relation or
adaptation of structure and habit to the life of the bird. Some of the
special subjects for such observation are pointed out in the following
paragraphs. A suggestive book, treating of the adaptive structure and
the life of birds is Baskett's "The Story of the Birds."
=Bills and feet.=--The interesting adaptation of structure to special
use is admirably shown in the varying character of the bills and feet of
birds. The various feeding habits and uses of the feet of different
birds are readily observed, and the accompanying modification of bills
and feet can be readily seen in birds either freshly killed or preserved
as "bird-skins." Such skins may be made as directed on p. 467, or may be
bought cheaply of taxidermists. A set of such skins, properly named,
will be of great help in studying birds, and should be in the
high-school collection. In some cases the general structure of feet and
bills may be seen in the live birds by the use of an opera-glass. The
characters of bills and feet are much used in the classification of
birds, so that any knowledge of them gained primarily in the study of
adaptations will have a secondary use in classification work.
[Illustration: FIG. 143.--Russet-backed thrush, _Turdus ustulatus_.
(Photograph from life by Eliz. and Jos. Grinnell.)]
Note the foot of the robin, bluebird, catbird, wrens, warblers and
other passerine or perching birds. It has three unwebbed toes in
front, and a long hind toe perfectly opposable to the middle front
one. This is the _perching_ foot. Note the so-called _zygodactyl_ foot
of the woodpecker, with two toes projecting in front and partly yoked
together, and two similarly yoked projecting behind. Note the webbed
swimming foot of the aquatic birds; note the different degrees of
webbing, from the _totipalmate_, where all four toes are completely
webbed, _palmate_, where the three front toes only are bound together
but the web runs out to the claws, to the _semi-palmate_, where the
web runs out only about half way. Note the _lobate_ foot of the coots
and phalaropes. Note the long slender wading legs of the sandpipers,
snipe and other shore birds; the short heavy strong leg of the divers;
the small weak leg of the swifts and humming-birds, almost always on
the wing; the stout heavily nailed foot of the scratchers, as the
hens, grouse, and turkeys; and the strong grasping talons, with their
sharp long curving nails, of the hawks and owls and other birds of
prey. In all these cases the fitness of the structure of the foot to
the special habits of the bird is apparent.
Similarly the shape and structural character of the bill should be
noted, as related to its use, this being chiefly concerned of course
with the feeding habits. Note the strong hooked and dentate bill of
the birds of prey; they tear their prey. Note the long slender
sensitive bill of the sandpipers; they probe the wet sand for worms.
Note the short weak bill and wide mouth of the night-hawk and
whippoorwill and of the swifts and swallows; they catch insects in
this wide mouth while on the wing. Note the flat lamellate bill of the
ducks; they scoop up mud and water and strain their food from it. Note
the firm chisel-like bill of the woodpeckers; they bore into hard wood
for insects. Note the peculiarly crossed mandibles of the cross-bills;
they tear open pine-cones for seeds. Note the long sharp slender bill
of the humming-birds; they get insects from the bottom of flower-cups.
Note the bill and foot of any bird you examine, and see if they are
specially adapted to the habits of the bird.
The tongues and tails of birds are two other structures the
modifications and special uses of which may be readily observed and
studied. Note the structure and special use of the tongue and tail of
the woodpeckers; note the tongue of the humming-bird; the tail of the
grackles.
=Flight and songs.=--The most casual observation of birds reveals
differences in the flight of different kinds, so characteristic and
distinctive as to give much aid in determining the identity of birds in
nature. Note the flight of the woodpeckers; it identifies them
unmistakably in the air. Note the rapid beating of the wings of quail
and grouse; also of wild ducks; the slow heavy flapping of the larger
hawks and owls and of the crows; and the splendid soaring of the
turkey-buzzard and of the gulls. This soaring has been the subject of
much observation and study but is still imperfectly understood. The
soaring bird evidently takes advantage of horizontal air-currents, and
some observers maintain that upward currents also must be present. The
principal hopes for the invention of a successful flying-machine rest on
the power of soaring possessed by birds. The speed of flight of some
birds is enormous, the passenger-pigeon having been estimated to attain
a speed of one hundred miles an hour. The long distances covered in a
single continuous flight by certain birds are also extraordinary, as is
also the total distance covered by some of the migrants. "It is said
that some plovers that nest in Labrador winter in Patagonia, their long
wings easily carrying them this great distance."
[Illustration: FIG. 144.--Oriole's nest with skeleton of blue jay
suspended from it; the blue jay probably came to the nest to eat the
eggs, became entangled in the strings composing the nest, and died by
hanging. (Photograph by S. J. Hunter.)]
Varying even more than the manner and power of flight among different
birds are the vocal utterances, the cries and calls and singing. By
their calls and songs alone many birds may be identified although they
remain unseen. The field-student of birds comes to know them by their
songs; knows what birds they are; knows what they are doing or not
doing; knows what time in their life-season it is, whether they are
mating, or brooding, or preparing to migrate; knows whether they are
frightened, or self-confident, whether in distress or happy. Little
urging and suggestion are needed to induce the student to attend to
the songs. But the naturalist should not only hear and enjoy them, but
by observation and the recording of repeated observations, he should
come to understand the significance of the calls and songs.
As to how these sounds are made, attention has already been called (see
p. 338) to the voice-organ or syrinx. The condition of this organ varies
much in birds, as would be expected from the differing character of
vocal utterances. Dissections will make these differences apparent.
=Nesting and care of young.=--Among the birds' most interesting
instincts and habits are those domestic ones which include mating,
nest-building, and care of the young. Birds' eggs and birds' nests are
always attractive objects of search and collection for boys, and most
boys have a considerable personal knowledge of the domestic habits of
the commoner summer birds of their region. With this interest and
unsystematized knowledge as a basis the teacher should be able to get
from the class much excellent field-work and personal observation. The
first thing to undertake in this study is the gathering of data
regarding the character of the nests of different species, their
situation, the time of nesting, the participation or non-participation
of the male in nest-building, etc.; also the number of eggs, their
size and color markings, the length of incubation, the help or lack of
help of the male in brooding, etc. In connection with this gathering
of data in the field by note-taking, sketching, and photographing,
nests and eggs can be collected (see directions on page 469). Let only
one clutch of eggs of each species be taken for the common high-school
collection, and if more than one nest is desired take used and
deserted nests. When the nestlings are hatched, the bringing of food,
the defence of the home, and the teaching of the young to fly should
all be observed and noted.
Some attempt should be made to systematize the miscellaneous data
obtained. Do all the members of a group have similar nesting habits?
Note the early nesting of birds of prey; note the nests of the
woodpeckers in holes in trees; note the nesting of the various
swallows. Is there any significance in the colors and markings of
eggs? Observe the protective coloration obvious in some (see Chap.
XXXI). Are there differences in the condition of the newly hatched
nestlings? Note the helpless altricial young of the robin; the
independent precocial young of the quail.
The strong influence of the mating passion will be made plain by
observations on the fighting, love-making, singing, and general
behavior of the birds in the mating season. The expression of the
mental and emotional traits, the psychic phenomena of birds, are most
emphasized at this time, and reveal the possession among animals lower
than man of many characteristics which are too commonly ascribed as
the exclusive attributes of the human species.
[Illustration: FIG. 145.--Western robin, _Merula migratoria
propinqua_. (Photograph from life by Eliz. and Jos. Grinnell.)]
=Local distribution and migration.=--As explained in Chapter XXXII,
the geographical distribution of animals is a subject of much
importance, and offers good opportunities in its more local features
for student field-work. The field-study of the birds of a given
locality will comprise much observation bearing directly on
zoogeography or the distribution of animals. Certain birds will be
found to be limited to certain parts of even a small region, the
swimmers will be found in ponds and streams and the long-legged
shore-birds on the pond- or stream-banks, or in the marshes and wet
meadows, although a few like the upland plover, curlews, and godwits
are common on the dry upland pastures. Distinguish the ground-birds
from the birds of the shrubs and hedge-rows and these again from the
strictly forest-birds. Find the special haunts of swallows and
kingfishers. Which are the shy birds driven constantly deeper into the
wild places or being exterminated by the advance of man; which birds
do not retreat but even find an advantage in man's seizure of the
land, obtaining food from his fields and gardens?
Make a map on large scale of the locality of the school, showing on it
the topographic features of the region, such as streams, ponds,
marshes, hills, woods, springs, wild pastures, etc., also roads and
paths, and such landmarks as schoolhouses, county churches, etc. On
this map indicate the local distribution of the birds, as determined
by the data gradually gathered; mark favorite nesting-places of
various species, roosting-places of crows and blackbirds,
feeding-places, and bathing- and drinking-places of certain kinds, the
exact spots of finding rare visitants, rare nests, etc., etc. The
making of such a zoogeographical map will be a source of great
interest and profit to the students.
As already mentioned, many of the birds of a locality are "migrants,"
that is, they breed farther north, but spend the winter in more
southern latitudes. These migrants pass through the locality twice
each year, going north in the spring and south in the autumn. They are
much more likely to be observed during the spring migration than in
the fall, as the flight south is usually more hurried. The observation
of the migration of birds is very interesting, and much can be done by
beginning students. Notes should be made recording the first time each
spring a migrating species is seen, the time when it is most abundant
and the last time it is seen the same spring. Similar records should
be made showing the movements of the birds in the fall. A series of
such records covering a few years will show which are the earliest
species to appear, which the later, and which the last. Such records
of appearance and disappearance should also be kept for the summer
residents, those birds that come from the South in the spring, breed
in the locality, and then depart for the South again in the autumn.
Notes on the kinds of days, as stormy, clear, cold, warm, etc., on
which the migration seems to be most active; on the greater prevalence
of migratory flights by day or by night; on the height from the earth
at which the migrants fly, etc., are all worth while. The Division of
Biological Survey, U. S. Department of Agriculture, keeps records of
notes on migration sent in by voluntary observers and furnishes blanks
to be filled out by each observer. A suggestive book about migration,
and one giving the records for many species at many points in the
Mississippi valley is Cooke's "Bird Migration in the Mississippi
Valley." Migration is discussed in most bird-books.
=Feeding habits, economics, and protection of birds.=--The feeding
habits of birds are not only interesting, but their determination
decides the economic relation of birds to man, that is, whether a
particular bird species is harmful or beneficial to man. Casual
observation shows that birds eat worms, grains, seeds, fruits,
insects. A single species often is both fruit-eating and
insect-eating. Do fruits or do insects compose the chief food-supply
of the species? To determine this more than casual observation is
necessary. The birds must be watched when feeding at different
seasons. The most effective way of determining the kind of food which
the bird takes is to examine the stomachs of many individuals taken at
various times and localities. Much work of this kind has been done,
especially by the investigators connected with the Division of
Biological Survey of the U. S. Department of Agriculture, and
pamphlets giving the results of these investigations can be had from
the Division. It has been distinctly shown that a great majority of
birds are chiefly beneficial to man by eating noxious insects and the
seeds of weeds. Many birds commonly reputed to be harmful, and for
that reason shot by farmers and fruit-growers, have been proved to do
much more good than harm. Some few birds have been proved to be, on
the whole, harmful. An investigation of the food habits of the crow, a
bird of ill-repute among farmers, based on an examination of 909
stomachs shows that about 29 per cent of the food for the year
consists of grain, of which corn constitutes something more than 21
per cent, the greatest quantity being eaten in the three winter
months. All of this must be either waste grain picked up in fields and
roads, or corn stolen from cribs and shocks. May, the month of
sprouting corn, shows a slight increase over the other spring and
summer months. On the other hand the loss of grain is offset by the
destruction of insects. These constitute more than 23 per cent of the
crow's yearly diet, and the larger part of them are noxious. The
remainder of the crow's food consists of wild fruit, seeds and various
animal substances which may on the whole be considered neutral.
[Illustration: FIG. 146.--Sickle-billed thrasher, _Harporhynchus
redevivus_. (Photograph from life by Eliz. and Jos. Grinnell.)]
The slaughter of birds for millinery purposes has become so fearful
and apparent in recent years that a strong movement for their
protection has been inaugurated. Rapacious egg-collecting, legislation
against birds wrongly thought to be harmful to grains and fruit, and
the selfish wholesale killing of birds by professional and amateur
hunters, help in the work of destruction. Apart from the brutality of
such slaughter, and the extermination of the most beautiful and
enjoyable of our animal companions, this destruction[18] works
strongly against our material interests. Birds are the natural enemies
of insect pests, and the destroying of the birds means the rapid
increase and spread, and the enhanced destructive power of the pests.
It is asserted by investigators that during the past fifteen years the
number of our common song-birds has been reduced to one-fourth. At the
present rate, says one author, extermination of many species will
occur during the lives of most of us. Already the passenger-pigeon and
Carolina paroquet, only a few years ago abundant, are practically
exterminated. Protect the birds!
FOOTNOTE:
[18] One of the most unfortunate and conspicuous examples of this
slaughter is the partial extermination of the song-birds of Japan in
the interests of European milliners. To meet their demands the country
people used birdlime throughout the woods with disastrous
effectiveness, as shown in the present exceeding scarcity of birds and
the abundance of insect pests.
CHAPTER XXVIII
BRANCH CHORDATA (_Continued_). CLASS MAMMALIA:
THE MAMMALS
THE MOUSE (_Mus musculus_)
TECHNICAL NOTE.--It is best to catch specimens alive in a good trap.
A live trap well baited and placed in some old granary should
furnish plenty for class use. White mice can often be obtained at
"bird-stores." When mice are not procurable, use rats. A rat is
perhaps preferable on account of its size, but all essential
structures can readily be made out in the mouse. Specimens should be
killed by chloroform as described for the toad, p. 5.
=Structure= (fig. 147).--Compare the external characters of the mouse
with those of the toad and sparrow. The mouse, unlike the other
vertebrates so far studied, is thickly covered with _hair_ all over its
body except on the tip of the nose and the soles of the feet. Where are
the _nostrils_ placed? What are the large leaf-like expansions called
_pinnæ_ situated just back of the eyes? Pull open the _mouth_ and note
the large _incisor teeth_ on the upper and lower jaws. Cut one corner of
the mouth back and observe the large flat-topped _molar teeth_ on both
jaws. How does the attachment of the large fleshy _tongue_ differ from
the condition in the toad? The toad's tongue is for snapping up insects,
whereas in the mouse this organ serves to move food about in the mouth.
On the tongue are numerous small _taste-papillæ_. Notice the long hairs,
"feelers," on each side of the nose. Note the similarity between the
front paws and our own hands; each has four fingers with a small
rudimentary thumb on the inner side of the paw. How does the hind foot
of the mouse differ from the foot of man? Posteriorly the body is
terminated by a long _tail_. At the root of the tail is a small
aperture, the _anus_, and just below, or ventral to it, is the opening
from the kidneys and reproductive organs.
TECHNICAL NOTE.--Place the mouse on its back in a dissecting-pan and
cut through the skin from anus to the lower jaw. Extend the legs,
pin down each foot and pin out the cut edges of the skin. Now
carefully cut forward through the body-wall from the anal region and
on through the breast-bones and ribs. Pin each side out.
Near the hindmost pair of _ribs_ note a sheet of muscles, the
_diaphragm_, which extends across the body-cavity, dividing it into an
anterior portion, the _thoracic cavity_, and a posterior, the _abdominal
cavity_. What are the most conspicuous organs in the thoracic cavity?
Leading anteriorly to the mouth-cavity is a long tube, the _trachea_,
composed of a series of cartilaginous parts of rings placed end to end.
Note at its anterior end the _glottis_ and _epiglottis_. Insert a
blowpipe into the glottis and inflate the _lungs_, which will fill all
the otherwise unfilled space in the thoracic cavity. The abdominal
cavity contains the _viscera_ suspended in a fold of the lining
membrane, as in the other vertebrates studied. Note lying against the
diaphragm a large, red, glandular structure, the _liver_. Separate the
two large lobes of the liver and expose the opalescent _gall-bladder_.
By passing a canula into this and ligaturing, the _cystic duct_ may be
injected. Beneath the liver is a large loop-shaped expansion of the
alimentary canal, the _stomach_. Arising from the right end of the
stomach is the narrow _duodenum_, which gradually merges into the very
much convoluted _small intestine_, or _ileum_, which is followed by the
_large intestine_, or _colon_, the last part of which is a straight
tube, the _rectum_. The small intestine occupies most of the space in
the peritoneal cavity. Within the loop of the pylorus will be found an
irregular pinkish mass of tissue, the _pancreas_. Beneath the stomach on
the left side of the body lies a very dark glandular mass not much
unlike the liver but altogether detached from it. This structure is the
_spleen_, a ductless gland.
Note dorsally of the trachea a long tube passing through the diaphragm
and connecting the mouth with the stomach. What is this tube? Note the
_Eustachian tubes_ extending from the mouth to the ears. The median
part of the roof of the mouth is the _palate_, hard in front, soft
behind. A pair of small bodies at the sides of the soft palate near
its hinder end are the _tonsils_. At the posterior angle of the lower
jaw are glandular bodies, the _sub-maxillary glands_, which lead by a
short _duct_ anteriorly to open on the floor of the mouth. On the
sides of the neck just below the ears are pink or yellowish bodies,
the _parotid glands_, opening anteriorly in the sides of the
mouth-cavity. These two sets of glands are collectively known as the
_salivary glands_, the function of which is to secrete the saliva.
Push apart the sub-maxillary glands and note below them overlying the
trachea on either side two dark-red lobes connected by a band of
tissue. These constitute the _thyroid gland_, another of the so-called
ductless glands. Within the thoracic cavity anterior to the heart note
a mass of pinkish tissue, the _thymus gland_. Observe the large
_masseter muscles_, which cover the jaws. What is their function? On
either side of the neck lies a large blood-vessel, the _external
jugular vein_, which collects blood from the head and carries it down
to the heart. Note the large _pectoral muscles_ which cover the breast
and extend out into the arms, and which are so strong and highly
developed in the sparrow. The head is supported by large muscles which
run down the back of the neck to the ribs. Others are attached to the
ribs, which they raise and lower. These movements, together with the
contraction of the diaphragm, cause the expansion and contraction of
the thoracic cavity whereby the lungs are regularly filled and
emptied. Note that the abdomen is covered by a double layer of
muscular tissue, the outer part made up of the _external oblique
muscles_, the inner by the _internal oblique muscles_.
[Illustration: FIG. 148.--Diagram of the circulation of the blood in a
mammal; _a_, auricles; _l_, lung; _lv_, liver; _p_, portal vein
bringing blood from the intestine; _v_, ventricles; the arrows show
the direction of the current; the shaded vessels carry venous blood,
the others arterial blood. (From Kingsley.)]
Examine the _heart_. How many _auricles_ has it? The _ventricles_ in the
mouse, as in the bird, are entirely separated, forming two complete
compartments, a _right_ and a _left ventricle_. The blood flowing from
the veins of the body is collected in the right auricle, thence it
passes into the right ventricle, whence it is conveyed to the lungs;
returning it flows through the left auricle into the left ventricle,
whence it is forced through the arteries of the body. For a study of the
circulatory system in mammals (fig. 148), a rat or a rabbit should be
injected by the teacher and an advanced text-book, as Parker's "Zootomy"
or Marshall and Hurst's "Practical Zoology," used as a guide. A sheep's
heart is very good to cut open for a class demonstration.
Make a drawing of the organs observed thus far in the dissection.
The _kidneys_ in the mouse are situated in the dorsal region next to
the backbone. They consist of two bean-shaped smooth glands. From them
a pair of ducts, the _ureters_, can be traced down to a median
thin-walled muscular sac, the _bladder_. The bladder opens to the
exterior of the body by means of a short tube, the _urethra_. Cut open
a kidney longitudinally and examine the cut surfaces.
The two egg-glands of the female mouse lie in the median portion of
the abdominal cavity, somewhat below the kidneys, and from the
vicinity of each runs an egg-tube. These tubes meet below the bladder,
and open to the exterior of the body through the aperture noted below
the anus. In the posterior parts of these tubes lie until birth the
developing embryos.
TECHNICAL NOTE.--For a study of the nervous system place the
specimen ventral side down and cut through the skull with the
bone-cutters or heavy scissors, exposing the brain and spinal cord.
Note the large _brain_ (fig. 149), composed of small _optic lobes_,
large _cerebrum_, _cerebellum_, and _medulla oblongata_, followed by
the long _spinal cord_. Note the nerves arising from the brain and
spinal cord.
[Illustration: FIG. 149.--Diagram of brains of vertebrates; _Olf. L._,
olfactory lobes; _Cbr._, cerebrum; _Md. Br._, midbrain (optic lobes);
_Cbl._, cerebellum; _Med. Ob._, medulla oblongata; _Sp. Cd._, spinal
cord. (From specimens.)]
For a careful dissection of the mammalian nervous system a larger
mammal, such as a cat or dog or rabbit, should be used. For guide use
a text-book such as, for the dog, Howell's "Dissection of the Dog";
for the cat, Reighard and Jennings' "Anatomy of the Cat"; and for the
rabbit, Parker's "Zootomy" or Marshall and Hurst's "Practical
Zoology." Make a good preparation of the brain and preserve it for
future use in some fluid like Fischer's fluid (see page 453).
TECHNICAL NOTE.--Prepare a well-cleaned skeleton by boiling a
specimen in a soap solution and thoroughly cleansing it (see p.
452).
Note the very compact _skeleton_ of the mouse. Note the closely sutured
_skull_. How many _cervical_ or _neck vertebræ_ are there? The _ribs_
are attached to the _thoracic vertebræ_. How many pairs of ribs? The
bony thorax supports the _shoulder-girdle_ and bones of the fore legs.
The thorax is followed by a series of ribless vertebræ, the _lumbar
vertebræ_, which in the posterior region of the body fuse with the
_pelvic girdle_ supporting the hind limbs. The body vertebræ are
succeeded by the very much smaller _caudal vertebræ_. Compare the
skeleton of the mouse with that of the bird; also with that of the toad.
For directions for a detailed study of the skeleton see in Parker's
"Zootomy" an account of the skeleton of the rabbit, pp. 263-286.
TECHNICAL NOTE.--For the study of the eye (fig. 150) the teacher
should obtain the eye of some large mammal, as the ox or sheep,
with which to make a class demonstration. The eye of a rabbit or
cat can of course be used. For an account of the vertebrate eye
see Parker and Haswell's "Text-book of Zoology," Vol. II. pp.
103-107. For a study of the ear use a bird or mammal, and see pp.
107-110 of the same book.
[Illustration: FIG. 150.--Diagram of vertebrate eye; _c_, choroid;
_i_, iris; _l_, lens; _n_, optic nerve; _r_, retina; _s_, sclerotic.
(From Kingsley.)]
=Life-history and habits.=--The house-mouse is not a native of North
America, but was introduced into this country from Europe, to which,
in turn, it came from Asia, its original habitat. The mouse came to
this country in the vessels of early explorers. Similarly the brown
and black rats, now so abundant all over North America, and members of
the same genus as the mouse, were introduced from Europe. Accompanying
man in his travels the mouse has spread from Asia until it is now to
be found over the whole world.
The habits of mice are well known; their fondness for living in our
homes and outbuildings makes them familiar acquaintances. Their food
is varied; they seem to thrive best, however, on a vegetable diet.
Grains and nuts are favorite foods. The house-cat is their greatest
enemy, but man takes advantage of their instinct to go into holes by
constructing traps with funnel or tunnel entrances which, baited with
cheese or other favorite food, are fatally attractive. In climbing,
mice are aided by the tail. Their strong hind legs enable them to
stand erect, and even to take several steps in this posture. They can
swim readily, although naturally they rarely take to water. Their
special senses are keen, the senses of hearing and taste being
unusually well developed. Their "singing," which has been the subject
of much discussion, seems to be actually a voluntary and normal
performance which, however, hardly deserves to be called singing, but
rather a slightly varied peeping or whistling.
The mouse is a prolific mammal, producing from four to six times a
year broods of from four to eight young. The mouse makes a cosy nest
of straw, bits of paper, feathers, wool or other soft materials, and
in this the young are born. The newly born mice are very small and are
blind and helpless. They are odd little creatures, being naked and
almost transparent. They grow rapidly, being covered with hair in a
week, although not opening their eyes for about two weeks. A day or
two after their eyes are open they begin to leave the nest, and hunt
for food for themselves.
OTHER MAMMALS
The mammals constitute the highest group of animals, including man,
the monkeys and apes, the quadrupeds, the bird-like bats and fish-like
seals and whales; in all about 2500 species. They are found everywhere
except on a few small South Sea islands. Only a few species, however,
have a world-wide distribution. The name Mammalia is derived from the
mammary or milk glands with which the females are provided and by the
secretion of which the young of this class, born free in all but a few
of the lowest forms, are nourished for some time after birth. In size
mammals range from the tiny pigmy-shrew and harvest mouse, which can
climb a stem of wheat, to the great sulphur-bottom whale of the
Pacific Ocean, which attains a length of a hundred feet and a weight
of many tons. Mammals differ from fishes and batrachians and agree
with reptiles and birds in never having external gills; they differ
from reptiles and agree with birds in being warm-blooded and in having
a heart with two distinct ventricles and a complete double
circulation; finally, they differ from both reptiles and birds in
having the skin more or less clothed with hair, the lungs freely
suspended in a thoracic cavity separated from the abdominal by a
muscular partition, the diaphragm, and in the possession by the
females of mammary glands. In economic uses to man mammals are the
most important of all animals. They furnish the greater portion of the
animal food of many human races, likewise a large amount of their
clothing. Horses, asses, oxen, camels, reindeer, elephants, and llamas
are beasts of burden and draught; swine, sheep, cattle, and goats
furnish flesh, and the two latter milk for food; the wool of sheep,
the furs of the carnivores, and the leather of cattle, horses, and
others are used for clothing, while the bones and horns of various
mammals serve various purposes.
[Illustration: FIG. 147.--Dissection of the Mouse, _Mus musculus_.]
=Body form and structure.=--The mammalian body varies greatly. Its
variety of form and general organization is explained by the facts
that, although most of the species live on the surface of the earth,
some are burrowers in the ground, some flyers in the air, and some
swimmers in the water. Mammals never have more than two pairs of
limbs; in most cases both pairs are well developed and adapted for
terrestrial progression. In the aerial bats the fore limbs are
modified into organs of flight; among the aquatic seals, sea-lions,
walruses, and whales both sets are modified to be swimming flippers or
paddles. In many of these aquatic forms the hind limbs are greatly
reduced or even completely wanting.
Most mammals are externally clothed with hair, which is a peculiarly
modified epidermal process. Each hair, usually cylindrical, is
composed of two parts, a central pith containing air, and an outer
more solid cortex; each hair rises from a short papilla sunk at the
bottom of a follicle lying in the true skin. In some mammals the hairs
assume the form of spines or "quills," as in the porcupine. The hairy
coat is virtually wanting in whales and is very sparse in certain
other forms, the elephant, for example, which has its skin greatly
thickened. The claws of beasts of prey, the hooves of the hoofed
mammals, and the outer horny sheaths of the hollow-horned ruminants
are all epidermal structures.
The bones of mammals are firmer than those of other vertebrates,
containing a larger proportion of salts of lime. Among the different
forms the spinal column varies largely in the number of vertebræ, this
variation being chiefly due to differences in length of tail. Apart
from the caudal vertebræ their usual number is about thirty. The
mammalian skull is very firm and rigid, all the bones composing it,
excepting the lower jaw, the tiny auditory ossicles, and the slender
bones of the hyoid arch, being immovably articulated together. The
correspondence between the bones of the two sets of limbs is very
apparent. The number of digits varies in different mammals, and also
in the fore and hind limbs of a single species. Among the Ungulates
the reduction in the number of digits is especially noticeable; the
forefoot of a pig has four digits, that of the cow two, and that of
the horse one. The two short "splint" bones in the horse are remnants
of lost digits. The teeth are important structures in mammals, being
used not only for tearing and masticating food, but as weapons of
offence and defence. A tooth consists of an inner soft pulp (in old
teeth the pulp may become converted into bone-like material)
surrounded by hard white dentine or ivory, which is covered by a thin
layer of enamel, the hardest tissue known in the animal body. A hard
cement sometimes covers as a thin layer the outer surface of the root,
and may also cover the enamel of the crown. The teeth in most forms
are of three groups: (_a_) the incisors, with sharp cutting edges and
simple roots, situated in the centre of the jaw; (_b_) the canines,
often conical and sharp-pointed, next to the incisors; (_c_) next the
molars, broad and flat-topped for grinding, and divided into premolars
and true molars. There is great variety in the character and
arrangement of these structures in mammals, their variations being
much used in classification. The number and arrangement of the teeth
is expressed by a dental formula, as, for example, in the case of man
2--2 1--1 2--2 3--3
_i_----, _c_----, _p_----, _m_---- = 32.
2--2 1--1 2--2 3--3
[Illustration: FIG. 151.--A group of Rocky Mountain sheep, or "big
horns," _Ovis canadensis_, including males, females and young.
(Photograph by E. Willis from specimens mounted by Prof. L. L. Dyche,
University of Kansas.)]
The mouth is bounded by fleshy lips. On the floor of the mouth is the
tongue, which bears the taste-buds or papillæ, the organs of taste.
The œsophagus is always a simple straight tube, but the stomach varies
greatly, being usually simple, but sometimes, as in the ruminants
and whales, divided into several distinct chambers. The intestine in
vegetarian mammals is very long, being in a cow twenty times the
length of the body. In the carnivores it is comparatively short--in a
tiger, for example, but two or three times the length of the body.
The blood of mammals is warm, having a temperature of from 35° C. to
40° C. (95° F. to 104° F.). It is red in color, owing to the
reddish-yellow, circular, non-nucleated blood-corpuscles. The
circulation is double, the heart being composed of two distinct
auricles and two distinct ventricles. Air is taken in through the
nostrils or mouth and carried through the windpipe (trachea) and a
pair of bronchi to the lungs, where it gives up its oxygen to the
blood, from which it takes up carbonic-acid gas in turn. At the upper
end of the trachea is the larynx or voice-box, consisting of several
cartilages attaching by one end to the vocal cords and by the other to
muscles. By the alteration of the relative position of these
cartilages the cords can be tightened or relaxed, brought together or
moved apart, as required to modulate the tone and volume of the voice.
The kidneys of mammals are more compact and definite in form than those
of other vertebrates. In all mammals except the Monotremes they
discharge their product through the paired ureters into a bladder,
whence the urine passes from the body by a single median urethra.
Mammary glands, secreting the milk by which the young are nourished
during the first period of their existence after birth, are present in
both sexes in all mammals, though usually functional in the female only.
[Illustration: FIG. 152.--A group of moose, _Alce americana_, showing
male, female, and young. (Photograph by E. Willis from specimens
mounted by Prof. L. L. Dyche, University of Kansas.)]
The nervous system and the organs of special sense reach their highest
development in the mammals. In them the brain is distinguished by its
large size, and by the special preponderance of the forebrain or
cerebral hemispheres over the mid- and hind-brain. Man's brain is
many times larger than that of all other known mammals of equal bulk
of body, and three times as large as that of the largest-brained ape.
In man and the higher mammals the surface of the forebrain is thrown
into many convolutions; among the lowest the surface is smooth. Of the
organs of special sense, those of touch consist of free nerve-endings
or minute tactile corpuscles in the skin. The tactile sense is
especially acute in certain regions, as the lips and end of the snout
in animals like hogs, the fingers in man, and the under surface of the
tail in certain monkeys. All the other sense-organs are situated on
the head. The organs of taste are certain so-called taste-buds located
in the mucous membrane covering certain papillæ on the surface of the
tongue. The organ of smell, absent only in certain whales, consists of
a ramification of the olfactory nerves over a moist mucous membrane in
the nose. The ears of mammals are more highly developed than those of
other vertebrates both in respect to the greater complexity of the
inner part and the size of the outer part. A large outer ear for
collecting the sound-waves is present in all but a few mammals. A
tympanic membrane separates it from the middle ear in which is a chain
of three tiny bones leading from the tympanum to the inner ear,
composed of the three semicircular canals and the spiral cochlea. The
eyes (fig. 150) have the structure characteristic of the vertebrate
eye, consisting of a movable eyeball composed of parts through which
the rays of light are admitted, regulated, and concentrated upon the
sensitive expansion, retina, of the optic nerve lining the posterior
part of the ball. The eye is protected by two movable lids. In almost
all mammals below the Primates there is a third lid, the nictitating
membrane. In some burrowing rodents and others the eye is quite
vestigial and even concealed beneath the skin.
=Development and life-history.=--All mammals except the Monotremes
give birth to free young. The two genera of Monotremes produce their
young from eggs hatched outside the body; _Tachyglossus_ lays one egg
which it carries in an external pouch, while _Ornithorhynchus_
deposits two eggs in its burrow. The embryo of other mammals develops
in the lower portion of the egg-tube, to the walls of which it is
intimately connected by a membrane called the placenta. (In the
kangaroos and opossums, Marsupialia, there is no placenta.) Through
this placenta blood-vessels extend from the body of the mother to the
embryo, the young developing mammal thus deriving its nourishment
directly from the parent.
The duration of gestation (embryonic or prenatal development in the
mother's body) varies from three weeks with the mouse, eight weeks with
the cat, nine months with the stag, to twenty months with the elephant.
Like the birds, the young of some mammals, the carnivores for example,
are helpless at birth, while those of others, as the hoofed mammals, are
very soon able to run about. But all are nourished for a longer or
shorter time by the milk secreted by the mammary gland of the mother.
=Habits, instinct, and reason.=--Despite the wonderful examples of
instinct and intelligence shown by many insects and by the other
vertebrates, especially the birds, it is among mammals that we find
the highest development of these qualities and of reason. In the wary
and patient hunting for prey by the carnivora, in the gregarious and
altruistic habits of the herding hoofed mammals, in the highly
developed and affectionate care of the young shown by most mammals,
and in the loyal friendship and self-sacrifice of dogs and horses in
their relations to man, we see the culmination among animals of the
development of the functions of the nervous system. In the
characteristics of intelligence and reason man of course stands
immensely superior to all other animals, but both intelligence and
reason are too often shown by many of the other mammals not to make us
aware that man's mental powers differ only in degree, not in kind,
from those of other animals.
Pure instinct is hereditary, and purely instinctive actions are common
to all the individuals of a species. Those actions which the individual
could not learn by teaching, imitation, or experience are instinctive.
The accurate pecking at food by chicks just hatched from an incubator is
purely instinctive. Purely instinctive also is the laying of eggs by a
butterfly on a certain species of plant which may have to be sought for
over wide acres, so that the caterpillars when hatched shall find
themselves on their own special food-plant. Yet the butterfly never ate
of this plant and will never see its young. Such elaborate instincts as
these have been developed from the simplest manifestations of sensation
and nervous function, just as the complex structures of the body have
been developed from simple structures (see Chapter XXIX).
The feeding and domestic habits and the whole general behavior of
animals are extremely interesting subjects of observation and study.
And such observation intelligently pursued will be of much value. The
point to be kept ever in mind is that all animal habits are connected
with certain conditions of life; that in every case there is an answer
to the question "why." This answer may not be found; in many cases it
is extremely difficult to get at, but often it is simple and obvious
and can be found by the veriest beginner.
=Classification.=--The mammals of North America represent eight orders.
Three additional mammalian orders, namely, the Monotremata, including
the extraordinary duck-bills (_Ornithorhynchus_) and a species of
_Tachyglossus_ in Australia and Tasmania; the Edentata, including the
sloths, armadillos, and ant-eaters found in tropical regions; and the
Sirenia, including the marine manatees and dugongs, are not represented
(except by a single manatee) in North America. In the following
paragraphs some of the more familiar mammals representing each of the
eight orders represented in North America are referred to.
=The opossums (Marsupialia).=--The opossum (_Didelphys virginiana_) is
the only North American representative of the order Marsupialia, the
other members of which are limited exclusively to Australia and
certain neighboring islands. The kangaroos are the best known of the
foreign marsupials. After birth the young are transferred to an
external pouch, the marsupium, on the ventral surface of the mother,
in which they are carried about and fed. The opossum lives in trees,
is about the size of a common cat, and has a dirty-yellowish woolly
fur. Its tail is long and scaly, like a rat's. Its food consists
chiefly of insects, although small reptiles, birds, and bird's eggs
are eaten. When ready to bear young the opossum makes a nest of dried
grass in the hollow of a tree, and produces about thirteen very small
(half an inch long) helpless creatures. These are then placed by the
mother in her pouch. Here they remain until two months or more after
birth. Probably all the North American opossums found from New York to
California and especially common in the Southern States belong to a
single species, but there is much variety among the individuals.
=The rodents or gnawers (Glires).=--The rabbits, porcupines, gophers,
chipmunks, beavers, squirrels, and rats and mice compose the largest
order among the mammals. They are called the rodents or gnawers
(Glires) because of their well-known gnawing powers and proclivities.
The special arrangement and character of the teeth are characteristic
of this order. There are no canines, a toothless space being left
between the incisors and molars on each side. There are only two
incisor teeth in each jaw (rarely four in the upper jaw), and these
teeth grow continuously and are kept sharp and of uniform length by
the gnawing on hard substances and the constant rubbing on each other.
The food of rodents is chiefly vegetable.
Of the hares and rabbits the cottontail (_Lepus nuttalii_) and the
common jack-rabbit (_L. campestris_) are the best known. The
cottontail is found all over the United States, but shows some
variation in the different regions. There are several species of
jack-rabbits, all limited to the plains and mountain regions west of
the Mississippi River. The food of rabbits is strictly vegetable,
consisting of succulent roots, branches, or leaves. Rabbits are very
prolific and yearly rear from three to six broods of from three to six
young each. There are two North American species of porcupines, an
Eastern one, _Erethizon dorsatus_, and a Western one, _E. epixanthus_.
The quills in both these species are short, being only an inch or two
in length, and are barbed. In some foreign porcupines they are a foot
long. They are loosely attached in the skin and may be readily pulled
out, but they cannot be shot out by the porcupine, as is popularly
told. The little guinea-pigs (_Cavia_), kept as pets, are South
American animals related to the porcupines.
The pocket gophers, of which there are several species mostly
inhabiting the central plains, are rodents found only in North
America. They all live underground, making extensive galleries and
feeding chiefly on bulbous roots. The mice and rats constitute a large
family of which the house-mice and rats, the various field-mice, the
wood-rat (_Neotoma pennsylvanica_) and the muskrat (_Fiber
zibethicus_) are familiar representatives. The common brown rat (_Mus
decumanus_) was introduced into this country from Europe about 1775,
and has now nearly wholly supplanted the black rat (_M. rattus_), also
a European species, introduced about 1544. The beaver (_Castor
canadensis_) is the largest rodent. It seems to be doomed to
extermination through the relentless hunting of it for its fur. The
woodchuck or ground-hog (_Arctomys monax_) is another familiar rodent
larger than most members of the order. The chipmunks and
ground-squirrels are commonly known rodents found all over the
country. They are the terrestrial members of the squirrel family, the
best known arboreal members of which are the red squirrel (_Sciurus
hudsonicus_), the fox-squirrel (_S. ludovicianus_), and the gray or
black squirrel (_S. carolinensis_). The little flying squirrel
(_Sciuropterus volans_) is abundant in the Eastern States.
=The shrews and moles (Insectivora).=--The shrews and moles are all
small carnivorous animals, which, because of their size, confine their
attacks chiefly to insects. The shrews are small and mouse-like; certain
kinds of them lead a semi-aquatic life. There are nearly a score of
species in North America. Of the moles, of which there are but few
species, the common mole (_Scalops aquaticus_) is well known, while the
star-nosed mole (_Condylura cristata_) is recognizable by the peculiar
rosette of about twenty cartilaginous rays at the tip of its snout.
Moles live underground and have the fore feet wide and shovel-like for
digging. The European hedgehogs are members of this order.
=The bats (Chiroptera).=--The bats (fig. 153), order Chiroptera,
differ from all other mammals in having the fore limbs modified for
flight by the elongation of the forearms and especially of four of the
fingers, all of which are connected by a thin leathery membrane which
includes also the hind feet and usually the tail. Bats are chiefly
nocturnal, hanging head downward by their hind claws in caves, hollow
trees, or dark rooms through the day. They feed chiefly on insects,
although some foreign kinds live on fruits. There are a dozen or more
species of bats in North America, the most abundant kinds in the
Eastern States being the little brown bat (_Myotis subulatus_), about
three inches long with small fox-like face, high slender ears, and a
uniform dull olive-brown color, and the red bat (_Lasiurus borealis_),
nearly four inches long, covered with long, silky, reddish-brown fur,
mostly white at tips of the hairs.
[Illustration: FIG. 153.--The hoary bat, _Lasiurus cinereus_.
(Photograph from life by J. O. Snyder.)]
=The dolphins, porpoises, and whales (Cete).=--The dolphins,
porpoises, and whales (Cete) compose an order of more or less
fish-like aquatic mammals, among which are the largest of living
animals. In all the posterior limbs are wanting, and the fore limbs
are developed as broad flattened paddles without distinct fingers or
nails. The tail ends in a broad horizontal fin or paddle. The Cete are
all predaceous, fish, pelagic crustaceans, and especially squids and
cuttlefishes forming their principal food. Most of the species are
gregarious, the individuals swimming together in "schools." The
dolphins and porpoises compose a family (Delphinidæ) including the
smaller and many of the most active and voracious of the Cete. The
whales compose two families, the sperm-whales (Physeteridæ) with
numerous teeth (in the lower jaw only) and the whalebone whales
(Balænidæ) without teeth, their place being taken in the upper jaw by
an array of parallel plates with fringed edges known as "whalebone."
The great sperm-whales or cachalots (_Physeter macrocephalus_) found
in southern oceans reach a length (males) of eighty feet, of which the
head forms nearly one-third. Of the whalebone whales, the
sulphur-bottom (_Balænoptera sulfurea_) of the Pacific Ocean, reaching
a length of nearly one hundred feet, is the largest, and hence the
largest of all living animals. The common large whale of the Eastern
coast and North Atlantic is the right whale (_Balæna glacialis_); a
near relative is the great bowhead (_B. mysticetus_) of the Arctic
seas, the most valuable of all whales to man. Whales are hunted for
their whalebone and the oil yielded by their fat or blubber. The story
of whale-fishing is an extremely interesting one, the great size and
strength of the "game" making the "fishing" a hazardous business.
[Illustration: FIG. 154.--Male elk or wapiti, _Cervus canadensis_.
(Photograph by E. Willis from specimen mounted by Prof. L. L. Dyche,
University of Kansas.)]
[Illustration: FIG. 155.--Antelope, male, female, and young,
_Antilocapra americana_. (Photograph by E. Willis from specimens
mounted by Prof. L. L. Dyche, University of Kansas.)]
=The hoofed mammals (Ungulata).=--The order Ungulata includes some of
the most familiar mammal forms. Most of the domestic animals, as the
horse, cow, hog, sheep, and goat, belong to this order, as well as the
familiar deer, antelope, and buffalo of our own land and the
elephant, rhinoceros, hippopotamus, giraffe, camel, zebra, etc.,
familiar in zoological gardens and menageries. The order is a large
one, its members being characterized by the presence of from one to
four hooves, which are the enlarged and thickened claws of the toes.
The Ungulates are all herbivorous, and have their molar teeth fitted
for grinding, the canines being absent or small. The order is divided
into the Perissodactyla or odd-toed forms, like the horse, zebra,
tapir, and rhinocerus, and the Artiodactyla or even-toed forms, like
the oxen, sheep, deer, camels, pigs, and hippopotami. The
Artiodactyls comprise two groups, the Ruminants and Non-ruminants. All
of the native Ungulata of our Northern States belong to the Ruminants,
so called because of their habit of chewing a cud. A ruminant first
presses its food into a ball, swallows it into a particular one of the
divisions of its four-chambered stomach, and later regurgitates it
into the mouth, thoroughly masticates it, and swallows it again, but
into another stomach-chamber. From this it passes through the other
two into the intestine.
The deer family (Cervidæ) comprises the familiar Virginia or red deer
(_Odocoileus americanus_) of the Eastern and Central States and the
white-tailed, black-tailed, and mule deers of the West, the
great-antlered elk or wapiti (_Cervus canadensis_) (fig. 154), the
great moose (_Alce americana_) (fig. 152), largest of the deer family,
and the American reindeer or caribou (_Rangifer caribou_). All species
of the Cervidæ have solid horns, more or less branched, which are shed
annually. Only the males (except with the reindeer) have horns. The
antelope (_Antilocapra americana_) (fig. 155) common on the Western
plains also sheds its horns, which, however, are not solid and do not
break off at the base as in the deer, but are composed of an inner
bony core and an outer horny sheath, the outer sheath only being shed.
The family Bovidæ includes the once abundant buffalo or bison (_Bison
bison_) (frontispiece), the big-horn or Rocky Mountain sheep (_Ovis
canadensis_) (fig. 151), and the strange pure-white Rocky Mountain
goat (_Oreamnos montanus_). The buffalo was once abundant on the
Western plains, travelling in enormous herds. But so relentlessly has
this fine animal been hunted for its skin and flesh that it is now
practically exterminated (fig. 156). A small herd is still to be found
in Yellowstone Park, and a few individuals live in parks and
zoological gardens. In all of the Bovidæ the horns are simple, hollow,
and permanent, each enclosing a bony core.
=The carnivorous mammals (Feræ).=--The order Feræ includes all those
mammals usually called the carnivora, such as the lions, tigers, cats,
wolves, dogs, bears, panthers, foxes, weasels, seals, etc. All of them
feed chiefly on animal substance and are predatory, pursuing and
killing their prey. They are mostly fur-covered and many are hunted
for their skin. They have never less than four toes, which are
provided with strong claws that are frequently more or less
retractile. The canine teeth are usually large, curved, and pointed.
[Illustration: FIG. 156.--A buffalo, _Bison bison_, killed for its
skin and tongue, on the plains of Western Kansas thirty years ago.
(Photograph by J. Lee Knight.)]
While most of the Feræ live on land, some are strictly aquatic. The true
seals, fur-seals, sea-lions, and walruses comprise the aquatic forms,
all being inhabitants of the ocean. The true seals, of which the common
harbor seal (_Phoca vitulina_) is our most familiar representative, have
the limbs so thoroughly modified for swimming that they are useless on
land. The fur-seals, sea-lions, and walruses use the hind legs to
scramble about on the rocks or beaches of the shore. The fur-seals
(fig. 157) live gregariously in great rookeries on the Pribilof or Fur
Seal Islands, and the Commander Islands in Bering Sea.
The bears are represented in our country by the widespread brown,
black, or cinnamon bear (_Ursus americanus_) and the huge grizzly bear
(_U. horribilis_) of the West. The great polar bear (_Thalarctos
maritimus_) lives in arctic regions. The otters, skunks, badgers,
wolverines, sables, minks, and weasels compose the family Mustelidæ,
which includes most of the valuable fur-bearing animals. Some of the
members of this family lead a semi-aquatic or even strictly aquatic
life and have webbed feet. The wolves, foxes, and dogs belong to the
family Canidæ. The coyote (_Canis latrans_), the gray wolf (_C.
nubilus_), and the red fox (_Vulpes pennsylvanicus_) are the most
familiar representatives of this family, in addition to the dog (_C.
familiaris_), which is closely allied to the wolf. "Most carnivorous
of the carnivora, formed to devour, with every offensive weapon
specialized to its utmost, the Felidæ, whether large or small, are,
relatively to their size, the fiercest, strongest, and most terrible
of beasts." The Felidæ or cat family includes the lions, tigers,
hyenas, leopards, jaguars, panthers, wildcats, and lynxes. In this
country the most formidable of the Felidæ is the American panther or
puma (_Felis concolor_). It reaches a length from nose to root of tail
of over four feet. Its tail is long. The wildcat (_Lynx rufus_) is
much smaller and has a short tail.
[Illustration: FIG. 157.--The Lukanin rookery of fur seals,
_Callorhinus alascanus_, on St. Paul Island, Pribilof Group, Bering
Sea. (Photograph from life by the Fur Seal Commission.)]
=The man-like mammals (Primates).=--The Primates, the highest order of
mammals, includes the lemurs, monkeys, baboons, apes, and men. Man
(_Homo sapiens_) is the only native representative of this order in
our country. All the races and kinds of men known, although really
showing much variety in appearance and body structure, are commonly
included in one species. The chief structural characteristics which
distinguish man from the other members of this order are the great
development of his brain and the non-opposability of his great toe.
Despite the similarity in general structure between him and the
anthropoid apes of the Old World, in particular the chimpanzee and
orang-outang, the disparity in size of brain is enormous.
[Illustration: FIG. 158.--"Bob Jordan," a monkey of the genus
_Cercopithecus_. (Photograph from life by D. S. Jordan.)]
The lowest Primates are the lemurs found in Madagascar, in which
island they include about one-half of all the mammalian species found
there. The brain is much less developed in the lemurs than in any of
the other monkeys. The monkeys and apes may be divided into two
groups, the lower, platyrrhine monkeys, found in the New World, and
the higher, catarrhine forms, limited to the Old World. The
platyrrhine monkeys have wide noses in which the nostrils are
separated by a broad septum and with the openings directed laterally.
These monkeys are mostly smaller and weaker than the Old World forms
and are always long-tailed, the tail being frequently prehensile. They
include the howling, squirrel, spider, and capuchin monkeys common in
the forests of tropical South America. The catarrhine monkeys have the
nose-septum narrow and the openings of the nostrils directed forwards,
and the tail is wanting in numerous members of the group. They include
the baboons, gorillas, orang-outangs, and chimpanzees. These apes have
a dentition approaching that of man, and in all ways are the animals
which most nearly resemble man in physical character.
PART III
ANIMAL ECOLOGY
CHAPTER XXIX
THE STRUGGLE FOR EXISTENCE, ADAPTATION,
AND SPECIES-FORMING
TECHNICAL NOTE.--Multiplication, or increase by geometric ratio,
among animals can be illustrated by noting the many eggs laid by a
single female moth or beetle or fly or mosquito or any other
common insect (or almost any other non-mammalian animal). The
production of many live young by each female rose aphid can be
readily seen; the number of young in a litter of kittens or pups
or rabbits is a good illustration. From this geometric increase it
is obvious that there must be a great crowding of animals and a
struggle among them for existence. This struggle and the downfall
of the many and success of the victorious few can be observed by
rearing in a small jar of water all the young of a single brood of
water-tigers (larva of _Dyticus_) or other aquatic predaceous
insect. The strongest young will live by killing and eating the
weaker of their own kind. In a spider's egg-sac the young after
hatching do not immediately leave the sac, but remain in it for
several days. During this time they live on each other, the
strongest feeding on the weaker. Thus out of many spiderlings
hatched in each sac comparatively few issue. This can be readily
observed. Open several egg-sacs and count the eggs in them. Let
the spiderlings hatch and issue from some other egg-sacs belonging
to the same species of spider. The number of issuing spiderlings
will always be much less than that of the eggs. The actual working
of natural selection and the forming of new species can of course
be seen only in results, and not in process. The great variety of
adaptation, the fitness of adaptive structures, can be readily
illustrated among the commonest animals. Animals showing certain
striking and unusual adaptations will perhaps make the matter more
obvious. To all teachers will occur numerous opportunities of
illustrating, by reference to actual processes or to obvious
results, the principles of this chapter.
=The multiplication and crowding of animals.=--In the reproduction or
multiplication of animals the production of young proceeds in
geometric ratio, that is, it is truly a multiplication. Any species of
animal, if its multiplication proceeded unchecked, would sooner or
later be sufficiently numerous to populate exclusively the whole
world. The elephant is reckoned the slowest breeder of all known
animals. It begins breeding when thirty years old and goes on breeding
until ninety years old, bringing forth six young in the interval, and
surviving until a hundred years old. Thus after about eight hundred
years there would be, if all the individuals lived to their normal age
limit, 19,000,000 elephants alive descended from the first pair. A few
years more of unchecked multiplication of the elephant and every foot
of land on the earth would be covered by them. But the rate of
multiplication of other animals varies from a little to very much
greater than that of the elephant. It has been shown that at the
normal rate in increase in English sparrows, if none were to die save
of old age, it would take but twenty years to give one sparrow to
every square inch in the State of Indiana. The rate of increase of an
animal, each pair producing ten pairs annually and each animal living
ten years, is shown in the following table:
Years. Pairs produced. Pairs alive at end of year.
1 10 11
2 110 121
3 1,210 1,331
4 13,310 14,641
5 146,410 161,051
10 ...... 25,937,424,600
20 ...... 700,000,000,000,000,000,000
Some animals produce vast numbers of eggs or young; for example, the
herring, 20,000; a certain eel, several millions; and the oyster from
500,000 to 16,000,000. Supposing we start with one oyster and let it
produce one million of eggs. Let each egg produce an oyster which in
turn produces[19] one million of eggs, and let these go on increasing
at the same rate. In the second generation there would be one million
million of oysters, and in the fourth, i.e. the great great
grandchildren of the first oyster, there would be one million million
million million of oysters. The shells of these oysters would just
about make a mass the size of the earth.
But it is obvious that all the new individuals of any animal produced do
not live their normal duration of life. All animals produce far more
young than can survive. As a matter of fact, which we may verify by
observation, the number of individuals of animals in a state of nature
is, in general, about stationary. There are about as many squirrels in
the forest one year as another, about as many butterflies in the field,
about as many frogs in the pond. Some species increase in numbers, as
for example, the rabbit in Australia, which was introduced there in 1860
and in fifteen years had become so abundant as to be a great pest. Other
species decrease, as the buffaloes, which once roamed our great plains
in enormous herds and are now represented by a total of a few hundred
individuals, and the passenger-pigeon, whose migrating flocks ten years
ago darkened the air for hours in parts of the Mississippi valley, where
now it is a rare bird. But the hand of man is the agent which has helped
to increase or to check the multiplication of these animals. In nature
such quick changes rarely occur.
=The struggle for existence.=--The numbers of animals are stationary
because of the tremendous mortality occasioned by the constant preying
on eggs and young and adults by other animals, because of strenuous and
destructive climatic and meteorological conditions, and because there is
not space and food for all born, not even, indeed, for all of a single
species, let alone all of the hundreds of thousands of species which now
inhabit the earth. There is thus constantly going on among animals a
fearful _struggle for existence_. In the case of any individual this
struggle is threefold: (1) with the other individuals of his own species
for food and space; (2) with the individuals of other species, which
prey on him, or serve as his prey, or for food and space; and (3)
finally with the conditions of life, as with the cold of winter, the
heat of summer, or drouth and flood. Sometimes one of these struggles is
the severer, sometimes another. With the communal animals the struggle
among individuals is lessened--they help each other; but when the
struggle with the conditions of life are easiest, as in the tropics or
in the ocean, the struggle among individuals becomes intensified. Each
strives to feed itself, to save its own life, to produce and safeguard
its young. But in spite of all their efforts only a few individuals out
of the hosts produced live to maturity. The great majority are destroyed
in the egg or in adolescence.
=Variation and natural selection.=--What individuals survive of the many
which are born? Those best fitted for life; those which are a little
stronger, a little swifter, a little hardier, a little less readily
perceived by their enemies, than the others. They are the winners in the
struggle for existence; they are the survivors. And this survival of the
fittest, as it is called, is practically a process of selection by
Nature. Nature selects the fittest to live and to perpetuate the
species. Their progeny again undergo the struggle and the selecting
process, and again the fittest live. And so on until adjustment or
harmonizing of animals' bodies and habits with the conditions of life,
with their environment, comes to be extremely fine and nearly perfect.
It is evident, of course, that such a natural selection or survival of
the fittest and consequent adaptation to environment presupposes
differences among the individuals of a species. And this is an
observed fact. No two individuals, although of the same brood, are
exactly alike at birth; there always exist slight variations in
structure and performance of functions. And these slight variations
are the differences which determine the fate of the individual. One
individual is a little larger or stronger or swifter or hardier than
its mates. The existence of this variation we know from our
observation of the young kittens or puppies of a brood. So it is with
all animals. Thus natural selection depends upon two factors, namely,
the excess in the production of new individuals and the consequent
struggle for existence among them, and the existence of variations
which give certain individuals slight advantages in this struggle.
=Adaptation and adjustment to surroundings.=--The action of natural
selection obviously must, and does, result in a fine adaptation and
adjustment of the structure and habits of animals to their
surroundings. If a certain species or group of individuals cannot
adapt itself to its environment, it will be crowded out by others that
can. A slight advantageous variation comes in time by the continuously
selective process to be a well-developed adaptation.
The diverse forms and habits possessed by animals are chiefly
adaptations to their special conditions of life. The talons and beak
of the eagle, the fishing-pouch of the pelican, the piercing
chisel-like bill of the woodpecker, and the sensitive probing-bill of
the snipe are adaptations connected with the special feeding habits of
these birds. The quills of the porcupine, the poison-fangs of the
rattlesnake, the sting of the yellow-jacket, and the antlers of the
deer are adaptations for self-defence. The fins and gills of fishes,
the shovel-like fore feet of the mole, the wings of birds and insects
and bats, the toe-pads of the tree-toad, the leaping-legs of the
grasshopper, all these are adaptations concerned with the special
life-surroundings of these animals.
Adaptations may relate to habits and behavior as well as to structure.
Plainly adaptive are such habits as the migration of birds and some
other animals, most of the habits connected with food-getting, and
especially striking and interesting those connected with the production
and care of the young, including nest-making and home-building.
=Species-forming.=--It is evident that through the cumulative action
of natural selection, animals of a structural type considerably (even
unlimitedly) different from any original type may in time be produced
by the gradual modification of the original type under new conditions.
If, for example, a few individuals of a mainland species should come
to be thrown as waifs of wave and storm upon an island, and if these
should be able to maintain themselves there and produce young,
increasing so as to occupy the new territory, there would be produced
in time a new type of individual conforming or adapted to the
conditions obtaining in the island, these conditions being, of course,
almost certainly different from those of the mainland. Thus as an
offshoot or derivation from the original type still existing on the
mainland we should have the new island-inhabiting type. Now when these
island individuals come to differ so much, structurally and
physiologically, from the mainland type that they cannot, even if
opportunity offers, successfully mate or interbreed with mainland
individuals the island type constitutes a new species. That is, our
distinction between species rests not only on structural differences,
but on the impossibility of interbreeding (at least for the production
of fertile young). Such a combination of the action of natural
selection and the condition of isolation (as illustrated by the case
of island animals), is probably the most potent factor in the
production of new species of animals (and plants).
For accounts of the struggle for existence, variations, adaptations,
natural selection and species-forming see Darwin's "Origin of Species,"
Wallace's "Island Life," and Romanes' "Darwin and After Darwin," I.
=Artificial selection.=--When a selection among the individuals of a
species, that is, the choosing and preserving of individuals which
show a certain trait or traits and the destroying of those individuals
not possessing this trait, is done by man, it is called artificial
selection. To artificial selection we chiefly owe all the many races
or varieties of our domesticated animals and plants. For example, from
the ancestral jungle fowl have been developed by artificial selection
(and by cross-breeding) all our kinds of domestic fowl, as Brahmas,
black Spanish, bantams, game-cocks, etc.; from the wild rock-dove have
been developed our various fancy pigeons, as carriers, pouters,
fantails, etc.
For an account of artificial selection see Darwin's "Plants and Animals
under Domestication," and Romanes' "Darwin and After Darwin," I.
FOOTNOTE:
[19] Oysters are hermaphroditic, each individual producing both sperm-
and egg-cells.
CHAPTER XXX
SOCIAL AND COMMUNAL LIFE, COMMENSALISM
AND PARASITISM
=Social life and gregariousness.=--TECHNICAL NOTE.--Students should
refer to examples of gregariousness from their own observations of
animals. The roosting together of crows and of blackbirds; the
gathering of swallows preparatory to migration; the flocking of
geese and ducks, with leaders, in their migratory flights, all can
be readily observed. From observation or general reading students
will be more or less familiar with prairie-dog villages, beaver-dams
and marshes, the one-time great herds of bison, etc.
The struggle for existence is always operative; but in some cases one
or more phases of it may be ameliorated. For example, the amelioration
of the struggle among individuals of one species obtains in a lesser
or greater degree in the case of those animals which exhibit a _social
life_, of which mutual aid and mutual dependence are the basis. The
honey-bee and the ants are familiar examples of animals which show a
high degree of social life. They live, indeed, a truly communal life,
where the fate of the individual is bound up in the fate of the
community. But there are many animals which show a much lower degree
of mutual aid and a far less coherent society. The simplest form of
social life exists among those animals in which many individuals of
one species keep together, forming a great band or herd. In this case
there is not nearly so much mutual aid or mutual dependence as in that
of the honey-bee, and the safety of the individual is not wholly bound
up in the fate of the herd. Such animals are said to be _gregarious_
in habit, and this gregariousness is undoubtedly advantageous to the
individuals of the band. The great herds of reindeer in the North, and
of the bison or buffalo which once ranged over the Western American
plains are examples of a gregariousness in which mutual protection
from enemies, like wolves, seems to be the principal advantage gained.
The bands of wolves which hunted the buffalo show the advantage of
mutual help in aggression as well as in protection. Prairie-dogs live
in great villages or communities which spread over many acres. By
shrill cries they tell each other of the approach of enemies, and they
seem to visit each other and to enjoy each other's society a great
deal, although that they are thus afforded much actual active help is
not apparent. The beavers furnish a well-known and very interesting
example of mutual help; they exhibit a communal life although a simple
one. They live in villages or communities, all helping to build the
dam across the stream which is necessary to form the marsh or pool in
which the nests or houses are built.
=Communal life.=--TECHNICAL NOTE.--See technical notes, pp. 212
_et seq_, for directions for work in connection with the study of
the communal life of ants, bees, and wasps.
When many individuals of a species live together in a community in which
the different kinds of work are divided more or less distinctly among
the different members and where each individual works primarily for the
whole and not for himself; where there is, in other words, a thorough
mutual help and mutual dependence among the members of the community
accompanied by a division of labor, the life of the species is truly
communal. Those animals which show the most elaborate and specialized
_communal life_ are the termites or white ants, the social bees and
wasps, and the true ants. Of these the ants and honey-bees stand first.
As already explained (see pp. 220 _et seq_), there are among these
communal insects several different kinds of individuals in each species.
With most animals there are two kinds only, males and females, which may
or may not show differences in color, form, etc., so that they are
readily distinguishable. Among all the communal insects, however, there
are always three kinds of individuals, males, females, and workers,
these last being infertile individuals. With the social wasps and social
bees the workers are all infertile females and are smaller than the
fertile forms; with the termites there are besides the fertile males and
females, which are winged, workers which are wingless, and also peculiar
wingless individuals called soldiers which have very large jaws and
whose business it is to fight off attacking enemies of the community.
Among the ants the workers are also wingless, while the males and
females are winged. The worker ants in many species are of two kinds,
so-called worker majors and worker minors, differing markedly in size.
All the ant workers are good soldiers, but with some the fighting is
done almost wholly by certain especially large-headed and large-jawed
ones which may be called soldier-workers.
Thus among all strictly communal animals there is a specialization or
differentiation of individuals accompanying the division of labor.
Special individuals have a certain part of the work of the community
to do, and they are specially modified in structure to do this work.
This structural modification may make them incapable of performing
certain other labor or work which is necessary for their living and
which must be done for them, therefore, by others. Thus the mutual
interdependence of the individuals composing a colony is very real.
The worker honey-bees cannot perpetuate the species; honey-bees would
die out were it not for the males and females. But the males and
females have given up the functions of food-getting and of caring for
their young; did not the workers do these things for them, the
community would die out quite as soon.
The advantages of communal or social life, of co-operation and mutual
aid are real. Those animals that have adopted such a life are among
the most successful of all in the struggle for existence. The termite
worker is one of the most defenseless and for those animals that prey
on insects one of the most toothsome insects, and yet the termite is
one of the most abundant and successfully living insect kinds in all
the tropics. Ants are everywhere and are everywhere successful. The
honey-bee is a popular type of successful life. The artificial
protection afforded it by man may aid it in its struggle for
existence, but it gains this protection because of certain features of
its communal life, and in nature the honey-bee takes care of itself
well. Co-operation and mutual aid are among the most important factors
which help in the struggle for existence.
=Commensalism.=--TECHNICAL NOTE.--Examine ants' nests to find
myrmecophilous insects. If on the seashore search for hermit-crabs
with sea-anemones on shell. If inland, try to have some preserved
specimens showing the crabs and sea-anemones.
The phases of living together and mutual help just discussed concerned
in each instance a single species of animal. All the members of a pack
of wolves or of a honey-bee community belong to a single species. But
there are numerous instances known of the mutually advantageous
association of individuals of two different species. Such an
association is called _commensalism_ or _symbiosis_.
The hermit-crabs live, as has been learned (p. 154), in the shells of
molluscs, most of the body of the crab being concealed within the
shell, only the head and the grasping and walking legs protruding. In
some species of hermit-crabs there is always to be found on the shell
near the opening a sea-anemone. "This sea-anemone is carried from
place to place by the crab, and in this way is much aided in obtaining
food. On the other hand, the crab is protected from its enemies by the
well-armed and dangerous tentacles of its companion. On the tentacles
there are many thousand long slender stinging threads, and the fish
that would eat the hermit-crab must first deal with the stinging
anemone." If the sea-anemone be torn away from the shell the crab will
wander about seeking another anemone. When he finds one, he struggles
to loosen it from the rock to which it is attached, and does not rest
until he has torn it loose and placed it on his shell.
In the case of the hermit-crab and the sea-anemone there is no doubt
of the mutual advantage derived from their communal life. But this
mutual advantage is not so obvious in some cases of commensalism,
where indeed most or all of the advantage often seems to lie with one
of the animals, while the other derives little or none, but on the
other hand suffers no injury. For example, "small fish of the genus
_Nomeus_ may often be found accompanying the beautiful Portuguese
man-of-war (_Physalia_) as it sails slowly about on the ocean's
surface. These little fish lurk underneath the float among the various
hanging thread-like parts of the man-of-war which are provided with
stinging cells. They are protected from their enemies by their
proximity to these stinging threads, but of what advantage to the
man-of-war their presence is is not understood." Similarly in the
nests of the various species of ants and termites many different kinds
of other insects have been found. "Some of these are harmful to their
hosts, in that they feed on the food-stores gathered by the
industrious and provident ant, but others appear to feed only on
refuse or useless substances in the nest. Some may be of help to their
hosts by acting as scavengers. Over one thousand species of these
myrmecophilous (ant-loving) and termitophilous (termite-loving)
insects have been recorded by collectors as living habitually in the
nests of ants and termites."
=Parasitism.=--TECHNICAL NOTE.--As examples of temporary external
parasites find and examine fleas and ticks on dogs and cats, red
mites on house-flies and grasshoppers (at the bases of the wings),
etc. As examples of permanent external parasites find bird-lice on
pigeons or domestic fowls or on other birds. Note the absence of
wings and the peculiarly modified body shape of these parasites.
Examine a bird-louse under the microscope; note the absence of
compound eyes (it has simple eyes) and absence of wings; note bits
of feathers, its food, in stomach, showing through the body. Find,
as examples of internal parasites, intestinal worms or flukes.
Examine trichinized pork to see _Trichinæ_ in muscles. Examine
preserved specimens of tapeworms. Collect pupæ of some common
butterfly or moth and keep them in the schoolroom until either the
butterflies or ichneumon flies issue. Some will surely be
parasitized, and yield ichneumon flies (parasites) instead of a
butterfly. As examples of degeneration by quiescence examine
barnacles (found on outer rocks of seashore at low tide; easily
obtained as preserved specimens by inland schools) and the females
of scale-insects. These insects may be found on oleanders (the
black scale, _Lecanium oleæ_) or fruit-trees (the San Jose scale,
_Aspidiotus perniciosus_). Note the great degeneration of the
adult female of the San Jose scale; it has no eyes, antennæ,
wings, or legs. The young may be found crawling about at certain
times of the year; they have eyes, antennæ and legs.
In addition to the various ways of living together among animals,
already described, namely, the social and communal life of individuals
of a single species and the commensal and symbiotic life of
individuals of different species, there is another and very common
kind of association among animals. This is the association of parasite
and host; the association between two sorts of animals whereby one,
the parasite, lives on or in the other, the host, and at the expense
of the host. In this association the parasite gains advantages great
or small, sometimes even obtaining all the necessities of life, while
the host gains nothing, but suffers corresponding disadvantage, often
even the loss of life itself. _Parasitism_ is a phenomenon common in
all the large groups of animals, though the parasites themselves are
mostly invertebrates. There are parasitic Protozoa, worms,
crustaceans, insects, and molluscs, and a few vertebrates.
Some parasites, like the fleas and lice, live on the surface of the body
of the host. These are called _external parasites_. Others, as the
tapeworms, live exclusively inside the body; such are called _internal
parasites_. Again, some, as the bird-lice, which are external parasites
feeding on the feathers of birds, spend their whole lifetime on the
host; they are called _permanent parasites_. Others, as a flea, which
leaps on or off its host as caprice directs, or like certain parasites
which as young live free and active lives, finally attaching themselves
to some host and remaining fixed there for the rest of their lives, are
called _temporary parasites_. Such a grouping is purely arbitrary and
exists simply for the sake of convenience. It is not rigid, nor does it
class parasites in their proper natural groups.
When various parasites are examined it will be noted that practically in
all cases the body of a parasite is simpler in structure than the body
of other animals closely related to it; that is, species which live
parasitically, obtaining their food from and being carried about by a
host, have simpler bodies than related forms that live free active
lives, competing for food with other animals about them. This simplicity
is not primitive, but results from the loss or atrophy of the structures
which the special mode of life of the parasite renders useless. Many
parasites are attached firmly by hooks or suckers to their host, and do
not move about independently of it. They have no need of the power of
locomotion, and accordingly are usually without wings, legs, or other
locomotory organs. Because they have no need of locomotion they have no
need of organs of orientation, those special sense organs like the eyes,
ears, and feelers which serve to guide and direct the moving animal; and
most fixed parasites will be found to have no eyes, or any of those
organs accessory to locomotion, and which serve for the detection of
food or of enemies. Because these important organs, which depend for
their successful activity on a well-organized nervous system, are
lacking, the nervous system of parasites is usually very simple. Again,
because the parasite usually feeds on the already digested food or the
blood of its host, most parasites have a very simple alimentary canal,
or even none at all. Finally, as the fixed parasite leads a wholly
sedentary and inactive life, the breaking down and rebuilding of tissue
in its body goes on very slowly and in minimum degree, so that there is
little need of highly developed respiratory and circulatory systems; and
most fixed and internal parasites have these systems of organs decidedly
simplified. Altogether the body of a fixed permanent parasite is so
simplified and so wanting in all those special structures which
characterize the active, complex animals that it often presents a very
different appearance from those forms with which we know it to be nearly
related. This simplicity due to loss or reduction of parts is called
_degeneration_. Such simplicity of body-structure due to degeneration
is, however, essentially different in its origin from the simplicity of
the lower simpler animals. In them the simplicity of body is primitive;
they are generalized animals; the simplicity of degeneration is
acquired; it is really an adaptation, or specialization.
An excellent example of body degeneration due to the adoption of a
parasitic habit is that of _Sacculina_ (fig. 159), a crustacean
parasitic on other crustaceans, namely, crabs. The young _Sacculina_
is an active, free-swimming larva essentially like a young prawn or
crab. After a short period of independent existence it attaches itself
to the abdomen of a crab, and lives as a parasite. It completes its
development under the influence of this parasitic life, and when adult
bears absolutely no resemblance to such a typical crustacean as a crab
or crayfish. Its body external to the host crab is simply a pulsating
tumor-like sac, with no mouth-parts, no legs, and internally hardly
any well-developed organs except those of reproduction. Degeneration
here is carried very far.
[Illustration: FIG. 159.--_Sacculina_, a parasitic crustacean; _A_,
attached to a crab, the root-like processes of the parasite
penetrating the body of the host; _B_, the active larval condition;
_C_, the adult removed from its host. (After Haeckel.)]
Various parasites have been referred to in Part II under their proper
branch and class. The worms include an unusually large number of them,
such as the tapeworms, trichinæ and other intestinal forms, all of
which live as internal parasites in the alimentary canal or in other
organs of higher animals, especially the vertebrates. Many crustaceans
are parasitic, usually living, like the fish-lice, as fixed external
parasites on fishes, other crustaceans, etc., but with a free and
active larval stage. Among the insects, on the contrary, many of the
parasitic forms (as the ichneumon flies) are free and active in the
adult stage, but live as internal grubs or maggots in the larval
stage. The ichneumon flies (of the order Hymenoptera) are four-winged,
slender-bodied insects which lay their eggs either on or in (by means
of a sharp piercing ovipositor) some caterpillar or beetle grub, into
the body of which the young grub-like ichneumon larvæ burrow on
hatching. The parasites feed on the body-tissues of the host, not
attacking, however, such organs as the heart or nervous system, which
would produce the immediate death of the host. The caterpillar lives
with the ichneumon grubs within it usually until nearly time for its
pupation. Often, indeed, it pupates with the parasite still in its
body. But it never comes to maturity. The larval ichneumons pupate
either within the body of its host, or in a tiny silken cocoons
outside of its body (fig. 160). From the cocoons the winged adult
ichneumons issue; and after mating the females find another
caterpillar on whose body to lay their eggs.
[Illustration: FIG. 160.--Larva of a sphinx-moth, with cocoons of a
parasitic ichneumon fly. (From specimen.)]
Degeneration can be produced by other causes than parasitism. It is
evident that if for any other reason an animal should adopt an
inactive fixed life it would degenerate. The barnacles (see fig. 37)
are excellent examples of degeneration through quiescence. They are
crustaceans related most nearly to the crabs and shrimps. The young
barnacle just from the egg is a six-legged, free-swimming larva
(nauplius) with a single eye, greatly like a young prawn or crab. It
develops during its independent life two compound eyes and two large
antennæ. But soon it attaches itself to some stone or shell, or pile
or ship's bottom, giving up its power of locomotion, and its further
development is a degeneration. It loses its compound eyes and antennæ,
and acquires a protecting shell. Its swimming feet become modified
into grasping organs, and it loses most of its outward resemblance to
the typical members of its class. The Tunicata or ascidians compose a
whole group of animals which are fixed in their adult condition and
have thus become degenerate. They have been likened to a "mere rooted
bag with a double neck." In their young stage they are free-swimming,
active, tadpole-like or fish-like larvæ, possessing organs much like
those of the adult simplest fish or fish-like animals. Their larval
structure reveals, however, the relationships of the ascidians to the
vertebrates, a relationship which is not at all apparent in the
degenerate adults. Certain insects live sedentary or fixed lives. All
the members of one large family, the Coccidæ, or scale-insects (figs.
62 and 63), have females which as adults are wingless and in some
cases have no legs, eyes, or antennæ, while the males are all winged
and have legs and the special sense organs. The males lead a free
active life, but the females have nearly or quite given up the power
of locomotion, attaching themselves by means of their sucking beak to
some plant, where they obtain a sufficient food-supply (plant-sap) and
lay their eggs. In both males and females the larvæ are little active
crawling six-legged creatures with legs, eyes, and antennæ.
We are accustomed perhaps to think of degeneration as necessarily
implying a disadvantage in life. It is true that a blind, footless,
degenerate animal could not cope with the active, keen-sighted, highly
organized non-degenerate in free competition. But free competition is
exactly what the degenerate animal has nothing to do with. Certainly the
_Sacculina_ and the scale-insects live well; they are admirably adapted
to the kind of life they lead. A parasite enjoys certain obvious
advantages in life, and even extreme degeneration is no drawback (except
as we shall see later), but gives it a body which demands less food and
care. As long as the host is successful in eluding its enemies and
avoiding accident and injury the parasite is safe. Its life is easy as
long as the host lives. But the disadvantages of parasitism and
degeneration are nevertheless obvious. The fate of the parasite is bound
up with the fate of the host. "When the enemy of the host crab prevails,
the _Sacculina_ goes down without a chance to struggle in its own
defence. But far more important than the disadvantage in such particular
or individual cases is the fact that the parasite cannot adapt itself in
any considerable degree to new conditions. It has become so modified, so
specialized to adapt itself to the very special conditions under which
it now lives, it has gone so far in giving up organs and functions, that
if present conditions change and new ones come to exist the parasite
cannot adapt itself to them. The independent free-living animal holds
itself, one may say, able and ready to adapt itself to any new
conditions of life. The parasite has risked everything for the sake of a
sure and easy life under the present existing conditions. Change of
conditions means its extinction."
[Illustration: FIG. 161.--Young fur seals, _Callorhinus ursinus_, of
the Tolstoi rookery, St. Paul Island, Bering Sea, killed by a
parasitic intestinal worm, _Uncinaria_ sp. (Photograph by the Fur Seal
Commission.)]
For an elementary account of commensalism and parasitism see Jordan
and Kellogg's "Animal Life," pp. 172-200. The account here given is
based on the author's previously written account in "Animal Life." See
also Van Beneden's "Animal Parasites and Messmates."
CHAPTER XXXI
COLOR AND PROTECTIVE RESEMBLANCES
TECHNICAL NOTE.--For an appreciation of the reality of protective
resemblances observations must be made in the field. Examples are
easily found. Locusts, katydids, green caterpillars, lizards,
crouching rabbits, and brooding birds are readily observed instances
of general protective resemblance. For examples of variable
resemblance examine specimens of a single locust species taken from
different localities; the individuals of the various species of the
genus _Trimerotropis_ show much variation to harmonize with their
surroundings. Collect a number of larvæ (caterpillars) of one of the
swallow-tail butterflies (_Papilio_), and when ready to pupate put
them separately into pasteboard boxes lined inside with differently
colored paper. The chrysalids will show in their coloration the
influence of the different colors of the lining paper, their
immediate environment. As examples of special protective resemblance
observe inch- or span-worms (larvæ of Geometrid moths). The
walking-stick is not uncommon; many spiders that inhabit flower-cups
show striking protective color patterns; and the Graptas or
comma-butterflies which resemble dead leaves may be examined.
To illustrate warning colors, find, if possible, the larvæ
(caterpillars) of the common milkweed or monarch butterfly (_Anosia
plexippus_), and offer them to birds, at the same time offering
other caterpillars, and note the results. For terrifying or
threatening appearance find specimens of the large green tobacco-
or tomato-worm (larva of the five-spotted sphinx-moth,
_Phlegethontius carolina_), or other sphingid larvæ.
The butterflies illustrating the striking example of mimicry,
described on p. 432, can be found in most parts of the country.
Syrphid and other flies which mimic bees and wasps can readily be
found on flowers.
Each student should search for himself for examples of protective
resemblance.
=Use of color.=--The prevalence of color and the oftentimes striking
and intricate coloration patterns of animals demand some explanation.
As naturalists are accustomed to find the frequently bizarre and
seemingly inexplicable shapes and general structure of animals readily
explained by the principle of adaptation, that is, special
modification of body-structure to fit special conditions of life, so
they look to use as the chief explanation of color and markings. Some
uses are obvious; bright colors and striking patterns may serve to
attract mates or to avail as recognition marks by which individuals of
a kind may readily recognize each other. The white color of arctic
animals probably serves to help keep them warm by preventing radiation
of heat from the body; on the other hand dark color may also help to
keep animals warm by absorbing heat. "But by far the most widespread
use of color is for another purpose, that of assisting the animal in
escaping from its enemies or in capturing its prey."
It is common knowledge that the young and old, too, of many kinds of
ground-inhabiting animals, when startled by an enemy will not run, but
crouching close to the ground remain immovable, trusting to remain
unperceived. But a blue or crimson rabbit, however still it might
keep, would be easily seen by its enemy and killed. Rabbits, however,
which are good examples of animals having this habit of lying close,
are neither blue nor green nor red, but are colored very much like the
ground on which they crouch. This harmonious coloration is as
necessary to the success of this habit as is the keeping still. A
grasshopper flying or leaping in the air is conspicuous; when it
alights how inconspicuous it is! Unless one has followed it closely in
its flight and has kept the eye fixed on it after alighting it is
usually impossible to distinguish it from its surroundings. And this
is greatly to the advantage of the grasshopper in its efforts to
escape its enemies, that is, in its struggle for existence. On the
other hand a green katydid would be very conspicuous in a dusty road.
But dusty roads are precisely where katydids do not rest. They alight
among the green leaves of a tree or shrub. The animals that live in
deserts are almost all obscurely mottled with gray and brownish and
sand-color so as to harmonize in color with their habitual
environment. The arctic hares and foxes and grouse which live in
regions of perpetual snow are pure white instead of red or brown or
gray like their cousins of temperate and warm regions.
These cases of an animal's color and markings harmonizing with the
usual environment are called instances of _protective resemblance_;
that is, they are resemblances for a purpose, that purpose being to
render the animal indistinguishable from its surroundings and thus to
aid it in escaping its enemies. Such protective resemblances are
obviously of great value to animals, and, like other advantageous
modifications, have been produced by the action of natural selection.
Those individuals of a species most conspicuous and hence most readily
perceived by enemies are the first (under ordinary circumstances) to
be captured and eaten. The less conspicuous live and produce young
like themselves. Of these young the least conspicuous are again saved
and so over and over again through thousands of generations until this
natural selecting of the protectively colored results in the
production of the wonderfully specialized examples of resemblance to
which attention is called in the following paragraphs.
=General, variable, and special protective resemblance.=--In the
brooks most fishes are dark olive or greenish above and white below.
To the birds and other enemies which look down on them they are
colored like the bottom. To their fish-enemies which look up from
below they are like the white light above them in color and their
forms are not clearly seen. The green tree-frogs and tree-snakes which
live habitually among green foliage; the mottled gray and tawny
lizards and birds and small mammals of the plains and deserts, and the
white hares and foxes and owls and ptarmigan of the snowy arctic
regions--all show a general protective resemblance.
[Illustration: FIG. 162.--The twig or walking-stick insect,
_Diapheromera femorata_. (From specimen.)]
Sometimes an animal changes color when its surroundings change.
Certain hares and grouse of northern latitudes are white in winter
when the snow covers all the ground, but in summer when much of the
snow melts, revealing the brown and gray rocks and withered leaves,
they put on a grayish and brownish coat of hair or feathers. A small
insect called the toad-bug (_Galgulus_) lives abundantly on the banks
of a pond on the campus of Stanford University. The shores of this
pond are covered in some places with bits of bluish rock, in others
with bits of reddish rock, and in still others with sand. Specimens of
the toad-bug collected from the blue rocks are bluish or leaden in
color, those from the red rocks are reddish, and those from the sand
are sand-colored. Changes of color to suit the surroundings can be
quickly made by some animals. The chameleons of the tropics change
momentarily from green to brown, blackish, or golden. There is a
little fish (_Oligocottus snyderi_) common in the tide-pools of the
Bay of Monterey in California whose color changes quickly to harmonize
with the rocks it happens to rest above. Such changing coloration to
suit the surroundings may be called _variable protective resemblance_.
Very striking are those cases of protective resemblance in which the
animal resembles in color and shape, sometimes in extraordinary
detail, some particular object or part of its usual environment. This
may be called _special protective resemblance_. The larvæ of the
Geometrid moths called inch-worms or span-worms are twig-like in
appearance, and have the habit, when disturbed, of standing out
stiffly from the twig or branch on which they rest, so as to resemble
in attitude as well as color and markings a short or broken twig. To
increase this simulation the body of the larva often has a few
irregular spots or humps resembling the scars left by fallen leaves,
and it also lacks the middle prop-legs of the body common to other
lepidopterous larvæ, which would tend to destroy the illusion so
successfully carried out by it. The common twig-insect or
walking-stick (fig. 162) with its wingless, greatly elongate, brown or
greenish body and legs is when at rest quite indistinguishable from
the twigs on which it lies. Another excellent example of special
protective resemblance is furnished by the famous green-leaf insect
(_Phyllium_) of the tropics, which has broad leaf-like wings and body
of a bright green color with markings which imitate the leaf-veins,
and small irregular yellowish spots which simulate decaying or stained
or fungus-covered spots in the leaf. Most striking of all, however, is
the large dead-leaf butterfly _Kallima_ (fig. 163) of the East Indian
region. The upper sides of the wing are dark with purplish and
orange markings not at all resembling a dead leaf. But the butterflies
when at rest hold their wings together over the back, so that only the
under sides of them are exposed. These are exactly the color of a dry
dead leaf with markings mimicking midrib and oblique veins, and, most
remarkable of all, what are apparently two holes like those made in
leaves by insects, but in the butterfly imitated by two small circular
spots free from scales and hence clear and transparent. When _Kallima_
alights it holds the wings in such position that the combination of
all four produces with remarkable fidelity the simulation of a dead
leaf still attached to the twig by a short pedicel or leaf-stalk
(imitated by a short "tail" on the hind wings). The head and legs of
the butterfly are concealed beneath the wings.
[Illustration: FIG. 163.--The dead-leaf butterfly, _Kallima_ sp., a
remarkable case of special protective resemblance. (From specimen.)]
=Warning colors, terrifying appearances, and mimicry.=--While many
animals are so colored as to harmonize with their habitual or usual
environment, others on the contrary are very brightly colored and marked
in such bizarre and striking pattern as to be conspicuous. There is no
attempt at concealment; it is obvious that conspicuousness is the object
sought or at least produced by the coloration. Animals like these, we
shall find, are in almost all cases specially protected by special
weapons of defence such as stings or poison-fangs, or by the secretion
of an acrid, ill-tasting fluid in the body. Many caterpillars have been
found, by observation in nature and by experiment, to be distasteful to
insectivorous birds. Now it is obvious that it would be advantageous to
these caterpillars to be readily recognized by birds. After a few trials
the bird learns by experience to let these distasteful larvæ alone;
their conspicuous markings serve as _warning colors_. The
black-and-yellow-banded caterpillar of the common milkweed or monarch
butterfly (_Anosia plexippus_) is a good example of such protection by a
combination of distastefulness and warning coloration. The little
lady-bird beetles are mostly distasteful to birds; they are brightly and
conspicuously marked. Certain little Nicaraguan frogs have a bright
livery of red and blue, in strong contrast to the dull concealing colors
of other frogs in their region. By offering these little blue and red
frogs to hens and ducks the naturalist Belt found that they are
distasteful to the birds.
[Illustration: FIG. 164.--The larva of the pen-marked sphinx-moth,
_Sphinx chersis_, showing terrifying attitude. (After Comstock.)]
Certain animals which are without special means of defence and are not
distasteful are yet so marked or shaped, and so behave as to present a
threatening or _terrifying appearance_. The large green caterpillars
of the sphinx-moths, the tomato- and tobacco-worms, are familiar
examples, each larva having a sharp horn on the back of the next to
last body-segment (fig. 164). When disturbed the caterpillar assumes a
threatening attitude, and the horn seems to be an effective weapon of
defence. As a matter of fact it is not at all a weapon of defence,
being weak, not provided with poison, and altogether harmless.
But it would plainly be to the advantage of a defenceless animal, one
without poison-fangs or sting and without an ill-tasting substance in
its body, to be so marked and shaped as to mimic some other specially
defended or inedible animal sufficiently to be mistaken for it and
thus to escape attack. Such cases have been noted, especially among
insects. This kind of protective resemblance may be called _mimicry_.
A most striking case is that presented by the familiar monarch and
viceroy butterflies (fig. 165). The monarch (_Anosia plexippus_) is
perhaps the most abundant and widespread butterfly of our country. It
is a fact well known to entomologists that it is distasteful to birds
and is let alone by them. It is conspicuous, being large and chiefly
red-brown in color. The viceroy (_Basilarchia archippus_), also
red-brown and patterned almost exactly like the monarch, is not, as
its appearance would seem to indicate, a very near relation of the
latter, but on the contrary it belongs to a genus of butterflies all
of which, except the viceroy and one other, are black and white in
color and of different pattern from the monarch. The viceroy is not
distasteful to birds, but by its extraordinary simulation or mimicking
of the monarch it is not distinguished from it and so is not molested.
In the tropics there have been discovered numerous examples of mimicry
among insects. The members of two large families of butterflies
(Danaidæ and Heliconidæ) are distasteful to birds and are mimicked by
members of other butterfly families (especially the Pieridæ).
[Illustration: FIG. 165.--The monarch butterfly, _Anosia plexippus_
(above), distasteful to birds, and the viceroy, _Basilarchia
archippus_ (below), which mimics it. (From specimens.)]
=Alluring coloration.=--A few animals show what is called alluring
coloration; that is, they display a color pattern so arranged as to
resemble or mimic a flower or other lure, and thus entice to them other
animals, their natural prey. Certain Brazilian fly-catching birds have a
brilliantly colored crest which can be displayed in the shape of a
flower-cup. The insects attracted by the false flower furnish the bird
with food. In the tribe of fishes called the "anglers" or "fishing
frogs," the front rays of the dorsal fin are prolonged in the shape of
long slender filaments, the foremost and longest of which has a
flattened and divided extremity. The angler conceals itself in the mud
or in the cavities of a coral reef, and waves the filament back and
forth. Small fish are attracted by the lure, mistaking it for worms
writhing about. When they approach they are engulfed in the mouth of the
angler, which in some species is of enormous size. One of these angler
species is known to fishermen as the "all-mouth."
For a fuller account of protective resemblances and mimicry see Jordan
and Kellogg's "Animal Life," pp. 201-223. For still more extended
accounts see Poulton's "Colours of Animals," and Beddard's "Animal
Coloration."
CHAPTER XXXII
THE DISTRIBUTION OF ANIMALS
TECHNICAL NOTE.--The larger aspects or phenomena of the distribution
of animals over the earth on land and in sea cannot be studied
personally in the field by the student, but many local features of
distribution can be so observed and studied. The restriction of
certain kinds of animals to certain kinds of habitat, the presence
and character and effectiveness of barriers, some of the modes of
distribution, the presence and successful life of introduced foreign
species such as the black and brown rats, the English sparrow, the
German and Asiatic cockroaches, the gradual change of range or
distribution of certain kinds of animals through the influence of a
change in environment (caused by man in cutting off forests,
cultivating heretofore wild pastures, etc.) all offer favorable and
profitable opportunities for personal observation.
An excellent and feasible piece of field-work in distribution is
the making of a zoological survey of the locality in which the
school is situated. A map of the locality should be made on a
generous scale, which should include all prominent physical
features of the region, such as streams, ponds, hills, woodlands,
marshes, etc., and on this map should be indicated the places where
the various animals of the local fauna occur. Some of the animal
species will have a limited range, and the limits of this range
should be shown. This map and faunal list can be added to and
perfected by successive classes. For fuller discussions of the
geographical distribution of animals see Jordan and Kellogg's
"Animal Life," Beddard's "Zoogeography," Heilprin's "The
Distribution of Animals," and Wallace's "Geographical
Distribution."
=Geographical distribution.=--It is a matter of common knowledge with
all of us that there are no wild lions or camels or kangaroos or monkeys
or ostriches or nightingales in North America. Ostriches are found only
in Africa and South America, kangaroos only in Australia, lions only in
Asia and Africa. On the other hand there are no opossums in Europe or
grizzly bears or rattlesnakes anywhere else in the world than in this
country. That is, certain kinds of animals have a certain limited range
of occurrence or distribution. It is obvious, too, that certain animals
live only on land, while others live only in water, and of these latter
some are restricted to the ocean, while others live only in fresh water.
All of the facts regarding the dispersion or diffusion of animals on
land and in water make up the science of the _geographical distribution
of animals_, or, as it is sometimes called, _zoogeography_. Under this
subject are included not only the facts of the present actual
distribution or occurrence of animals over the world, but the facts
concerning the reasons for this distribution, the modes of travel and
dispersion, the character and influence of barriers to the spread, and
the results, in the adaptation of old forms and the production of new
forms, of the phenomena of distribution.
Just as maps are made to show graphically the facts of political
geography, which concerns the position and extent of the various
powers and States which claim the allegiance of the people, and the
facts of physical geography, which concerns the physical character of
the earth's surface, so maps are made to show the geographical
distribution of animals. Because of the great numbers of animal
species no one map can show the distribution of all species, but a
series of maps of the world or of a continent or of a State or county
or more limited region could be made (and many such have been made)
showing the distribution of selected species. In a map of a limited
locality, say of a few square miles, the occurrence and distribution
of most of the commoner and more familiar animals can be shown, and
each high school should possess such a map (see technical note at
beginning of this chapter).
=Laws of distribution.=--The laws governing the distribution of animals
over the earth's surface have been recently[20] expressed in a simple
statement as follows: Every species of animal is found in every part of
the earth unless (_a_) its individuals have been unable to reach this
region on account of barriers of some sort; or (_b_) having reached it,
the species is unable to maintain itself, through lack of capacity for
adaptation, through severity of competition with other forms, or through
destructive conditions of environment; or (_c_) having entered and
maintained itself it has become so altered in the process of adaptation
as to become a species distinct from the original type.
=Modes of migration and distribution.=--That animals should be
continually trying to extend their range is obvious from what we know
of their rapid increase by multiplication. In a region which can
provide food for but one thousand wolves, there is a production each
year of several times one thousand. These new wolves must struggle
among themselves for food, or migrate, if possible, to new regions as
yet not inhabited by wolves. The wolf's mode of migration or
distribution is walking or running, and so with other mammals except
the bats and aquatic forms. Birds and bats can fly, and can thus
migrate more swiftly, farther, and over barriers which would stop
mammals. Most insects can fly. Worms can only crawl and very slowly at
that. Fishes can swim, but if they are in a landlocked sheet of water,
they cannot go beyond its confines. Marine animals can migrate from
ocean to ocean, and land animals from continent to continent unless
checked by barriers (see next paragraph).
But besides such voluntary and independent modes of distribution long
journeyings may be made involuntarily, or by a passive migration as it
may be called. Parasites, for example, are carried by their hosts in
all their travels; the tiny Tardigrada and Rotifera, which can be
desiccated and yet restored to active life by coming again into water,
are carried in the dried mud on the feet of birds or other animals. On
floating objects in rivers or in ocean currents land-animals may be
carried long distances. Man, the greatest traveller of all, is
responsible for the widened distribution of many animals. Thus have
come to us in ships from Europe the black and brown rats, the English
sparrow, the Hessian fly, the commonest cockroaches of our houses and
many other forms. And these animals have been carried involuntarily
all over the United States in railway-cars and wagons.
=Barriers to distribution.=--As is indicated in the paragraph on the
modes of migration, a considerable sheet of water is obviously a
barrier to the further travelling of a walking or crawling
land-animal, although no barrier to a winged form. Similarly a strip
of land is a barrier to a strictly aquatic animal as a fish. Or a high
fall in the stream may serve as an insuperable barrier, making it
impossible for any fish below the fall to reach the upper part of the
stream. Numerous cases of this kind are known in the Rocky Mountains
and Sierra Nevada, where a stream may be well supplied with trout
below a fall, and quite bare of these fish above the fall. In the
Yosemite Valley in California trout live in the Merced River below the
great Vernal and Nevada falls, but above these falls the Merced
contains no trout. To fresh-water swimming animals salt water may be a
barrier; thus some kinds of fresh-water fishes are limited to one of
two near-by streams although the mouths of these streams empty near
each other into the ocean. The amphibious batrachians, at home in
fresh water and on land, are killed by contact with sea-water.
Earthworms also are killed by salt water. Thus the narrowest ocean
strait is as effective a barrier to these animals as a whole sea.
High mountain ranges and broad deserts are barriers to many
land-animals, partly because of the physical obstacles, partly because
of the differences in temperature and in food-supply.
Temperature and climate (as distinct from temperature) and the ocean are
the three great barriers when we consider the animal kingdom as a whole,
and look for the causes which determine the chief zoogeographical
divisions of the earth's surface. Most of the tropical animals cannot
endure frost, hence the isothermal line of frost is a line across which
few tropical animals venture. Most arctic animals are enfeebled by heat,
and the isothermal line which marks off the region in which frost occurs
the year round is another great zoogeographical boundary. But while
these lines are limits for localized species, some animals, as birds,
especially, keep within a relatively uniform temperature by migrations
across these lines. It should be borne in mind that the gradual decrease
in temperature met with in going north or south from the tropics is also
met in the ascent of high mountains. The summits of lofty peaks, even in
the tropics, are truly arctic in character; they are snow-covered, and
the animals and plants on them are truly arctic. Thus in the ascent of a
single mountain a whole series of life-zones from tropical to arctic can
be traversed.
Climate, as distinct from temperature, establishes limits of
distribution. The animals of Eastern North America accustomed to a humid
atmosphere cannot live in the dry plains and deserts of the West.
Closely associated with climate is the nature of the plant-growth
covering the land; here are forests and luxuriant meadows, there are
sparse tough grasses of the dry plateau. The limits of a special kind of
plant-growth often are the limits of distribution of certain animals.
The third great barrier, the ocean, is perhaps the most obvious of all
in its influence. It is only in rare cases that any land-animal can
independently cross a great ocean. Thus the land-animals of Australia
differ from those of all other countries, and those of Africa and
South America have developed almost independently of one another. The
ocean is, as already mentioned, also a barrier for fresh-water aquatic
animals, and even marine fishes which live normally in shallow waters
along the shore rarely venture across the great depths of mid-ocean.
The obstacles or barriers met with determine the limits of a species.
Each species broadens its range as far as it can. It attempts
unwittingly, through natural processes of increase, to overcome the
obstacles of ocean or river, of mountain or plain, of woodland or
prairie or desert, of cold or heat, of lack of food or abundance of
enemies--whatever the barriers may be. The degree of hindrance offered
by any barrier differs with the nature of the animal trying to pass
it. That which forms an impassable obstacle to one species may be a
great aid to the spread of another. "The river which blocks the monkey
or the cat is the highway of the fish and turtle. The waterfall which
limits the ascent of the trout is the chosen home of the ouzel."
=Faunæ and zoogeographic areas.=--The term _fauna_ is applied to the
animals of any region considered collectively. Thus the fauna of
Illinois includes the entire list of animals found naturally in that
State. The fauna of a schoolyard comprises all the animals found
living naturally in the yard. The fauna of a pond includes all the
animal inhabitants of the pond. (_Flora_ is used similarly of all the
plants in a given region.) The relation of one fauna to another
depends on the character and effectiveness of the barriers between,
and the physical character of the two regions. Thus the fauna of
Illinois differs but little from that of Indiana or Iowa, because
there are no barriers between the States, and they are alike
physically. On the other hand the fauna of California differs much
from that of the Eastern States because of the great barriers (the
desert and the Sierra Nevada Mountains) which lie between it and these
States, and because of the great differences in the physical and
climatic conditions of the two regions.
The land-surface of the earth has been divided by zoogeographers into
seven great realms of animal life, based on the distributional
characters shown by these various regions. These realms are separated
by barriers of which the chief are the presence of the sea and the
occurrence of frost. These realms are named, from their geographical
region, the Arctic, the North Temperate, the South American, the
Indo-African, the Madagascar, the Patagonian, and the Australian. Of
these the Australian alone is sharply defined. Most of the others are
surrounded by a broad fringe of debatable ground that forms a
transition to some other zone.
=Habitat and species.=--The habitat of a species of animal is the
region in which it is found in a state of nature. It is currently
believed that the habitat of any animal is the whole of that region
for which it is best adapted. But this is not necessarily true. In
fact in most cases it is not true. The trout naturally debarred from
the rivers in Yellowstone Park by the waterfalls could live there well
if the barrier could be passed. In the case of one stream it has been
passed and the trout flourish above the fall. The success of the black
and brown rats and the English sparrow in America, of the rabbit in
Australia, of bumblebees and house-flies in New Zealand, all of which
animals had a natural habitat not including these regions, but by
artificial means have been carried over the barriers and into the new
territory, prove that "habitat" is not necessarily coincident with
"only fit region." Shad, striped bass, and catfish from the Potomac
River have been introduced into and now thrive in the Sacramento River
in California. In fact the whole work of the introduction and
diffusion of valuable food-animals in territory not naturally included
in the habitat of the species is based on our knowledge that the
habitat of a species is often determined by physical barriers rather
than by exclusive fitness of environment. Within the natural habitat
the environment _is_ fit for the species' existence, outside of it the
environment _may_ be fit.
But there occur numerous instances where a species successful in leaving
its original habitat is unsuccessful in attempting to maintain itself on
new ground. Man has introduced various animals from one country to
another. The English sparrow (naturally debarred from this country by
the ocean barrier), brought to America from Europe, has covered its new
territory rapidly and maintains itself with brilliant success. But the
nightingale, the starling and skylark which have been repeatedly
introduced and set free are unable to maintain themselves here.
=Species-extinguishing and species-forming.=--Accompanying the
constant slow migrating of species into new habitats and the constant
slow changing of environment and conditions everywhere is to be seen a
constant modification of the fauna of any region due to the inability
of some species to maintain their ground, the predominating increase
of others, and the modifying or adaptive changing of others into new
forms. In 1544 the black rat of Europe was introduced into America and
it soon crowded out the native rats, being in its turn crowded out by
the European brown rat (the present common rat in buildings),
introduced about 1775. Here we have the original native species unable
to maintain itself in competition with introduced forms.
With a change of environing conditions, certain species are unable to
maintain themselves. With the destruction of the forests going on in
parts of our country the great host of wood-creatures, the bears,
squirrels, the wood-birds and insects, can no longer maintain
themselves, and grow rare and disappear. Man often also influences the
status of a species by checking its increase either by actual
slaughter, as with the bison and passenger-pigeon, or by making
adverse changes in its environment, as by destroying forests, or
putting the plains under cultivation.
In the discussion of "species-forming" (see p. 408) it was shown that
adaptation may lead to the altering of species, and to the formation
of new ones (under the influence of natural selection). With the
gradual change of conditions, or with the facing of new conditions
because of an unusual migration to or invasion of new territory, those
individuals of the species exposed to the new conditions must adapt
themselves in structure and habit in order to meet successfully the
new demands. By the cumulative action of natural selection these
adaptive changes are emphasized; and this emphasis may come to be so
pronounced that the part of the species represented in this newly
acquired territory, if isolated from the original stock, is so altered
as to be quite distinct in appearance from the old. If these changed
individuals are also physiologically distinct from the old stock, i.e.
are unable to mate with them, a new species is established. As already
mentioned, the peopling of islands from mainlands is an excellent and
readily observable example of the phenomena referred to in the third
law of distribution.
FOOTNOTE:
[20] Jordan and Kellogg's "Animal Life," 1900, p. 274.
APPENDICES
EQUIPMENT AND METHODS
APPENDIX I
EQUIPMENT AND NOTES OF PUPILS
=Equipment of pupils.=--Each pupil should have a laboratory note-book
of about 8 × 10 inches, opening at the end, in which both drawings and
notes can be made. The paper should be unruled and of good quality
(not too soft). Each pupil should have also instruments of his own as
follows: scalpel, pair of small scissors, spring forceps, pair of
dissecting-needles, small glass pipette, and paper of ribbon-pins for
pinning out specimens. The cost of this outfit need not exceed $1.00.
The laboratory should furnish him with a dissecting-dish and a
dissecting-microscope, or at least a lens.
=Laboratory drawings and notes.=--Each pupil should make the drawings
called for in the directions for the laboratory exercises. These
drawings should be in outline, and put in by pencil; the lines may be
inked over if preferred. Shading should be used sparingly, if at all.
Each drawing and all the organs and animal parts represented in it
should be fully named. See the anatomical plates in this book for
example. With such complete "labelling," little note-taking need be
done in connection with the dissections.
Notes should be made of any observations which cannot be represented
in the drawings; for example, on the behavior of the living animals.
All notes referring to matters of life-history should be dated.
=Field-observations and notes.=--Scattered through this book will be
found numerous suggestions for student field-work, for the observation
of the life-history and habits and conditions of animals in nature. As
explained in the Preface, the initiation and direction of such work
must be left to the teacher. But its importance both because of its
instructiveness and its interest is great. Pupils should not only be
incited to make individual observations whenever and wherever they
can, but the teacher should make little field-excursions with the
class or with parts of it at various times, to ponds or streams or
woods, and "show things" to all. The life-history and feeding-habits
of insects, the web-making of spiders, the flight, songs, nesting, and
care of young of birds, the haunts of fishes, the development of
frogs, toads, and salamanders, the home-building and feeding-habits of
squirrels, mice, and other familiar mammals are all (as has been
called attention to at proper places in the book) specially fit
subjects for field-observation.
Each pupil should keep a field note-book, recording from day to day,
under exact date, any observations he may make. Let the most trivial
things be noted; when referred to later in connection with other notes
they may not seem so trivial. The field note-book should be smaller than
the laboratory note- and drawing-book, small enough to be carried in the
pocket. Notes should be made on the spot of observation; do not wait to
get home. Sketches, even rough ones, may be advantageously put into the
book. Students with photographic cameras can do some very interesting
and valuable field-work in making photographs of animals, their nests
and favorite haunts. Such photographic work is very effectively used now
in the illustration of books about animals and plants (see the
reproductions of photographs in this book). If the class is making a
collection the collecting notes or data made in the field-books of the
different pupil collectors should all be transferred to a common "Notes
on Collections" book kept by the whole class.
APPENDIX II
LABORATORY EQUIPMENT AND METHODS
=Equipment of laboratory.=--The equipment of the laboratory or
classroom will, of necessity, depend upon the opportunities afforded
the teacher by the school officers to provide such facilities as
instruments, books, and charts. If dissections are to be seriously and
properly made, however, some equipment is indispensable. Flat-topped
tables, not over 30 inches high, a few compound microscopes (one is
much better than none), as many simple lenses, or, far better, simple
dissecting-microscopes, as there are students, dissecting-dishes, a
pair of bone-clippers, one injecting-syringe, a bunch of bristles,
water, a few simple reagents and some inexpensive glassware, as
slides, cover-glasses, watch-crystals, and fruit- or battery-jars for
live cages and aquaria, make up a sufficient equipment for good work.
Much can be done with less, and perhaps a little more with some
additional facilities.
The dissecting-pans should be of galvanized iron or tin, oblong, about
6 × 8 inches by 2 inches deep, with slightly flaring sides. If an iron
wire be run around the margin, and the margin bent back over it, it
will strengthen the dish, and make a broader and smoother edge for the
hands to rest on. Diagonally across the dish, about one-fourth inch
from the bottom, should run a thick wire. A layer of paraffin one-half
inch thick should cover the bottom. It should be poured in melted,
when the diagonal wire will be imbedded in it and will hold it in
place. Acids must not be put into the pan.
The reagents necessary are alcohol of 95 per cent and 85 per cent, and
formalin of 4 per cent (the formaldehyde sold by druggists is 40 per
cent and should be diluted ten times with water), these for preserving
material for dissection; chloroform for killing specimens; glycerin for
making temporary microscopic mounts, and 20 per cent nitric acid for
preparing specimens for study of the nervous system. In addition there
will be needed the few other materials mentioned in the following
paragraphs as necessary in the preparation of injecting-fluids, the
staining of fresh tissue and preserving by special methods.
A list of reference books desirable in the laboratory is appended as a
separate paragraph (see p. 454).
=Collecting and preparing material for use in the laboratory.=--As
directions have been given in the "technical notes" scattered through
the book for the collecting and preparing of all the various kinds of
animals chosen as subjects of the laboratory exercises, it will only be
necessary to give here directions for making certain special mixtures
and for the special preparation of specimens by injection, etc.
Specimens to be used for dissection should be kept in alcohol of 85 per
cent or in formalin of 4 per cent. Alcohol is better for the earthworm,
but for the other examples formalin is either better or as good, and as
it is much cheaper it may well be chosen for the general preservative.
_Methyl green_, a stain used for coloring fresh tissues. Dissolve the
methyl green powder in water, using about as much powder as the water
will take up. Add a few drops of acetic acid.
_Injecting-masses._--Injections are best made with preparations of
French gelatine, but white glue will answer most purposes. For fine
injection use a combination of the following: 1 part of a solution of
gelatine, 1 part to 4 parts of water; 1 part of a saturated solution of
lead acetate in water, and 1 part of a saturated solution of potassium
bichromate in water. A mixture of these when hot gives a beautiful
yellow injection-mass which, filtered, will pass through the finest
capillaries. For different colorings use dry paints, which come in
ultramarine blue, vermilion, and green. The gelatine should be
thoroughly soaked before the coloring-matter is added. A mistake is
generally made in using the injection-mass too thick. One part by weight
of gelatine to six or even more parts of water is a good proportion. The
gelatine as well as glue-masses should be made in a water-bath, which
consists of one dish placed within another outer one containing warm
water. The mass should be injected warm, _not hot_, after which the
injected specimen is to be placed in cold water until the injecting-mass
has set. Glue (the ordinary white kind) can be used for most injections
just as the gelatine was used, but should not be so much diluted. All
injection-masses should be filtered through a cloth before using.
_Preparing skeletons._--In general, skeletons are best cleaned by
boiling. After most of the flesh has been cut away the skeleton should
be boiled in a soap solution until the remaining parts of the muscles
are thoroughly softened. The soap solution is made of 2,000 c.c. of
water, preferably distilled, 12 grams of saltpetre, and 75 grams of
hard soap (white). Heat these until dissolved, then add 150 c.c. of
strong ammonia. This stock solution is mixed with four or five parts
of water, when the mixture is ready for use. The bones after boiling
are rinsed in cold water, brushed and picked clean, then left to dry
on a clean surface.
_Preserving anatomical preparations._--Many specimens worth keeping
will be found, and for them a solution known as Fischer's formula is
suggested as good, especially for brains. Fischer's formula is made up
as follows: 2,000 c.c. of water, 50 c.c. of formalin, 100 grams of
sodium chloride, and 15 grams of zinc chloride. These are mixed
together until thoroughly dissolved. Open preparations well before
placing them in the liquid and use about twenty times the volume of
the object to be preserved.
_To keep fresh dissections._--For materials which are dissected fresh
and must be kept over for several days in a fresh condition add a few
drops of carbolic acid to the water which covers them. Carbolized
water (2 per cent in water) will preserve a great many tissues for a
long time. Hearts will remain for years in a supple condition in this
solution.
_Obtaining marine animals, microscopic preparations, etc._--For
schools not on the seashore the marine animals such as starfishes,
etc., which are to be dissected or examined as examples of the
branches to which they belong must be obtained as preserved specimens
from dealers in such supplies. Among such dealers on the Atlantic
coast are the Marine Biological Laboratory, Woods Holl, Mass.; F. W.
Walmsley, Academy of Natural Sciences, Philadelphia, Pa.; and H. H.
and C. S. Brimley, Raleigh, N. C.; on the Pacific coast the Supply
Department, Hopkins Seaside Laboratory, Stanford University,
California. Ward's Natural Science Establishment, Rochester, N. Y.,
supplies almost any biological specimens asked for. This establishment
furnishes already made dissections and sets illustrating life-history
and metamorphosis. The few permanent microscopic preparations which
are mentioned in the book as desirable to have can be made by the
teacher if he has had any training in microscopical technic. If not,
they may be bought cheaply of such dealers in natural history
supplies as the Bausch & Lomb Optical Co., Rochester, N. Y.; the
Kny-Scheerer Co., 17 Park Place, New York City; Queen & Co., 1010
Chestnut Street, Philadelphia, Pa., and numerous others. From these
dealers also can be bought all of the laboratory supplies, such as
lenses, slides, cover-glasses, dissecting-scalpels, scissors and
needles, etc., mentioned in this book.
_Reference books._--Throughout the preceding chapters exact references
have been made to various books, as many of which as possible should
be in the school-library. Some of these references have been made with
special regard to the teacher, but most with special regard to the
pupil. All of the books referred to are included in the following
list. For the convenience of the prospective buyer, the names of the
publishers and prices of the books are appended. In buying books, it
is of course not necessary to order from the various publishers. A
list of the books desired may be handed to any book-dealer, who will
order them and who should in most cases be able to get them for a
little less than publisher's list prices.
=Baskett, J. N.= The Story of the Birds. 1899, D. Appleton & Co.
$0.65.
=Beddard, Frank.= Animal Coloration. 1892, Macmillan Co. $3.50.
---- Zoogeography. 1895, Macmillan Co. $1.60.
=Bendire, Chas.= Directions for Collecting, Preparing, and
Preserving Birds' Eggs and Nests. Distributed by U. S. National
Museum.
=Bird Lore=, an Illustrated Journal about Birds. Macmillan Co.
$1.00 a year.
=Cambridge Natural History=, Vols. V (Peripatus), $4.00, VI
(Insects), $3.50. Macmillan Co.
=Chapman, Frank.= Handbook of the Birds of Eastern North America.
1899. D. Appleton & Co. $3.00.
=Comstock, J. H.= Manual for the Study of Insects. 1897, Comstock
Publishing Co. $3.75.
---- Insect Life. 1901, D. Appleton & Co. $1.50.
---- =and Kellogg, V. L.= Elements of Insect Anatomy. 1901,
Comstock Publishing Co. $1.00.
=Cooke, W. W.= Bird Migration in the Mississippi Valley.
Distributed by the Division of Biological Survey, U. S. Dept.
Agric.
=Cowan, T. W.= Natural History of the Honey-bee. 1890, London:
Houlston. 1s. 6d.
=Coues, Elliott.= Key to North American Birds. 1890, Estes and
Lauriat. $7.50.
=Darwin, Chas.= The Formation of Vegetable Mold through the
action of Worms. D. Appleton & Co. $1.50.
---- Origin of Species. 1896, Caldwell. $0.75.
---- The Structure and Distribution of Coral Reefs. D. Appleton &
Co. $2.00.
---- Plants and Animals under Domestication. D. Appleton & Co.
=Davie, Oliver.= Methods in the Art of Taxidermy. 1894, Oliver
Davie & Co., Columbus, O. $10 net.
=Gage, S. H.= Life History of the Toad. Teacher's Leaflets No. 9,
April, 1898, prepared by College of Agriculture, Cornell
University, Ithaca, N. Y.
=Heilprin, A.= The Distribution of Animals. 1886, D. Appleton &
Co. $2.00.
=Hodge, C. F.= The Common Toad. Nature Study Leaflet, Biology
Series No. 1. 1898, published by C. H. Hodge, Worcester, Mass.
=Holland, W. J.= The Butterfly Book. 1899, Doubleday and McClure
Co. $3.00.
=Hornaday, W. T.= Taxidermy and Zoological Collecting. 1897,
Chas. Scribner's Sons. $2.50 net.
=Howell, W. H.= Dissection of the Dog. 1889, Henry Holt & Co.
$1.00.
=Huxley, T. H.= The Crayfish: an introduction to the Study of
Zoology. D. Appleton & Co. $1.75.
=Jordan, D. S.= Manual of Vertebrate Animals of the Northern
United States, 8th ed. 1899. A. C. McClurg & Co. $2.50.
---- =and Evermann, B. W.= Fishes of North and Middle America, 4
vols. 1898-1900, Distributed by U. S. National Museum.
---- =and Kellogg, V. L.= Animal Life. 1900, D. Appleton & Co.
$1.20.
=Lubbock, John.= Ants, Bees, and Wasps. 1882. D. Appleton & Co.
$2.00.
=Marshall, H. M., and Hurst, C. H.= Practical Biology, 5th ed. G.
P. Putnam's Sons. $3.50.
=Martin, H. W., and Moale, W. A.= Handbook of Vertebrate
Dissection, 3 parts. 1881, Macmillan Co.
Part 1. How to dissect a Chelonian (red-bellied slider terrapin);
Part 2. How to dissect a bird (pigeon);
Part 3. How to dissect a rodent (rat).
=McCook, Henry.= American Spiders and their Spinning Work, 3
vols. 1889-1893, H. C. McCook, Phila., Pa. $30.00.
=Miall, L. C.= The Natural History of Aquatic Insects. 1895,
Macmillan Co. $1.75.
=Parker, T. J.= A Course of Instruction in Zootomy. 1884,
Macmillan Co. $2.25.
---- Lessons in Elementary Biology. 1897, Macmillan Co. $2.65.
---- =and Haswell, W. A.= Textbook of Zoology, 2 vols. 1897,
Macmillan Co. $9.00.
=Peckham, George W. and E. J.= On the Instincts and Habits of the
Solitary Wasps. 1898, sold by Des Forges & Co., Milwaukee, Wis.
$2.00.
=Potts, E.= Fresh-water Sponges. 1887, Phil. Acad. of Sciences.
=Poulton, E. B.= The Colors of Animals. 1890, D. Appleton & Co.
$1.75.
=Reighard, J. E., and Jennings, H. S.= The Anatomy of the Cat.
1901, Henry Holt & Co. $4.00.
=Ridgway, R.= Directions for Collecting Birds. Distributed by U.
S. National Museum.
=Riverside Natural History=, 6 vols. Houghton, Mifflin & Co.
$30.00.
=Romanes, Geo.= Darwin and After Darwin, I. 1895-97, Open Court
Publishing Co.
=Scudder, S. H.= The Life of a Butterfly. 1893, Henry Holt & Co.
$1.00.
=Van Beneden, E.= Animal Parasites and Messmates. 1876, D.
Appleton & Co. $1.50.
=Wallace, A. R.= The Geographical Distribution of Animals. 1876,
Harper & Bros. $10.00.
=Wallace, A. R.= Island Life. 1881, Harper & Bros. $4.00.
APPENDIX III
REARING ANIMALS AND MAKING COLLECTIONS
Much good work in observing the behavior and life-history of some
kinds of animals can be done by keeping them alive in the schoolroom
under conditions simulating those to which they are exposed in nature.
The growth and development of frogs and toads from egg to adult, as
well as their feeding habits and general behavior, can all be observed
in the schoolroom as explained in Chapter XII. Harmless snakes are
easily kept in glass-covered boxes; snails and slugs are contented
dwellers indoors; certain fish live well in small aquaria, and many
other familiar forms can be kept alive under observation for a longer
or shorter time. But from the ease with which they are obtained and
cared for, the inexpensiveness of their live-cages, and the
interesting character of their life-history and general habits,
insects are, of all animals, the ones which specially commend
themselves for the schoolroom menagerie. In the technical notes in the
chapter (XXI) devoted to insects are numerous suggestions regarding
the obtaining and care of certain kinds of insects which may be reared
and studied to advantage in the schoolroom. In the following
paragraphs are given directions for making the necessary live-cages
and aquaria for these insects.
=Live-cages and aquaria.=--Prof. J. H. Comstock has so well described
the making of simple and inexpensive cages and aquaria in his book,
"Insect Life," that, with his permission, his account is quoted here.
_Live-cages._--"A good home-made cage can be built by fitting a pane of
glass into one side of an empty soap-box. A board, three or four inches
wide, should be fastened below the glass so as to admit of a layer of
soil being placed in the lower part of the cage, and the glass can be
made to slide, so as to serve as a door (fig. 166). The glass should fit
closely when shut, to prevent the escape of the insects.
[Illustration: FIG. 166.--Soap-box breeding-cage for insects. (From
Jenkins and Kellogg.)]
"In rearing caterpillars and other leaf-eating larvæ, branches of the
food-plant should be stuck into bottles or cans which are filled with
sand saturated with water. By keeping the sand wet the plants can be
kept fresh longer than in water alone, and the danger of the larvæ
being drowned is avoided by the use of sand.
"Many larvæ when full-grown enter the ground to pass the pupal state;
on this account a layer of loose soil should be kept in the bottom of
a breeding-cage. This soil should not be allowed to become dry,
neither should it be soaked with water. If the soil is too dry the
pupæ will not mature, or if they do so the wings will not expand
fully; if the soil is too damp the pupæ are liable to be drowned or to
be killed by mold.
"It is often necessary to keep pupæ over winter, for a large
proportion of insects pass the winter in the pupal state. Hibernating
pupæ may be left in the breeding-cages or removed and packed in moss
in small boxes. Great care should be taken to keep moist the soil in
the breeding-cages, or the moss if that be used. The cages or boxes
containing the pupæ should be stored in a cool cellar, or in an
unheated room, or in a large box placed out of doors where the sun
cannot strike it. Low temperature is not so much to be feared as great
and frequent changes of temperature.
"Hibernating pupæ can be kept in a warm room if care be taken to keep
them moist, but under such treatment the mature insects are apt to
emerge in midwinter.
[Illustration: FIG. 167.--Lamp-chimney and flower-pot breeding-cage
for insects. (From Jenkins and Kellogg.)]
"An excellent breeding-cage is represented by fig. 167. It is made by
combining a flower-pot and a lantern-globe. When practicable, the
food-plant of the insects to be bred is planted in the flower-pot; in
other cases a bottle or tin can filled with wet sand is sunk into the
soil in the flower-pot, and the stems of the plant are stuck into this
wet sand. The top of the lantern-globe is covered with Swiss muslin.
These breeding-cages are inexpensive, and especially so when the pots
and globes are bought in considerable quantities. A modification of
this style of breeding-cage that is used by the writer differs only in
that large glass cylinders take the place of the lantern-globes. These
cylinders were made especially for us by a manufacturer of glass, and
cost from six to eight dollars per dozen, according to size, when made
in lots of fifty.
"When the transformation of small insects or of a small number of
larger ones are to be studied, a convenient cage can be made by
combining a large lamp-chimney with a small flower-pot.
"_The root-cage._--For the study of insects that infest the roots of
plants, the writer has devised a special form of breeding-cage known as
the root-cage. In its simplest form this cage consists of a frame
holding two plates of glass in a vertical position and only a short
distance apart. The space between the plates of glass is filled with
soil in which seeds are planted or small plants set. The width of the
space between the plates of glass depends on the width of two strips of
wood placed between them, one at each end, and should be only wide
enough to allow the insects under observation to move freely through the
soil. If it is too wide the insects will be able to conceal themselves.
Immediately outside of each glass there is a piece of blackened zinc
which slips into grooves in the ends of the cage, and which can be
easily removed when it is desired to observe the insects in the soil.
"_Aquaria._--For the breeding of aquatic insects aquaria are needed.
As the ordinary rectangular aquaria are expensive and are liable to
leak we use glass vessels instead.
"Small aquaria can be made of jelly-tumblers, glass finger-bowls, and
glass fruit-cans, and larger aquaria can be obtained of dealers. A
good substitute for these is what is known as a battery-jar (fig.
168). There are several sizes of these, which can be obtained of most
dealers in scientific apparatus.
"To prepare an aquarium, place in the jar a layer of sand; plant some
water-plants in this sand, cover the sand with a layer of gravel or
small stones, and then add the required amount of water carefully, so
as not to disturb the plants or to roil the water unduly. The growing
plants will keep the water in good condition for aquatic animal life,
and render changing of the water unnecessary, if the animals in it
live naturally in quiet water. Among the more available plants for use
in aquaria are the following:
"Waterweed, _Elodea canadensis_.
"Bladderwort, _Utricularia_ (several species).
"Water-starwort, _Callitriche_ (several species).
"Watercress, _Nasturtium officinale_.
"Stoneworts, _Chara and Nitella_ (several species of each).
"Frog-spittle or water-silk, _Spirogyra_.
"A small quantity of duckweed, _Lemna_, placed on the surface of the
water adds to the beauty of an aquarium.
[Illustration: FIG. 168.--Battery-jar aquarium. (From Jenkins and
Kellogg.)]
"When it is necessary to add water to an aquarium on account of loss
by evaporation, rain water should be used to prevent an undue
accumulation of the mineral-water held in solution in other water."
=Making collections.=--Much is to be learned about animals by
"collecting" them. But the collecting should be done chiefly with the
idea of learning about the animals rather than with the notion of
getting as many specimens as possible. To collect, it is necessary to
find the animals alive; one learns thus their haunts, their local
distribution, and something of their habits, while by continued work
one comes to know how many and what different kinds or species of each
group being collected occur in the region collected over. Collecting
requires the sacrifice of life, however, and this will always be kept
well in mind by the humane teacher and pupil. Where one set of
specimens will do, no more should be collected. The author believes
that high-school work in this line should be almost exclusively
limited to the building up of a common school collection. Let a single
set of specimens be brought together by the combined efforts of all
the members of the class, and let it be well housed and cared for
permanently. Each succeeding class will add to it; it may come in time
to be a really representative exhibition of the local fauna.
The high-school collection should include not only adult specimens of
the various kinds of animals, forming a systematic collection, as it
is called, but also all kinds of specimens which illustrate the
structure and habits of the animals in question and which will
constitute a so-called biological collection. Specimens of the eggs
and all immature stages; dissections preserved in alcohol or formalin
showing the external and internal anatomy; nests, cocoons, and all
specimens showing the work and industries of the various animals; in
short, any specimen of the animal itself in embryonic or postembryonic
condition, or any parts of the animal, or anything illustrating what
the animal does or how it lives, all these should be collected as
assiduously as the adult individuals. Each specimen in the collection
should be labelled with the name of the animal, the date, and
locality, and the name of the collector, with any particular
information which will make it more instructive. If such special data
are too voluminous for a label, they should be written in a general
note-book called "Notes on Collections" (kept in the schoolroom with
the collection), the specimen and corresponding data being given a
common number so that their association may be recognized. In the
following paragraphs are given brief directions for catching, pinning
up, and caring for insects, for making skins of birds and mammals, and
for the alcoholic preservation of other kinds of animals.
_Insects._--For catching insects there are needed a net, a
killing-bottle, a few small vials of alcohol, and a few small boxes to
carry home live specimens, cocoons, galls, etc. For preparing and
preserving the insects there are needed insect-pins, cork- or pith-lined
drawers or boxes, and small wide-mouthed bottles of alcohol.
[Illustration: FIG. 169.--Insect killing-bottle; cyanide of potassium
at bottom, covered with plaster of Paris. (From Jenkins and Kellogg.)]
The net, about 2 feet deep, tapering and rounded at its lower end, is
made of cheesecloth or bobinet (not mosquito-netting, which is too
frail), attached to a ring, one foot in diameter, of No. 3 galvanized
iron wire, which in turn is fitted into a light wooden or cane handle
about three and a half feet long.
The killing-bottle (fig. 169) is prepared by putting a few small lumps
(about a teaspoonful) of cyanide of potassium into the bottom of a
wide-mouthed bottle holding about four ounces, and covering this
cyanide with wet plaster of Paris. When the plaster sets it will hold
the cyanide in place, and allow the fumes given off by its gradual
volatilization to fill the bottle. Insects dropped into it will be
killed in from two or three to ten minutes. Keep a little tissue paper
in the bottle to soak up moisture and to prevent the specimens from
rubbing. Also keep the bottle well corked. Label it "Poison," and do
not breathe the fumes (hydrocyanic gas). Insects may be left in it
over night without injury to them.
Butterflies or dragon-flies too large to drop into the killing-bottle
may be killed by dropping a little chloroform or benzine on a piece of
cotton, to be placed in a tight box with them. Larvæ (caterpillars,
grubs, etc.) and pupæ (chrysalids) should be dropped into the vials of
alcohol.
In collecting, visit flowers, sweep the net back and forth over the
small flowers and grasses of meadows and pastures, look under stones,
break up old logs and stumps, poke about decaying matter, jar and
shake small trees and shrubs, and visit ponds and streams. Many
insects can be collected in summer at night about electric lights, or
a lamp by an open window.
When the insects are brought home or to the schoolroom they must be
"pinned up." Buy insect-pins, long, slender, small-headed, sharp-pointed
pins, of a dealer in naturalists' supplies (see p. 453). These pins cost
ten cents a hundred. Order Klaeger pins, No. 3, or Carlsbaeder pins, No.
5. These are the most useful sizes. For larger pins order Klaeger No. 5
(Carlsbaeder No. 8); for smaller order Klaeger No. 1 (Carlsbaeder No.
2). Pin each insect straight down through the thorax (fig. 170) (except
beetles, which pin through the right wing-cover near the middle of the
body). On each pin below the insect place a small label with date and
locality of capture. Insects too small to be pinned may be gummed on to
small slips of cardboard, which should be then pinned up. Keep the
insects in drawers or boxes lined on the bottom with a thin layer of
cork, or pith of some kind. (Corn-pith can be used; also in the West,
the pith of the flowering stalk of the century plant.) The cheapest
insect-boxes and very good ones, too, are cigar-boxes. But unless well
looked after they let in tiny live insects which feed on the dead
specimens. For a permanent collection, therefore, it will be necessary
to have made some tight boxes or drawers. Glass-topped ones are best, so
that the specimens may be examined without opening them. A "moth-ball"
(naphthaline) fastened in one corner of the box will help keep out the
marauding insects.
[Illustration: FIG. 170.--Insect properly "pinned up." (From Jenkins
and Kellogg.)]
Butterflies, dragon-flies, and other larger and beautiful-winged
insects should be "spread," that is, should be allowed to dry with
wings expanded. To do this spreading- or setting-boards (figs. 171 and
172) are necessary. Such a board consists of two strips of wood
fastened a short distance apart so as to leave between them a groove
for the body of the insect, and upon which the wings are held in
position until the insect is dry. A narrow strip of pith or cork
should be fastened to the lower side of the two strips of wood,
closing the groove below. Into this cork is thrust the pin on which
the insect is mounted. Another strip of wood is fastened to the lower
sides of the cleats to which the two strips are nailed. This serves as
a bottom and protects the points of the pins which project through the
piece of cork. The wings are held down, after having been outspread
with the hinder margins of the fore wings about at right angles to the
body, by strips of paper pinned down over them.
"Soft specimens" such as insect larvæ, myriapods, and spiders should
be preserved in bottles of alcohol (85 per cent). Nests, galls, stems,
and leaves partly eaten by insects, and other dry specimens can be
kept in small pasteboard boxes.
For a good and full account of insect-collecting and preserving, with
directions for making insect-cases, etc., see Comstock's "Insect
Life," pp. 284-314.
[Illustration: FIG. 171.--Setting-board with butterflies properly
"spread." (After Comstock.)]
_Birds._--In collecting birds, shooting is chiefly to be relied on. Use
dust-shot (the smallest shot made) in small loads. For shooting small
birds it is extremely desirable to have an auxiliary barrel of much
smaller bore than the usual shotgun which can be fitted into one of the
regular gun-barrels. In such an auxiliary barrel use 32-calibre shells
loaded with dust-shot instead of bullets. Plug up the throat and vent of
shot birds with cotton, and thrust each bird head downward into a
cornucopia of paper. This will keep the feathers unsoiled and smooth.
[Illustration: FIG. 172.--Setting-board in cross-section to show
construction. (After Comstock.)]
Birds should be skinned soon after bringing home, after they have become
relaxed, but before evidences of decomposition are manifest. The tools
and materials necessary to make skins are scalpel, strong sharp-pointed
scissors, bone-cutters, forceps, corn-meal, a mixture of two parts white
arsenic and one part powdered alum, cotton, and metric-system measure.
Before skinning, the bird should be measured. With a metric-system
measure carefully take the alar extent, i.e. spread from tip to tip of
outstretched wings; length of wing, i.e. length from wrist-joint to tip;
length of bill in straight line from base (on dorsal aspect) to tip;
length of tarsus, and length of middle toe and claw.
To skin the bird, cut from anus to point of breast-bone through the skin
only. Work skin away on each side to legs; push each leg up, cut off at
knee-joint, skin down to next joint, remove all flesh from bone, and
pull leg back into place; loosen skin at base of tail, cut through
vertebral column at last joint, being careful not to cut through bases
of tail-feathers; work skin forward, turning it inside out, loosening it
carefully all around, without stretching, to wings; cut off wings at
elbow-joint, skin down to next joint and remove flesh from wing-bones;
push skin forward to base of skull, and if skull is not too large (it is
in ducks, woodpeckers, and some other birds), on over it to ears and
eyes; be very careful in loosening the membrane of ears and in cutting
nictitating membrane of eyes; do not cut into eyeball; remove eyeballs
without breaking; cut off base of skull, and scoop out brain; remove
flesh from skull, and "poison" the skin by dusting it thoroughly with
the powdered arsenic and alum mixture. Turn skin right side out, and
clean off fresh blood-stains by soaking them up with corn-meal; wash off
dried blood with water, and dry with corn-meal. Corn-meal may be used
during skinning to soak up blood and grease.
There remains to stuff the skin. Fill orbits of eyes with cotton (this
can be advantageously done before skin is reversed); thrust into neck a
moderately compact, elastic, smooth roll of cotton about thickness of
the natural neck; make a loose oval ball of size and general shape of
bird's body and put into body-cavity with anterior end under the
posterior end of neck-roll; pull two edges of abdominal incision
together over the cotton, fasten, if necessary, with a single stitch of
thread, smooth feathers, fold wings in natural position, wrap skin, not
tightly, in thin sheet of cotton (opportunity for delicate handling
here) and put away in a drawer or box to dry. Before putting away tie
label to leg, giving date and locality of capture, sex and measurements
of bird, and name of collector. Before bird is put into permanent
collection it should be labelled with its common and scientific name.
The mounting of birds in lifelike shape and attitude is hard to do
successfully; and a collection of mounted birds demands much more
room and more expensive cabinets than one of skins. For instructions
for the mounting of birds see Davie's "Methods in the Art of
Taxidermy," pp. 39-57; or Hornaday's "Taxidermy and Zoological
Collecting." For a more detailed account of making bird-skins, see
also these books, or Ridgway's "Directions for Collecting Birds."
In collecting birds' nests cut off the branch or branches on which the
nest is placed a few inches above and below the nest, leaving it in
its natural position. Ground-nests should have the section of the sod
on which they are placed taken up and preserved with them. If the
inner lining of the nest consists of feathers or fur put in a
"moth-ball" (naphthaline).
To preserve birds' eggs they should be emptied through a single small
hole on one side by blowing. Prick a hole with a needle and enlarge with
an egg-drill (obtain of dealers in naturalists' supplies, see p. 453.)
Blow with a simple bent blowpipe with point smaller than the hole. After
removing contents clean by blowing in a little water, and blowing it out
again. After cleaning, place the egg, hole downward, on a layer of
corn-meal to dry. Label each egg by writing on it near the hole a
number. Use a soft pencil for writing. This number should refer to a
record (book) under similar number, or to an "egg-blank," containing the
following data: name of bird, number of eggs in set, date and locality,
name of collector, and any special information about the eggs or nest
which the collector may think advisable. The eggs may be kept in drawers
or boxes lined with cotton, and divided into little compartments.
For detailed directions for collecting and preserving birds' eggs and
nests, see Bendire's "Directions for Collecting, Preparing, and
Preserving Birds' Eggs and Nests" or Davie's "Methods in the Art of
Taxidermy," pp. 74-78. [21] _Mammals._--Any mammal intended for a
scientific specimen should be measured in the flesh, before skinning,
and as soon after death as practicable, when the muscles are still
flexible. (This is particularly true of larger species, such as foxes,
wildcats, etc.) The measurements are taken in millimetres, a rule or
steel tape being used. (1) Total length: stretch the animal on its
back along the rule or tape and measure from the tip of the nose (head
extended as far as possible) to the tip of the fleshy part of tail
(not to end of hairs). (2) Tail: bend tail at right angles from body
backward and place end of ruler in the angle, holding the tail taut
against the ruler. Measure only to tip of flesh (make this measurement
with a pair of dividers). (3) Hind foot: place sole of foot flat on
ruler and measure from heel to tip of longest toe-nail (in certain
small mammals it is necessary to use dividers for accuracy). The
measurements should be entered on the label, along with such necessary
data as sex, locality, date, and collector's name.
Skin a mammal as soon after death as possible. Lay mammal on back and
with scissors or scalpel open the skin along belly from about midway
between fore and hind legs to vent, taking care not to cut muscles of
abdomen. Skin down on either side of the body by working the skin from
flesh with fingers till hind legs appear. Use corn-meal to stanch
blood or moisture. With left hand grasp a leg and work the knee from
without into the opening just made; cut the bone at the knee, skin leg
to heel and clean meat off the bone (leaving it attached of course to
foot). In animals larger than squirrels skin down to tips of toes. Do
the same with other leg. Skin around base of tail till the skin is
free all around so that a grip can be secured on body; then with thumb
and forefinger hold the skin tight at base of tail and slowly pull out
the tail. In small mammals this can be done readily, but in foxes it
is often necessary to split the skin up along the under side and
dissect it off the tail-bones. After the tail is free skin down the
body, using the fingers (except in large mammals) till the fore legs
are reached; treat the fore legs in the same manner as hind legs,
thrusting elbow out of the skin much as a person would do in taking
off a coat; cut bone at elbow; clean fore-arm bone. Skin over neck to
base of ears. With scalpel cut through ears close to skull. With
scalpel dissect off skin over the head (taking care not to injure
eyelids) down to tip of nose, severing its cartilage and hence freeing
skin from body. Sew mouth by passing needle through under lip and then
across through two sides of the upper lip; draw taut and tie thread.
Poison skin thoroughly. Turn skin right side out. Next sever the skull
carefully from body, just where the last neck-vertebra joins the back
of the skull. It is necessary to keep the skull, because characters of
bone and teeth are much used in classification. Remove superfluous
meat from the skull and take out brain with a little spoon made of a
piece of wire with loop at end. Tag the skull with a number
corresponding to that on skin, and hang up to dry. A finished specimen
skull is made by boiling it a short time and picking the meat off with
forceps, further cleaning it with an old tooth-brush, when it is
placed in the sun to bleach. Care must be taken always not to injure
bones or dislodge teeth.
Mammals are stuffed with cotton or tow; the latter is used in species
from a gray squirrel up. Large mammals stuffed with cotton do not dry
readily, and often spoil. Being much thicker-skinned than birds,
mammals require more care in drying and ordinarily require a much
longer period. Soft hay may be substituted for tow; never use feathers
or hair. Roll a longish wad of cotton about the size of body and
insert with forceps, taking care to form the head nearly as in life.
Split the back end of the cotton and stuff each hind leg with the two
branches thus formed. Roll a piece of cotton around end of forceps and
stuff fore legs. Place a stout straight piece of wire in the tail,
wrapping it slightly to give the tail the plump appearance of life.
(If the cotton cannot be reeled on to the wire evenly, leave it off
entirely.) Make the wire long enough to extend half way up belly. Sew
up slit in belly. Lay mammal on belly and pin out on a board by legs,
with the fore legs close beside head, and hind legs parallel behind,
soles downward. Be sure the label is tied securely on right hind leg.
For directions for preparing and mounting skeletons of birds, mammals,
and other vertebrates, see the books of Davie and Hornaday already
referred to.
_Fishes, batrachians, reptiles, and other animals._--The most
convenient and usual way of preserving the other vertebrates (not
birds or mammals) is to put the whole body into 85 per cent alcohol or
4 per cent formalin. Batrachians should be kept in alcohol not
exceeding 60 per cent strength. Several incisions should always be
made in the body, at least one of which should penetrate the abdominal
cavity. Anatomical preparations are similarly preserved. By keeping
the specimens in glass jars they may be examined without removal.
Fishes should not be kept in formalin more than a few months, as they
absorb water, swell, and grow fragile.
Of the invertebrates all, except the insects, are preserved in alcohol
or formalin. The shells of molluscs can be preserved dry, of course,
in drawers or boxes divided into small compartments.
FOOTNOTE:
[21] The following directions for making skins of mammals were written
for this book by Mr. W. K. Fisher of Stanford University, an
experienced collector.
INDEX
--Illustrations are indicated by an asterisk
_Acanthia lectularia_, *188.
Acarina, 230.
_Acmara spectorum_, *248.
Actinozoa, 97, 102.
Adaptation, 407.
Adder, spreading, 321.
_Ægialitis vocifera_, 349.
Agalenidæ, 235.
_Agkistrodon piscivorous_, 323.
_Aix sponsa_, 347.
Albatross, 346.
_Alca impennis_, 345.
_Alce americana_, *385, 396.
Alligator, 326.
_Alligator mississippensis_, 326.
Alternation of generations, 96.
_Amblophtes rupestris_, 282.
_Amblystoma_, 297, 298.
_Amblystoma maculatum_, 299.
_Ameiurus_, 282.
_Amœba_, *32;
structure and life of, 31.
_Amphioxus_, 278.
Anaconda, 324.
_Anas boschas_, 347.
Anatomy defined, 3.
_Anguilla_, 284.
_Anguillula_, 140.
_Anolis principalis_, 319.
_Anosia plexippus_, anatomy of larva of, 177;
external structure of, 171, *172;
life of, 175;
mimicked by _Basilarchia archippus_, *433.
Anseres, 347.
Ant, little black, *224;
little brown, 223.
Antelope, *395, 396.
Antenna of carrion beetle, *184.
_Antilocapra americana_, *395, 396.
_Antrostomus vociferus_, 356.
Ant, 212, 218, 223.
Anura, 299.
Ape, 401.
Aphidiæ, 200.
_Apis florea_, comb of, *222.
_Apis mellifica_, *218.
Appearance, terrifying, 430.
Aquarium, 457.
Aquarium, battery-jar, *461.
_Aquila chrysætos_, 342.
Arachnida, 144, 229.
_Arctomys monax_, 391.
_Ardea herodias_, 358.
_Ardea virescens_, 347.
_Argiope_ sp., *236.
Argonaut, 257.
_Argonauta argo_, 257.
_Ariolimax californica_, *252.
Arthogastra, 230.
Arthropoda, 144.
Ascidian, 259, *261.
_Aspidiotus aurantii_, *198.
_Asterias_ sp., structure and life of, 108.
_Asterias_, *109;
cross-section of, *112.
_Asterias ocracia_, *122.
_Asterina mineata_, *122.
Asteroidea, 120, 121.
Attidæ, 235.
Auk, great, 345.
Aves, 327.
_Aythya vallisneria_, 347.
Ayu, 283.
Back-swimmer, 197, 199.
_Balanus_, *153.
_Balæna glacialis_, 393.
_Balæna mysticetus_, 393.
Barbadoes earth, 82.
Barnacle, *153, 155;
sessile, 155;
stalked, 155.
Barn-owl, 353.
_Bartramia longicauda_, 349.
_Bascanium constrictor_, 321.
Bass, 282.
Bat, hoary, *392.
Batrachia, 291.
Batrachians, 291;
body form and structure of, 292;
classification of, 295;
life-history and habits of, 295;
structure of, 292.
Bat, 391.
Bead-snake, 322.
Bear, 398.
Beaver, 391.
Bed-bug, *188.
Bee, 212;
solitary, 216.
Beetle, great water-scavenger, 163;
external structure, *164;
internal structure, *167;
antenna of carrion, *184;
Colorado potato, 209.
Beetle, 206;
carrion, 209;
whirligig, 206.
Bell-animalcule, structure and life of, 75.
Bills of birds, 362.
Bipinnaria, 119.
Bird, frigate, 346;
man-of-war, 346;
outline of body showing external regions, *330;
ruby-throat humming, nest and eggs of, *357.
Bird-louse, *194.
Birds, 327;
bills and feet of, 362;
body form and structure of, 336;
care of young, 366;
classification of, 340;
collecting, 466;
determining, 359;
development and life-history of, 339;
feeding habits of, 370;
flight and songs of, 364;
migration of, 367;
molting of, 361;
nesting of, 366;
protection of, 370.
Bird-skins, making, 466.
_Bison bison_, 396, *397.
Bittern, 348.
Blacksnake, 321.
_Blissus leucopterus_, 198.
Blood, circulation of, in mammal, *376.
Blood of toad, structure of, 40.
Blow-fly, 201, 202;
section through compound eye of, *185.
"Bob Jordan" (monkey), *400.
Bobwhite, 350.
_Bombus_, 216.
_Bombyx mori_, anatomy of larva of, *178.
_Bonasa umbellus_, 350.
Books, reference, 454.
Borer, peach-tree, *210.
_Botaurus lentiginosus_, 348.
Box tortoise, 315.
_Brachynotus nudus_, *153.
Brains of vertebrates, *378.
Branch, defined, 73.
_Branta canadensis_, 347.
Breeding cage, 458, 459.
Brittle-stars, 120, 121, 122.
_Bubo virginianus_, 353.
Buffalo, 396, *397.
_Bufo lentiginosus_, 301;
dissection of, 5.
Bullfrog, 299.
Bumblebee, 216.
_Bunodes californica_, 103.
_Buteo_, 353.
Butterfly, external structure of, 171, *172;
life of, 175;
monarch, anatomy of larva of, 177;
dead leaf, *429;
mimicked by viceroy, *433.
Butterflies, 205;
setting-board for, *466, 467.
Buzzard, turkey, 352.
Cachalot, 393.
Cage, lamp-chimney and flower-pot breeding, *459;
soap-box breeding, *458.
Cake-urchin, 124.
Calcarea, 91.
_Calliphora vomitoria_, 202;
section through compound eye of, *185.
_Callorhinus alascanus_, *399.
_Callorhinus ursinus_, parasitized, *422.
_Cambarus_ sp., dissection of, 18;
life of, 146.
_Camphephilus principalis_, 355.
_Cancer productus_, *153.
_Canis familiaris_, 398.
_Canis latrans_, 398.
_Canis nubilus_, 398.
Canvas-back, 347.
_Carcharodon_, 280.
Caribou, 396.
Cassowary, 343.
_Castor canadensis_, 391.
Caterpillar, apple tent, 208;
forest tent, 209.
Catfish, 282.
_Cathartes aura_, 352.
_Cavia_, 390.
Cell, defined, 37.
Cell differentiation, degrees of, 54.
Cell products, 38.
Cell wall, 38.
Centiped, *228, 229;
skein, *228.
Centipeds, 226.
_Centrocercus urophasianus_, 350.
_Centrurus_ sp., *236.
Cephalpoda, 246.
_Cercopithicus_, *400.
_Cervus canadensis_, *394, 395.
_Ceryle alcyon_, 354.
Cete, 393.
_Cetorhinus_, 270, 280.
_Chætura pelagica_, 356.
Chain-snake, 320.
Chalk, 81.
Chameleon, green, 318.
Chelonia, 313, 314.
_Chelonia mydas_, 315.
_Chelydra serpentina_, 314.
_Chen hyperborea_, 247.
Chicken-hawk, 353.
Chimney-swift, 356.
Chinch bug, 198.
Chipmunk, 391.
Chiroptera, 391.
Chitin, 145, 158.
_Chlorostomum funebrale_, *248.
Chordata, 259;
classification of, 260.
_Chordeiles virginianus_, 356.
Chromatophore, 256.
Chub, 282.
_Chrysemys_, 314.
_Cicada_, 199;
seventeen-year, *200;
_septendecim_, *200, 197.
Circulation of blood in mammal, *376.
_Circus hudsonius_, 352.
_Cistudo carolina_, 314.
Clams, 246;
hard shell, 247;
soft-shell, 247.
Class, defined, 73.
Classification, basis and signification of, 65;
defined, 3;
example of, 68.
_Clisiocampa americana_, larvæ, *208.
_Clisiocampa disstria_, caterpillars, *209;
life-history of, 207.
_Clupea harengus_, 284.
Cobra-da-capello, 324.
Coccidæ, 198.
Coccyges, 354.
_Coccyzus_, 354.
Cock, chapparal, 354.
Cockroach, 192.
Codfish, 284.
Cœcilians, 302.
Cœlenterata, 92;
classification of, 96;
development and life-history of, 95;
form of, 93;
skeleton of, 95;
structure of, 94.
_Colaptes auratus_, *355.
_Colaptes cafer_, 355.
Coleoptera, 206.
_Colinus virginianus_, 350.
Collections, making, 461.
Color, use of, 424.
Colors, warning, 430.
Colubridæ, 319.
_Columba fasciata_, 351.
_Columba livia_, 351.
Columbæ, 351.
_Colymbus auritus_, 343.
Comb-building of honey-bee, *221.
Comb of East Indian honey-bee, *222.
Commensalism, 155, 413.
Communal life, 411.
Condor, California, 352.
_Condylura cristata_, 391.
Conjugation, 35, 60.
_Conotrachelus cratægi_, *212, 213.
_Conotrachelus nenuphar_, *214.
Constrictor, boa, 324.
_Conurus carolinensis_, 353.
Coot, American, 349.
Copperhead, 322.
Coral, 95;
branching, *104;
red, 106.
Coral islands, 104, 106.
Coral polyps, 104.
Coral reefs, 106.
Corals, 92, 102, 104.
_Coregonus_, 283.
_Corisa_, 197.
_Corisa_ sp., *199.
Cormorant, 346.
Cornea of eye of horse-fly, *186.
Cottontail, 390.
Coyote, 398.
Crab, 151, 152;
soft-shelled, 154.
Crabs, *153.
Crane, sand-hill, 348;
whooping, 348.
Crayfish, dissection of, 18, *18, 22;
life of, 146.
Cricket, house, *193.
Cricket, 192.
Crinoid, *126.
Crinoidea, 121, 125.
Crocodile, 326.
Crocodilea, 313, 325.
_Crocodilus americanus_, 326.
_Crotalus_, 322.
Crustacea, 144, 146;
form and structure of, 147.
_Cryptobranchus_, 298.
Ctenophora, 97, 107.
Cuckoos, 354.
_Cucumaria_, 124.
Cucumber-beetles, 209.
_Culex_ sp., 204, *205.
Curculio plum, *214.
Curculio quince, *212, 213.
Curlew, long-billed, 350.
Cuttlefishes, 255.
_Cyclas_, 247.
_Cyclophis æstivus_, 320.
_Cyclops_, 148, *149.
Cyclostomata, 278.
Cytoplasm, 38.
Dabchick, 343.
_Dactylus_ sp., *249.
Damp bug, *151.
Darters, 282.
_Dasyatis_, 281.
Decapoda, 151.
Decapods, 256.
Deer, 396.
Degeneration, 417.
_Dendrostomium cronjhelmi_, *134.
Development, defined, 3;
embryonic, defined, 62;
post-embryonic, defined, 62;
simplest, 59.
_Diapheromera femorata_, *427.
_Diaspis rosæ_, *198.
Dictynidæ, 235.
_Didelphis virginiana_, 390.
_Diemystylus torosus_, *299.
_Diemystylus viridescens_, 297.
Dimorphism, 96.
Diptera, 201.
Distribution, barriers to, 437;
geographical,435;
laws of, 436;
local, of birds, 367;
modes of, 437.
Diver, great northern, 343.
Dolphins, 393.
_Doris tuberculata_, *254.
_Draco_, 319.
Dragon-flies, 294.
Dragon-fly, *196.
Dragon, flying, 318.
Drawings, 447.
_Dryobates pubescens_, 355.
_Dryobates villosus_, 355.
Duck, ruddy, 347.
_Dyticus_ sp., 210.
Dytiscidæ, 207.
Eagle, bald, 352;
golden, 352.
Ear of locust, *187.
Earthworm, anatomy of, *126;
alimentary canal of, *126;
cross-section of, *131;
reproductive organs of, *130;
structure and life of, 127.
Earthworms, 136.
Echinoderm, development of, 119;
structure of, 117;
shape of, 116.
Echinodermata, 108;
classification of, 120.
_Echinodoris_ sp., *254.
Echinoidea, 121, 122, 123.
_Eciton_, 225.
Ecology, animal, 403.
_Ectopistes migratorius_, 351.
Eel, 284.
Eft, green, 297;
western brown, *299.
Eggs of birds, collecting, 469.
Eider, 347.
Elasmobranchii, 279.
_Elassoma_, 271.
Elk, *394, 396.
Epeiridæ, 236.
Ephemerida, 194.
_Epialtus productus_, *153.
Equipment of laboratory, 450.
Equipment of pupil, 447.
_Erethizon dorsatus_, 391.
_Erethizon epixanthus_, 391.
_Eretmochelys imbricata_, 215.
_Erismatura rubida_, 347.
_Eumeces skeltonianus_, *316.
_Eupomotic gibbosuc_, dissection of, (facing) *263;
life of, 270;
structure of, 263.
_Exocætus_, 285.
Eye, cornea of compound, of horse-fly, *186;
section through compound, of blow-fly, *185.
Eye of vertebrate, *378.
_Falco sparverius_, 353.
Family, defined, 72.
Fauna, 440.
Feather-stars, 121, 125, *126.
Feet of birds, 362.
_Felis concolor_, 398.
Feræ, 397.
Fever, yellow, and mosquitoes, 205.
_Fiber zibethicus_, 391.
Fire-flies, 209.
Fishes, 263;
body form and structure of, 271;
classification of, 277;
development and life-history of, 276;
habits and adaptations of, 285.
Fish-hatcheries, 288.
Flat-worms, 137.
Flea, house, *204.
Flickers, 355.
Flies, 201;
chalcid, 214;
ichneumon, 212.
Flight of birds, 366.
Flying fishes, 285.
Food-fishes, 288.
Food of birds, 370.
Foraminifera, 80.
Fox, 398.
_Fregata aquila_, 346.
Frogs, 299.
_Fulica americana_, 349.
Fulmars, 345.
Function, defined, 14.
Functions, essential, 15.
Fur-seals, 398, *399.
Fur-seals, parasitized, *422.
_Gadus callarias_, 284.
Galley-worm, *227.
Gall-flies, 214.
Gallinæ, 350.
Gastropoda, 246.
_Gavia imber_, 347.
Gavial, 326.
Generation, spontaneous, 58.
Genmules, 85.
Genus, defined, 70.
_Geococcyx californianus_, 354.
Gephyrean, *134.
Girdler, currant stem, *215.
Glass-snake, 317.
Glires, 391.
Goat, Rocky Mt., 397.
_Gonionema vertens_, *101.
Goose, Canada, 347.
Gophers, pocket, 391.
_Gordius_, 140.
_Grantia_, *47.
_Grantia_ sp., 85.
Grayling, 284.
Grebe,
horned, 343;
pied-billed, 343.
Green, methyl, 351.
Greensnake, 320.
Gregariousness, 410.
Grouse, ruffed, 350.
_Grus americana_, 348.
_Grus mexicana_, 348.
Guillemot, 345.
Guinea-pig, 391.
Guinea-worm, 140.
Gull, great black-backed, 345.
Gulls, 345.
Gymnophiona, 302.
Gyrinidæ, 206.
Habitat, 441.
Hag-fishes, 279.
Hair-worms, 140.
_Haliætus leucocephalus_, 352.
_Halictus_, 216.
_Harporhynchus redivivus_, *371.
_Hatteria_, 312.
Hawk, marsh, 352.
Helmet shells, 255.
_Heloderma horridum_, *317.
Hemiptera, 197.
Hermit-crab, 154, *153.
Herodiones, 347.
Heron,
great blue, 348;
green, 347.
Herring, 284.
_Heteredon platirhinos_, 321.
_Hippocampus hippocampus_, *285.
Holothuroidea, 121, 124.
_Homo sapiens_, 398.
Honey-bee, *218;
brood-cells of, *219;
building comb, *221;
comb of East Indian, *222;
cross-section of body of pupa of, *191.
Honey-bees, 212.
Honey-dew, food of ants, 223.
Hornets, 217.
Horse-fly, cornea of eye of, *186.
House-fly, 202.
Humming-birds, 356.
_Hydra_, *47;
structure and life of, 46.
Hydrozoa, 96, 97.
Hydrophilidæ, 207.
_Hydrophilus_ sp.,
external structure of, *164;
internal structure of, *167.
_Hygrotrechus_, 198, *199.
_Hyla pickeringii_, 300.
_Hyla versicolor_, 300.
Hymenoptera, 212.
_Hyptiotes_ sp., and web, *238.
Iguana, 318.
Imago, 190.
Injecting-masses, 451.
Insect, pinned, *465;
twig, *427;
wingless, *181.
Insecta, 157.
Insectivora, 391.
Insects, classification of, 191;
collecting, 463;
communal, 215;
development and life-history of, 188;
form and structure of, 181;
killing-bottle for, *463;
social, 215.
Invertebrate, defined, 30.
Islands, coral, 104, 106.
Isopod, *151.
Isopoda, 150.
Jack rabbit, 390.
_Janus integer_, *215.
Jellyfish, *101.
Jellyfishes, 92;
colonial, 97.
Joint-snake, 318.
_Julus_, *327.
June-beetle, 212.
June beetles, 206.
_Kallima_, *429.
Kangaroo, 389.
Katydids, 192.
Kelp-crab, *152.
Kill-deer, 349.
Killing-bottle for insects, *463.
Kingfisher, belted, 354.
Laboratory, equipment of, 450.
_Lachnosterna_, 212.
Lady-birds, 209.
_Lagopus_, 350.
Lake-lamprey, 279.
_Lampetra wilderi_, 279.
Lamprey, *278;
brook, 279.
_Lampropeltis boylii_, *321.
_Lampropeltis getulus_, 320.
Lancelet, 278.
Larks, horned, *358.
_Larus marinus_, 345.
Larva, 189;
of Monarch butterfly, anatomy of, 177;
parasitized, *420.
_Lasiurus borealis_, 392.
_Lasiurus cinereus_, *392.
_Lasius flavus_, 223.
Leeches, 136.
Lemurs, 401.
_Leucania unipuncta_, *211.
_Lepidocyrtus americanus_, *181.
Lepidoptera, 205.
Leptocardii, 277.
_Leptoplana californica_, *138.
_Lepus campestris_, 390.
_Lepus nuttali_, 390.
Life-history, defined, 62.
Life-processes, essential, 15.
Limicolæ, 349.
Limpets, 255, *248.
_Littorina scutulata_, *248.
Live cages, 457.
Liver of toad, structure of, 41.
Lizard, *309.
Lizards, 316.
Lobster, 151, 152.
Locust, differential, 156;
ear of, *187;
red-legged, 156, *157;
Rocky Mt., 156;
structure and life of, 156;
two-striped, 156.
Locusts, 192.
_Loligo_, 257.
Longipennes, 345.
Loon, 343.
_Lumbricus_ sp., alimentary canal of, *131;
cross-section of, *132;
structure and life of, 127.
Lung-fish, 285.
_Lycena_, scales of wings of, *206.
Lycosidæ, 235.
_Lynx rufus_, 398.
Macrocheira, 154.
_Madrepora cervicornis_, *105.
_Malaclemmys palustris_, 314.
Malaria and mosquitoes, 205.
Mallard, 347.
Mammal, circulation of blood in, *376.
Mammalia, 373.
Mammals, 373;
body form and structure of, 381;
classification of, 389;
development and life-history of, 388;
habits, instinct and reason of, 388;
making skins of, 470.
Man, 398.
Man-of-war, Portuguese, 98, *97.
Marsupialia, 389.
_Martesia xylophaga_, *251.
Massasauga, 32.
May-flies, 194.
May-fly, nymph of, *197.
Medusa, *101.
Megalobatrachus, 298.
_Megascops asio_, *352, 353.
_Melanerpes erythrocephalus_, 355.
_Melanerpes formicivorus_, 356.
_Melanoplus_ sp., ear of, *187;
structure and life-history of, 157.
_Melanoplus vibittatus_, 157.
_Melanoplus differentialis_, 157.
_Melanoplus femur-rubrum_, 157, *158.
_Melanoplus spretus_, 157.
_Meleagrina margaritifera_, 250.
_Merula migratoria propinqua_, *368.
Metamorphosis, complete, 171, 188, 189;
incomplete, 171, 189.
Metazoa, defined, 43.
Mice, 391.
_Micropterus dolomien_, 282.
_Micropterus salmoides_, 282.
Migration of birds, 367.
Mimicry, 430.
Millipeds, 226.
Mite, cheese, *230.
Mites, 229.
Modifications of structure and function, 29.
Moles, 391.
Mollusca, 239.
Molluscs, 239;
classification of, 246;
development of, 246;
form and structure of, 245.
Molting, 361;
of birds, 361.
Monitor, 318.
Monkey, *400.
_Momomerium minutum_, *224.
Monster, Gila, *317.
Moose, *385, 396.
Morphology, defined, 3.
Mosquito, 202, *203.
Mosquitoes, 201;
and malaria, 205;
and yellow fever, 205.
Moth, forest tent-caterpillar, life-history of, *207.
Moths, 205.
Mourning dove, 351.
Mouse, life-history and habits of, 379;
structure of, 373.
Mud-eel, 297.
Mud-hen, 349.
Mud-puppies, 297.
Mud-turtle, 313.
Multiplication of one-celled animals, 59;
of many-celled animals, 61.
Murres, *344.
Muscles of toad, structure of, 41.
_Mus decumanus_, 391.
_Mus musculus_, structure of, 373.
_Mus rattus_, 391.
Musk-rat, 391.
Mussel, fresh-water, life-history and habits of, 243;
structure of, 239.
_Mya arenaria_, 247.
_Myotis subulatus_, 392.
Myriapoda, 144, 226.
_Mytilus californianus_, *248.
_Myxine_, 279.
Names, scientific, 68.
_Narcobatis_, 281.
_Natrix sipedon_, 320.
Nautilus, 255;
pearly, 258.
_Nautilus pompilius_, 258.
_Necturus_, 297, 298.
Nemathelminthes, 140.
_Neotoma pennsylvanica_, 391.
Nereid, *134.
_Nereis_ sp., *134.
Nesting of birds, 366.
Nest of oriole, *365.
_Nettion carolinense_, 347.
Night-hawk, 356.
Night-heron, 348.
_Nirmus præstans_, *194.
Non-calcarea, 91.
Notes, 447, 448.
Notochord, 259.
_Notonecta_, 197, 199.
Nucleus, 38.
Nudibranchs, 252, *254.
_Numenius longirostris_, 350.
_Nyctea nyctea_, 353.
_Nycticorax_, 348.
Octopi, 255.
Octopods, 256.
_Odocoileus americanus_, 396.
Odonata, 194.
Oligochætæ, 136.
_Olor_, 347.
_Ommatostrephes californica_, *257.
_Oncorhynchus tschawtscha_, 283.
One-celled animals, multiplication of, 57.
Ooze, foraminifera, 81;
radiolaria, 81.
_Opheosaurus ventralis_, 317.
Ophiuroidea, 121, 122.
Opossum, 389.
Order, defined, 72.
_Oreamnos montanus_, 397.
Organ, defined, 14.
Orthoptera, 192;
sound-making of, 193.
Orb-web of Epeiridæ, 236.
_Ostrea virginiana_, 248.
Ostriches, 341, *342.
_Otocoris alpestris_, *358.
_Ovis canadensis_, *383, 396.
Owl, burrowing, 353;
great gray, 353;
great horned, 353;
snowy, 353.
Oyster, 248.
Oyster-crab, 154.
Oyster-drills, 255.
Oysters, 246;
"seed" of, 249;
"spat" of, 249.
_Pagurus samuelis_, *153.
Paludicolæ, 348.
Panther, 398.
_Paramœcium_, *35;
multiplication of, 60;
structure and life of, 34.
Parasitism, 415.
Paroquet, Carolina, 353.
Parrots, 353.
_Passer domesticus_, dissection of, (facing) *327;
life-history and habits of, 335;
structure of, 327.
Passeres, 357.
Pearl-oyster, *249.
_Pelecanus californicus_, 346.
_Pelecanus erythrorhynchus_, 346.
_Pelecanus fuscus_, 346.
Pelecypoda, 246.
Pelican, brown, 346;
white, 346.
_Pentacrinus_ sp., *126.
_Pentacta frondosa_, *125.
_Peripatus eiseni_, *226.
_Perla_ sp., *182.
Petrels, 345.
_Petromyzon marinus_, *278.
_Phalacrocorax_, 346.
Pheasants, 350.
_Phoca vitulina_, 397.
Phœbe, black, nest and eggs of, *340.
_Pholas_ sp., *250.
_Phrynosoma_, 318.
Phylloxera, grape, 198, 201.
_Phylloxera vastatrix_, 198, 201.
Phylum, defined, 73.
_Physalia_ sp., *97.
_Physeter macrocephalus_, 393.
Physiology, defined, 3.
Pici, 354.
Pickerel-frog, 300.
Pigeon, band-tailed, 351;
passenger, 351.
_Pinnotheres_, 154.
Pipe-fish, 285.
Pisces, 263.
_Pituophis bellona_, *323.
_Planaria_ sp., *138.
Planarian, fresh-water, *138;
marine, *138.
Planarians, 137.
Plant-lice, 197, 200.
Planula, 96
Platyhelminthes, 137.
_Plectrophenax nivalis_, *358.
_Plethodon_, 297.
Plover, field, 349.
Pluteus, 119.
_Podilymbus podiceps_, 343.
Poison-fangs of rattlesnake, *324.
_Pollicipes polymenus_, *153.
Polymorphism, 96.
_Polynœ brevisetosa_, *134.
Polyps, 92, 97.
_Pomoxis annularis_, 282.
_Pomoxis separoides_, 282.
Pond-snails, 252.
Porcupine, 390.
Porcupine-fish, 285.
Porifera, 84.
Porpoises, 393.
_Porzana carolina_, 349.
Prairie-chicken, 350.
Prawns, 152.
Preparations, preserving anatomical, 452.
Primates, 398.
_Pristis pectinatis_, 281.
Protophyta, 82.
Protoplasm, described, 39.
_Protopterus_, 288.
Protozoa, defined, 43, 75;
form of, 78;
marine, 80.
_Pseudemys_, 313.
_Pseudogryphus californianus_, 352.
Psittaci, 353.
Ptarmigan, 356.
Puff-adder, 325.
Puffins, 345.
_Pulex irritans_, *204.
Pulmonata, 253.
Puma, 398.
Pumpkin seed, life of, 270;
structure of, 263.
Pupa, 189;
cross-section of body of, honey-bee, *191.
Pupation, 189.
_Purpura saxicola_, *248.
Pygopodes, 343.
Python, 324.
Quail, 350.
_Querquedula discors_, 347.
Rabbits, 390.
Radiolaria, 80.
Rail, Carolina, 349.
_Raja erinacea_, 280, *281.
_Raja lævis_, 281.
_Rana catesbiana_, 299.
_Rana palustris_, 300.
_Rana sylvatica_, 300.
_Rangifer caribou_, 396.
Raptores, 351.
Ratitæ, 341.
Rats, 391.
Rattlesnake poison-fangs, *324.
Rattlesnakes, 321.
Rattles of rattlesnake, *223.
Reefs, coral, 106.
Reindeer, 396.
Remora, *287.
_Remoropsis brachyptera_, *287.
Root-cage, 460.
Reptiles, body form and organization of, 309;
classification of, 312;
life-history of, 312;
structure of, 310.
Reptilia, 303.
Resemblance, protective, 326.
Rheas, 343.
Road-runner, 354.
Robber-ant, 225.
Robin, Western, *368.
Rock-bass, 282.
Rock-crab, *153.
Rock-dove, 351.
Rodents, 390.
_Rosalina varians_, *81.
_Rotifer_ sp., *143.
Round worms, 140.
Ruminants, 395.
_Sacculina_, 67, *418.
Sage-hen, 350.
Salamander, red-backed, 297;
tiger, *292.
Salamanders, 297.
_Salmo irideus_, *283.
Salmon, king, 284.
Sand-dollar, 124.
Sand-pipers, 345.
_Sanninoidea existiosa_, *212.
Sap-sucker, downy, 355;
hairy, 355.
Saw-fish, 281.
_Sayornis nigricans_, nest and eggs of, *340.
Scale insect, red-orange, *198.
Scale insects, 198.
Scale rose, *198.
Scales of wings of Lycæna, *206;
wing of Monarch butterfly, *174.
Scallops, 246.
_Scalops aquaticus_, 391.
_Scaphiopus_, 300.
_Sceloporus_, 317.
_Sciuropterus volans_, 391.
_Sciurus carolinensis_, 391.
_Sciurus hudsonicus_, 391.
_Sciurus ludovicianus_, 391.
_Scolopendra_ sp., 228, 229.
Scorpion, *230.
Scorpions, 229.
_Scotiaptex cinera_, 353.
Screech-owl, *352, 353.
_Scutigera forceps_, *228.
Scyphozoa, 97, 101.
Sea anemones, 92, 102, *103.
Sea cucumber, *125.
Sea-cucumbers, 108, 121, 124.
Sea-fan, 107.
Sea-feather, 106.
Sea-horse, *285.
Sea-lamprey, 279.
Sea-lily, 118.
Sea-pen, 106.
Sea-shells, 252.
Sea-slugs, 255.
Sea-snakes, 325.
Sea-squirt, *261.
Sea-turtles, 315.
Sea-urchin, *114;
structure of, 113.
Sea-urchins, 108, 121, 123.
Seals, 397.
Selection, artificial, 409;
natural, 406.
Sembling of insects, 176.
Sepias, 256.
Setting-board for butterflies, *466, 467.
Shark, basking, 270, 280;
hammer-headed, 280;
man-eating, 280.
Sharks, 280.
Shearwaters, 345.
Sheep-fluke, 138.
Sheep, Rocky Mt., *383, 397.
Shipworm, 251.
Shoveller, 347.
Shrews, 391.
Shrimp, 151, 152.
Silk-worm, anatomy of, *178.
Siphonophore, 98.
_Siren_, 297, 298.
_Sistrurus_, 322.
Skate, barn-door, 281;
common, 280, *281.
Skates, 280.
Skeleton of coral, 105.
Skeletons, preparing, 452.
Skink, blue tailed, *317.
Skin of toad, structure of, 40.
Slipper-animalcule, *35;
structure and life of, 34.
Slug, giant yellow, *252.
Slugs, 252, 253.
Snake coral, 321.
Snake, garter, *320;
life of, 307;
structure of, 303;
gopher, *322.
Snake king, *321.
Snakes, 316.
Snails, 252, 253.
Snapping-turtle, 314.
Snipes, 349.
Snowflakes, *358.
Snow-goose, 347.
Social life, 410.
_Somateria_, 347.
Songs of birds, 364.
Sora, 349.
Sound making of orthoptera, 193.
Spadefoot, 300.
Sparrow, English, dissection of, (facing), *327;
life-history and habits of, 335;
structure of, 327;
western chipping, *360.
Sparrow-hawk, 353.
_Spatula clypeata_, 347.
Species, defined, 69.
Species-extinguishing, 442.
Species-forming, 408, 442.
_Speotyto cunicularia_, 353.
_Sphinx chersis_ larva, *431.
Sphinx-moth, pen-marked, larva, *431
_Sphyma_, 280.
Spicules, sponge, 85.
Spider and web, *237.
Spider, crab, *235;
jumping, *235;
long-legged, *233;
running, *234;
running with egg-sac, *234;
triangle, and web, *238.
Spider-crab, 154.
Spiders, 229;
hunting, 233;
sedentary, 233;
trap-door, 233;
wandering 233;
web weaving, 233.
Spinnerets of spider, *233.
_Spizella socialis arizonæ_, *360.
Sponge, commercial, 86;
fresh-water, 84;
glass, *87;
skeleton of, 88;
structure of, 88.
Sponges, 84;
calcareous ocean, 85;
classification of, 91;
development and life-history of, 89;
feeding habits of, 88;
form and size of, 87;
of commerce, 90.
Spongin, 86.
_Spongilla_ sp., 84.
Springtail, American, *181.
Squamata, 312, 316.
Squid, great, *257.
Squids, 255, 257.
Squirrels, 391.
Starfish, *109;
cross-section of, *112.
Starfishes, 108, 121.
_Stentor_ sp., *79.
_Sterna_, 345.
_Sterna maxima_, 194.
Sting-ray, 281.
Stone-fly, *182.
_Strix pratincola_, 353.
_Strongylocentrotus_ sp., structure of, 113.
_Strongylocentrotus franciscanus_, *115, 122;
structure of, 115.
Struggle for existence, 406.
_Struthio camelus_, 342.
Sub-species, 69.
Suckers, 283.
Sucking-bugs, 197.
Sun animalcule, *78.
Sunfish, dwarf, 271;
golden, dissection of, (facing) *263;
life of, 270;
structure of, 263.
Supplies, obtaining laboratory, 453.
Swans, 347.
Swarming of honey-bee, 219.
Swell-fish, 285.
Swift, common, 316.
Sword-fish, 285.
Symbiosis, 155, 413.
Symmetry, bilateral, 5;
radial, 108.
_Sympetrum illotum_, *196.
_Syngnathus fuscum_, 285.
Tadpole, 55.
Tadpoles, *296.
_Tænia solium_, 139.
Tapeworm, 139.
Teal, blue-winged, 347;
green-winged, 347.
Teleostomi, 282.
Tell-tale, 349.
Teredo, 251.
Tern, 194.
Terns, 345.
Terrapin, diamond-back, 314;
red-bellied, 314;
yellow-bellied, 314.
_Testudo_ sp., *315.
_Tetragnatha_ sp., *233.
_Thalarctos maritimus_, 398.
_Thamnophis_ sp., life of, 307;
structure of, 303.
_Thamnophis parietalis_, *320.
Theridiæ, 235.
_Therioplectes_ sp., cornea of compound eye of, *186.
Thomisidæ, 235.
Thrasher, sickle-billed, *371.
Thrush, russet-backed, *363.
_Thymallus signifer_, 284.
Ticks, 229.
Tiger-beetles, 209.
Toad, cellular structure of, 40;
development of, 55;
garden, dissection of, 5;
horned, 317;
skeleton of, *11.
Toads, 299, 300.
Torpedo, 281.
Tortoise, Galapagos giant, *315.
Tortoises, 313.
Tortoise-shell, 315.
_Totanus melanoleucus_, 349.
Trachea, *184.
Tree-frogs, 300.
Tree-toads, 300.
Trepang, 124.
Trichina, 140.
_Trichina spiralis_, 141, *141.
Trichinosis, 141.
_Triopha modesta_, *254.
Tripoli rock, 82.
Triton, green, 297.
_Trochilus colubris_, 356;
nest and eggs of, *357.
Trout, rainbow, *283.
Tumble bugs, 209.
_Turdus ustulaius_, *363.
Turkeys, wild, 350.
Turtle, green, 315;
hawk-bill, 315;
logger-head, 315.
Turtle-dove, 351.
Turtles, 313.
_Tympanuchus americanus_, 350.
_Tyroglyphus siro_, *230.
Ungulata, 393.
_Unio_ sp., life-history and habits of, 243;
structure of, 239.
_Uria troile californica_, *344.
_Ursus americanus_, 398.
_Ursus horribilis_, 398.
_Varanus niloticus_, 318.
Variation, 406.
Variety, 69.
Venation of wings of insects, 174;
of wings of Monarch butterfly, *175.
_Venus mercenaria_, 247.
Vermes, 127;
life-history and habits of, 132;
classification of, 135.
Vertebrate, defined, 30;
brains of, 376;
eye of, *379.
Vertebrates, 259;
structure of, 259.
Vespidæ, 217.
Vinegar-eel, 140, *140.
Viper, 324;
blowing, 321.
_Vipera cerasta_, 324.
_Vorticella_ sp., *76.
_Vorticella_, structure and life of, 75.
_Vulpes pennsylvanicus_, 398.
Walking-stick, 193, 194.
Wapiti, *394.
Wasps, 212;
digger, 217;
solitary, 217.
Water-beetle, predaceous, 210.
Water-beetles, 206.
Water-boatman, *199.
Water-boatmen, 197.
Water-flea, 148, *149.
Water-dog, 298.
Water-snake, 320.
Water-strider, 197, 199, *199.
Water tiger, 212, *214.
Weevils, 209.
Whalebone, 393.
Whales, 393.
Wheel-animalcule, *143.
Whip-poor-will, 356.
Whitefish, 284.
Wild-cat, 398.
Wings of Monarch butterfly showing venation, *175.
Wolf, 398.
Woodchuck, 391.
Wood-duck, 347.
Wood-frog, 300.
Wood-lice, 150.
Woodpecker, California, 356;
ivory-billed, 355;
red-headed, 355.
Woodpeckers, 354.
Wood-rat, 391.
Worm, army, *211.
Worms, 127;
life-history and habits of, 133;
classification of, 135;
marine, *134.
_Xiphias gladius_, 285.
Yellow-hammer, *355.
Yellow-jackets, 217.
Yellow-shank, 349.
_Zenaidura macroura_, 351.
Zoogeography, 436.
Zooids, 98.
Zoology, a first course in, 3;
defined, 3;
divisions of, 2;
systematic, defined, 3.
Zoophytes, 92.
Transcriber's Notes:
Obvious punctuation and spelling errors have been fixed throughout.
Inconsistent hyphenation has been left as in the original text.
There is no figure 27, the original text goes from figure 26 to 28,
left as in the original text.
Page 245: There is no closing parenthesis for the sentence starting
"(Where the typical...".
*** End of this LibraryBlog Digital Book "Elementary Zoology, Second Edition" ***
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