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Title: The Crayfish - An Introduction to the Study of Zoology. The International Scientific Series, Vol. XXVIII
Author: Huxley, Thomas Henry
Language: English
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THE CRAYFISH, AN INTRODUCTION TO THE STUDY OF ZOOLOGY

BY T. H. HUXLEY, F.R.S.


      *      *      *      *      *      *

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London: KEGAN PAUL, TRENCH, TRÜBNER, & CO., LTD.

      *      *      *      *      *      *


The International Scientific Series.

Vol. XXVIII.


[Illustration: THE COMMON CRAYFISH.

(_Astacus fluviatilis_, Male.)

_Frontispiece._]


THE CRAYFISH

An Introduction to the Study of Zoology

by

T. H. HUXLEY, F.R.S.

With Eighty-two Illustrations

Sixth Edition



London
Kegan Paul, Trench,Trübner & Co., Lt^d.
1896



“Διὸ δεῖ μὴ δυσχεραίνειν παιδικῶς τὴν περὶ τῶν ἀτιμοτέρων ζῴων
ἐπίσκεψιν· ἐν πᾶσι γὰρ τοῖς φυσικοῖς ἔνεστί τι θαυμαστὸν.”—ARISTOTLE,
_De Partibus_, I. 5.

       *       *       *       *       *

“Qui enim Autorum verba legentes, rerum ipsarum imagines (eorum
verbis comprehensa) sensibus propriis non abstrahunt, hi non veras
Ideas, sed falsa Idola et phantasmata inania mente concipiunt
. . . . . . .

“Insusurro itaque in aurem tibi (amice Lector!) ut quæcunque à
nobis in hisce . . . . exercitationibus tractabuntur, ad exactam
experientiæ trutinam pensites: fidemque iis non aliter adhibeas, nisi
quatenus eadem indubitato sensuum testimonio firmissime stabiliri
deprehenderis.”—HARVEY. _Exercitationes de Generatione. Præfatio._

       *       *       *       *       *

“La seule et vraie Science est la connaissance des faits: l’esprit
ne peut pas y suppléer et les faits sont dans les sciences ce qu’est
l’expérience dans la vie civile.”

“Le seul et le vrai moyen d’avancer la science est de travailler à
la description et à l’histoire des differentes choses qui en font
l’objet.”—BUFFON. _Discours de la manière d’étudier et de traiter
l’Histoire Naturelle._

       *       *       *       *       *

“Ebenso hat mich auch die genäuere Untersuchung unsers Krebses
gelehret, dass, so gemein und geringschätzig solcher auch den
meisten zu seyn scheinet, sich an selbigem doch so viel Wunderbares
findet, dass es auch den grossten Naturforscher schwer fallen
sollte solches ailes deutlich zu beschreiben.”—ROESEL V. ROSENHOF.
_Insecten Belustigungen._—“_Der Flusskrebs hiesiges Landes mit seinen
merkwurdigen Eigenschaften._”



PREFACE.


In writing this book about Crayfishes it has not been my intention
to compose a zoological monograph on that group of animals. Such a
work, to be worthy of the name, would require the devotion of years
of patient study to a mass of materials collected from many parts of
the world. Nor has it been my ambition to write a treatise upon our
English crayfish, which should in any way provoke comparison with the
memorable labours of Lyonet, Bojanus, or Strauss Durckheim, upon the
willow caterpillar, the tortoise, and the cockchafer. What I have
had in view is a much humbler, though perhaps, in the present state
of science, not less useful object. I have desired, in fact, to show
how the careful study of one of the commonest and most insignificant
of animals, leads us, step by step, from every-day knowledge to the
widest generalizations and the most difficult problems of zoology;
and, indeed, of biological science in general.

It is for this reason that I have termed the book an “Introduction
to Zoology.” For, whoever will follow its pages, crayfish in hand,
and will try to verify for himself the statements which it contains,
will find himself brought face to face with all the great zoological
questions which excite so lively an interest at the present day;
he will understand the method by which alone we can hope to attain
to satisfactory answers of these questions; and, finally, he will
appreciate the justice of Diderot’s remark, “Il faut être profond
dans l’art ou dans la science pour en bien posséder les éléments.”

And these benefits will accrue to the student whatever shortcomings
and errors in the work itself may be made apparent by the process
of verification. “Common and lowly as most may think the crayfish,”
well says Roesel von Rosenhof, “it is yet so full of wonders that the
greatest naturalist may be puzzled to give a clear account of it.”
But only the broad facts of the case are of fundamental importance;
and, so far as these are concerned, I venture to hope that no
error has slipped into my statement of them. As for the details,
it must be remembered, not only that some omission or mistake is
almost unavoidable, but that new lights come with new methods of
investigation; and that better modes of statement follow upon the
improvement of our general views introduced by the gradual widening
of our knowledge.

I sincerely hope that such amplifications and rectifications may
speedily abound; and that this sketch may be the means of directing
the attention of observers in all parts of the world to the
crayfishes. Combined efforts will soon furnish the answers to many
questions which a single worker can merely state; and, by completing
the history of one group of animals, secure the foundation of the
whole of biological science.

In the Appendix, I have added a few notes respecting points of
detail with which I thought it unnecessary to burden the text; and,
under the head of Bibliography, I have given some references to the
literature of the subject which may be useful to those who wish to
follow it out more fully.

I am indebted to Mr. T. J. Parker, demonstrator of my biological
class, for several anatomical drawings; and for valuable aid in
supervising the execution of the woodcuts, and in seeing the work
through the press.

Mr. Cooper has had charge of the illustrations, and I am indebted
to him and to Mr. Coombs, the accurate and skilful draughtsman to
whom the more difficult subjects were entrusted, for such excellent
specimens of xylographic art as the figures of the Crab, Lobster,
Rock Lobster, and Norway Lobster.

 T. H. H.

 LONDON,
 _November, 1879_.



CONTENTS.


 PREFACE • v

 LIST OF WOODCUTS • xi


 CHAPTER I.

 The Natural History of the Common Crayfish • 1


 CHAPTER II.

 The Physiology of the Common Crayfish. The Mechanism by which the
 Parts of the Living Engine are supplied with the materials necessary
 for their maintenance and growth • 46


 CHAPTER III.

 The Physiology of the Common Crayfish. The Mechanism by which
 the Living Organism adjusts itself to surrounding conditions and
 reproduces itself • 87


 CHAPTER IV.

 The Morphology of the Common Crayfish. The structure and the
 development of the individual • 137


 CHAPTER V.

 The Comparative Morphology of the Crayfish. The structure and the
 development of the Crayfish compared with those of other living
 beings • 227


 CHAPTER VI.

 The Distribution and the Ætiology of the Crayfishes • 288


 NOTES • 347

 BIBLIOGRAPHY • 357

 INDEX • 363



LIST OF WOODCUTS.


 _Frontispiece._ THE COMMON CRAYFISH, _Astacus fluviatilis_, (MALE)

  1. _Astacus fluviatilis._ SIDE VIEW OF THE MALE • 6

  2. — — DORSAL VIEWS OF MALE AND FEMALE • 18

  3. — — VENTRAL VIEWS OF MALE AND FEMALE • 21

  4. — — THE GILLS • 26

  5. — — DISSECTION FROM THE DORSAL SIDE (MALE) • 28

  6. — — LONGITUDINAL VERTICAL SECTION OF THE ALIMENTARY CANAL • 29

  7. — — A GASTROLITH OR “CRAB’S EYE” • 30

  8. — — ATTACHMENT OF YOUNG TO SWIMMERET OF MOTHER • 41

  9. — — STRUCTURE OF THE STOMACH • 53

 10. — — LONGITUDINAL SECTION OF THE STOMACH • 56

 11. — — ROOF OF THE STOMACH, FROM WITHIN • 60

 12. — — DISSECTION FROM THE SIDE (MALE) • 62

 13. — — ALIMENTARY CANAL FROM ABOVE • 65

 14. — — BLOOD CORPUSCLES • 68

 15. — — TRANSVERSE SECTION OF THORAX • 70

 16. — — THE HEART • 72

 17. — — STRUCTURE OF THE GILLS • 76

 18. — — THE GREEN GLAND • 83

 19. — — MUSCULAR TISSUE • 91

 20. — — MUSCLES OF CHELA • 93

 21. — — ARTICULATION OF ABDOMINAL SOMITES • 97

 22. — — MUSCULAR SYSTEM • 100

 23. — — NERVE FIBRES • 102

 24. — — NERVE GANGLIA • 103

 25. — — NERVOUS SYSTEM • 104

 26. — — OLFACTORY AND AUDITORY ORGANS • 114

 27. — — AUDITORY SAC • 117

 28. — — STRUCTURE OF EYE • 119

 29. — — DIAGRAM OF EYE • 123

 30. — — FEMALE REPRODUCTIVE ORGANS • 129

 31. — — MALE REPRODUCTIVE ORGANS • 130

 32. — — STRUCTURE OF OVARY • 131

 33. — — STRUCTURE OF TESTIS • 132

 34. — — SPERMATOZOA • 134

 35. — — THE LAST THORACIC STERNUM IN THE MALE AND FEMALE • 136

 36. — — TRANSVERSE SECTION OF ABDOMEN • 142

 37. — — ABDOMINAL APPENDAGES • 144

 38. — — CONNECTION BETWEEN THORAX AND ABDOMEN • 151

 39. — — CEPHALOTHORACIC STERNA AND ENDOPHRAGMAL SYSTEM • 153

 40. — — OPHTHALMIC AND ANTENNULARY SOMITES • 156

 41. — — THE ROSTRUM • 157

 42. — — A SEGMENT OF THE ENDOPHRAGMAL SYSTEM • 159

 43. — — LONGITUDINAL SECTION OF CEPHALOTHORAX • 162

 44. — — THE THIRD MAXILLIPEDE • 164

 45. — — THE FIRST AND SECOND MAXILLIPEDES • 166

 46. — — THE SECOND AMBULATORY LEG • 169

 47. — — THE MANDIBLE AND MAXILLÆ • 171

 48. — — THE EYE-STALK, ANTENNULE, AND ANTENNA • 172

 49. — — BLOOD CORPUSCLES • 176

 50. — — EPITHELIUM • 178

 51. — — CONNECTIVE TISSUE • 179

 52. — — MUSCULAR TISSUE • 181

 53. — — MUSCULAR TISSUE • 182

 54. — — NERVE GANGLIA • 188

 55. — — NERVE FIBRES • 189

 56. — — CUTICULAR TISSUE • 191

 57. — — SECTIONS OF EMBRYOS • 208

 58. — — EARLIER STAGES OF DEVELOPMENT • 210

 59. — — LATER STAGES OF DEVELOPMENT • 216

 60. — — NEWLY HATCHED YOUNG • 220

 61. _Astacus torrentium_, _Astacus nobilis_, and _Astacus
 nigrescens._ COMPARATIVE VIEWS OF THE CARAPACE, THIRD ABDOMINAL
 SOMITE, AND TELSON • 233

 62. — —, — —, and — — COMPARATIVE VIEWS OF THE FIRST AND SECOND
 ABDOMINAL APPENDAGES OF THE MALE • 245

 63. _Cambarus Clarkii_ • 248

 64. _Parastacus brasiliensis_ • 250

 65. _Astacoides madagascarensis_ • 251

 66. DIAGRAM OF THE MORPHOLOGICAL RELATIONS OF THE _Astacidæ_ • 253

 67. _Homarus vulgaris_ • 258

 68. _Parastacus_, _Nephrops_, and _Palæmon._ PODOBRANCHIÆ • 259

 69. _Nephrops norvegicus_ • 260

 70. _Palinurus vulgaris_ • 262

 71. _Palæmon jamaicensis_ • 269

 72. _Cancer pagurus_ • 273

 73. _Penæus_ • 281

 74. _Cancer pagurus._ DEVELOPMENT • 282

 75. _Astacus leptodactylis_ • 301

 76. _Australian Crayfish_ • 307

 77. MAP OF THE DISTRIBUTION OF CRAYFISHES • 309

 78. _Cambarus._ WALKING LEG • 312

 79. _Palæmon jamaicensis_ • 329

 80. _Pseudastacus pustulosus_ and _Eryma modestiformis_ • 340

 81. _Hoploparia longimana_ • 342



THE CRAYFISH:

AN INTRODUCTION TO THE STUDY OF ZOOLOGY.



CHAPTER I.

THE NATURAL HISTORY OF THE COMMON CRAYFISH

(_Astacus fluviatilis._)


Many persons seem to believe that what is termed Science is of a
widely different nature from ordinary knowledge, and that the methods
by which scientific truths are ascertained involve mental operations
of a recondite and mysterious nature, comprehensible only by the
initiated, and as distinct in their character as in their subject
matter, from the processes by which we discriminate between fact and
fancy in ordinary life.

But any one who looks into the matter attentively will soon perceive
that there is no solid foundation for the belief that the realm of
science is thus shut off from that of common sense; or that the
mode of investigation which yields such wonderful results to the
scientific investigator, is different in kind from that which is
employed {2} for the commonest purposes of everyday existence. Common
sense is science exactly in so far as it fulfils the ideal of common
sense; that is, sees facts as they are, or, at any rate, without the
distortion of prejudice, and reasons from them in accordance with the
dictates of sound judgment. And science is simply common sense at
its best; that is, rigidly accurate in observation, and merciless to
fallacy in logic.

Whoso will question the validity of the conclusions of sound science,
must be prepared to carry his scepticism a long way; for it may be
safely affirmed, that there is hardly any of those decisions of
common sense on which men stake their all in practical life, which
can justify itself so thoroughly on common sense principles, as the
broad truths of science can be justified.

The conclusion drawn from due consideration of the nature of the case
is verified by historical inquiry; and the historian of every science
traces back its roots to the primary stock of common information
possessed by all mankind.

In its earliest development knowledge is self-sown. Impressions force
themselves upon men’s senses whether they will or not, and often
against their will. The amount of interest which these impressions
awaken is determined by the coarser pains and pleasures which they
carry in their train, or by mere curiosity; and reason deals with the
materials supplied to it as far as that interest carries it, and no
farther. Such common {3} knowledge is rather brought than sought;
and such ratiocination is little more than the working of a blind
intellectual instinct.

It is only when the mind passes beyond this condition that it begins
to evolve science. When simple curiosity passes into the love of
knowledge as such, and the gratification of the æsthetic sense of
the beauty of completeness and accuracy seems more desirable than
the easy indolence of ignorance; when the finding out of the causes
of things becomes a source of joy, and he is counted happy who is
successful in the search; common knowledge of nature passes into what
our forefathers called Natural History, from whence there is but a
step to that which used to be termed Natural Philosophy, and now
passes by the name of Physical Science.

In this final stage of knowledge, the phenomena of nature are
regarded as one continuous series of causes and effects; and the
ultimate object of science is to trace out that series, from the
term which is nearest to us, to that which is at the furthest limit
accessible to our means of investigation.

The course of nature as it is, as it has been, and as it will be, is
the object of scientific inquiry; whatever lies beyond, above, or
below this, is outside science. But the philosopher need not despair
at the limitation of his field of labour: in relation to the human
mind Nature is boundless; and, though nowhere inaccessible, she is
everywhere unfathomable. {4}

The Biological Sciences embody the great multitude of truths which
have been ascertained respecting living beings; and as there are two
chief kinds of living things, animals and plants, so Biology is, for
convenience sake, divided into two main branches, Zoology and Botany.

Each of these branches of Biology has passed through the three stages
of development, which are common to all the sciences; and, at the
present time, each is in these different stages in different minds.
Every country boy possesses more or less information respecting the
plants and animals which come under his notice, in the stage of
common knowledge; a good many persons have acquired more or less of
that accurate, but necessarily incomplete and unmethodised knowledge,
which is understood by Natural History; while a few have reached the
purely scientific stage, and, as Zoologists and Botanists, strive
towards the perfection of Biology as a branch of Physical Science.

Historically, common knowledge is represented by the allusions to
animals and plants in ancient literature; while Natural History, more
or less grading into Biology, meets us in the works of Aristotle,
and his continuators in the Middle Ages, Rondoletius, Aldrovandus,
and their contemporaries and successors. But the conscious attempt
to construct a complete science of Biology hardly dates further back
than Treviranus and Lamarck, at the beginning of this century, while
it has received its strongest impulse, in our own day, from Darwin.
{5}

My purpose, in the present work, is to exemplify the general truths
respecting the development of zoological science which have just
been stated by the study of a special case; and, to this end, I have
selected an animal, the Common Crayfish, which, taking it altogether,
is better fitted for my purpose than any other.

It is readily obtained,[1] and all the most important points of
its construction are easily deciphered; hence, those who read
what follows will have no difficulty in ascertaining whether the
statements correspond with facts or not. And unless my readers are
prepared to take this much trouble, they may almost as well shut the
book; for nothing is truer than Harvey’s dictum, that those who read
without acquiring distinct images of the things about which they
read, by the help of their own senses, gather no real knowledge, but
conceive mere phantoms and idola.

     [1] If crayfish are not to be had, a lobster will be found to
     answer to the description of the former, in almost all points;
     but the gills and the abdominal appendages present differences;
     and the last thoracic somite is united with the rest in the
     lobster. (_See_ Chap. V.)

       *       *       *       *       *

It is a matter of common information that a number of our streams and
rivulets harbour small animals, rarely more than three or four inches
long, which are very similar to little lobsters, except that they are
usually of a dull, greenish or brownish colour, generally diversified
with pale yellow on the under side of the body, and sometimes with
red on the limbs. In rare cases, their {6} general hue may be red or
blue. These are “crayfishes,” and they cannot possibly be mistaken
for any other inhabitants of our fresh waters.

[Illustration: FIG. 1.—_Astacus fluviatilis._—Side view of a male
specimen (nat. size):—_bg_, branchiostegite; _cg_, cervical groove;
_r_, rostrum; _t_, telson.—1, eye-stalk; 2, antennule; 3, antenna;
9, external maxillipede; 10, forceps; 14, last ambulatory leg; 17,
third abdominal appendage; 20, lateral lobe of the tail-fin, or
sixth abdominal appendage; XV, the first; and XX, the last abdominal
somite. In this and in succeeding figures the numbers of the somites
are given in Roman, those of the appendages in ordinary numerals.]

The animals may be seen walking along the bottom of the shallow
waters which they prefer, by means of four pairs of jointed legs
(fig. 1); but, if alarmed, they swim {7} backwards with rapid jerks,
propelled by the strokes of a broad, fan-shaped flipper, which
terminates the hinder end of the body (fig. 1, _t_, _20_). In front
of the four pairs of legs, which are used in walking, there is a pair
of limbs of a much more massive character, each of which ends in two
claws disposed in such a manner as to constitute a powerful pincer
(fig. 1; _10_). These claws are the chief weapons of offence and
defence of the crayfish, and those who handle them incautiously will
discover that their grip is by no means to be despised, and indicates
a good deal of disposable energy. A sort of shield covers the front
part of the body, and ends in a sharp projecting spine in the middle
line (_r_). On each side of this is an eye, mounted on a movable
stalk (_1_), which can be turned in any direction: behind the eyes
follow two pairs of feelers; in one of these, the feeler ends in two,
short, jointed filaments (_2_); while, in the other, it terminates
in a single, many-jointed filament, like a whip-lash, which is more
than half the length of the body (_3_). Sometimes turned backwards,
sometimes sweeping forwards, these long feelers continually explore a
considerable area around the body of the crayfish.

If a number of crayfishes, of about the same size, are compared
together, it will easily be seen that they fall into two sets; the
jointed tail being much broader, especially in the middle, in the one
set than in the other (fig. 2). The broad-tailed crayfishes are the
{8} females, the others the males. And the latter may be still more
easily known by the possession of four curved styles, attached to
the under face of the first two rings of the tail, which are turned
forwards between the hinder legs, on the under side of the body (fig.
3, A; _15_, _16_). In the female, there are mere soft filaments in
the place of the first pair of styles (fig. 3, B; _15_).

Crayfishes do not inhabit every British river, and even where they
are known to abound, it is not easy to find them at all times of
the year. In granite districts and others, in which the soil yields
little or no calcareous matter to the waters which flow over it,
crayfishes do not occur. They are intolerant of great heat and of
much sunshine; they are therefore most active towards the evening,
while they shelter themselves under the shade of stones and banks
during the day. It has been observed that they frequent those parts
of a river which run north and south, less than those which have an
easterly and westerly direction, inasmuch as the latter yield more
shade from the mid-day sun.

During the depth of winter, crayfishes are rarely to be seen about
in a stream; but they may be found in abundance in its banks, in
natural crevices and in burrows which they dig for themselves. The
burrows may be from a few inches to more than a yard deep, and it has
been noticed that, if the waters are liable to freeze, the burrows
are deeper and further from the surface than otherwise. Where the
soil, through {9} which a stream haunted by crayfishes runs, is soft
and peaty, the crayfishes work their way into it in all directions,
and thousands of them, of all sizes, may be dug out, even at a
considerable distance from the banks.

It does not appear that crayfishes fall into a state of torpor in
the winter, and thus “hybernate” in the strict sense of the word.
At any rate, so long as the weather is open, the crayfish lies at
the mouth of his burrow, barring the entrance with his great claws,
and with protruded feelers keeps careful watch on the passers-by.
Larvæ of insects, water-snails, tadpoles, or frogs, which come
within reach, are suddenly seized and devoured, and it is averred
that the water-rat is liable to the same fate. Passing too near the
fatal den, possibly in search of a stray crayfish, whose flavour he
highly appreciates, the vole is himself seized and held till he is
suffocated, when his captor easily reverses the conditions of the
anticipated meal.

In fact, few things in the way of food are amiss to the crayfish;
living or dead, fresh or carrion, animal or vegetable, it is all
one. Calcareous plants, such as the stoneworts (_Chara_), are highly
acceptable; so are any kinds of succulent roots, such as carrots;
and it is said that crayfish sometimes make short excursions inland,
in search of vegetable food. Snails are devoured, shells and all;
the cast coats of other crayfish are turned to account as supplies
of needful calcareous matter; and the unprotected or weakly member
of the family is {10} not spared. Crayfishes, in fact, are guilty
of cannibalism in its worst form; and a French observer pathetically
remarks, that, under certain circumstances, the males “_méconnaissent
les plus saints devoirs;_” and, not content with mutilating or
killing their spouses, after the fashion of animals of higher moral
pretensions, they descend to the lowest depths of utilitarian
turpitude, and finish by eating them.

In the depth of winter, however, the most alert of crayfish can
find little enough food; and hence, when they emerge from their
hiding-places in the first warm days of spring, usually about March,
the crayfishes are in poor condition.

At this time, the females are found to be laden with eggs, of which
from one to two hundred are attached beneath the tail, and look like
a mass of minute berries (fig. 3, B). In May or June, these eggs are
hatched, and give rise to minute young, which are sometimes to be
found attached beneath the tail of the mother, under whose protection
they spend the first few days of their existence.

In this country, we do not set much store upon crayfishes as an
article of food, but on the Continent, and especially in France,
they are in great request. Paris alone, with its two millions
of inhabitants, consumes annually from five to six millions of
crayfishes, and pays about £16,000 for them. The natural productivity
of the rivers of France has long been inadequate to supply the {11}
demand for these delicacies; and hence, not only are large quantities
imported from Germany, and elsewhere, but the artificial cultivation
of crayfish has been successfully attempted on a considerable scale.

Crayfishes are caught in various ways; sometimes the fisherman
simply wades in the water and drags them out of their burrows; more
commonly, hoop-nets baited with frogs are let down into the water
and rapidly drawn up, when there is reason to think that crayfish
have been attracted to the bait; or fires are lighted on the banks
at night, and the crayfish, which are attracted, like moths, to the
unwonted illumination, are scooped out with the hand or with nets.

       *       *       *       *       *

Thus far, our information respecting the crayfish is such as would be
forced upon anyone who dealt in crayfishes, or lived in a district in
which they were commonly used for food. It is common knowledge. Let
us now try to push our acquaintance with what is to be learned about
the animal a little further, so as to be able to give an account of
its Natural History, such as might have been furnished by Buffon if
he had dealt with the subject.

There is an inquiry which does not strictly lie within the province
of physical science, and yet suggests itself naturally enough at the
outset of a natural history.

The animal we are considering has two names, one common, _Crayfish_,
the other technical, _Astacus fluviatilis_. How has it come by
these two names, and why, {12} having a common English name for it
already, should naturalists call it by another appellation derived
from a foreign tongue?

The origin of the common name, “crayfish,” involves some curious
questions of etymology, and indeed, of history. It might readily
be supposed that the word “cray” had a meaning of its own, and
qualified the substantive “fish”—as “jelly” and “cod” in “jellyfish”
and “codfish.” But this certainly is not the case. The old English
method of writing the word was “crevis” or “crevice,” and the “cray”
is simply a phonetic spelling of the syllable “cre,” in which the
“e” was formerly pronounced as all the world, except ourselves, now
pronounce that vowel. While “fish” is the “vis” insensibly modified
to suit our knowledge of the thing as an aquatic animal.

Now “crevis” is clearly one of two things. Either it is a
modification of the French name “écrevisse,” or of the Low Dutch
name “crevik,” by which the crayfish is known in these languages.
The former derivation is that usually given, and, if it be correct,
we must refer “crayfish” to the same category as “mutton,” “beef,”
and “pork,” all of which are French equivalents, introduced by the
Normans, for the “sheep’s flesh,” “ox flesh,” and “swine’s flesh,”
of their English subjects. In this case, we should not have called a
crayfish, a crayfish, except for the Norman conquest.

On the other hand, if “crevik” is the source of our {13} word, it
may have come to us straight from the Angle and Saxon contingent of
our mixed ancestry.

As to the origin of the technical name; ἀστακός, _astakos_, was the
name by which the Greeks knew the lobster; and it has been handed
down to us in the works of Aristotle, who does not seem to have taken
any special notice of the crayfish. At the revival of learning, the
early naturalists noted the close general similarity between the
lobster and the crayfish; but, as the latter lives in fresh water,
while the former is a marine animal, they called the crayfish, in
their Latin, _Astacus fluviatilis_, or the “river-lobster,” by way
of distinction; and this nomenclature was retained until, about
forty-five years ago, an eminent French Naturalist, M. Milne-Edwards,
pointed out that there are far more extensive differences between
lobsters and crayfish than had been supposed; and that it would be
advisable to mark the distinctness of the things by a corresponding
difference in their names. Leaving _Astacus_ for the crayfishes, he
proposed to change the technical name of the lobster into _Homarus_,
by latinising the old French name “_Omar_,” or “_Homar_” (now
_Homard_), for that animal.

At the present time, therefore, while the recognised technical name
of the crayfish is _Astacus fluviatilis_, that of the lobster is
_Homarus vulgaris_. And as this nomenclature is generally received,
it is desirable that it should not be altered; though it is attended
by the inconvenience, that _Astacus_, as we now employ the name,
does not {14} denote that which the Greeks, ancient and modern,
signify, by its original, _astakos_; and does signify something quite
different.

Finally, as to why it is needful to have two names for the same
thing, one vernacular, and one technical. Many people imagine that
scientific terminology is a needless burden imposed upon the novice,
and ask us why we cannot be content with plain English. In reply, I
would suggest to such an objector to open a conversation about his
own business with a carpenter, or an engineer, or, still better,
with a sailor, and try how far plain English will go. The interview
will not have lasted long before he will find himself lost in a maze
of unintelligible technicalities. Every calling has its technical
terminology; and every artisan uses terms of art, which sound like
gibberish to those who know nothing of the art, but are exceedingly
convenient to those who practise it.

In fact, every art is full of conceptions which are special to
itself; and, as the use of language is to convey our conceptions
to one another, language must supply signs for those conceptions.
There are two ways of doing this: either existing signs may be
combined in loose and cumbrous periphrases; or new signs, having a
well-understood and definite signification, may be invented. The
practice of sensible people shows the advantage of the latter course;
and here, as elsewhere, science has simply followed and improved upon
common sense. {15}

Moreover, while English, French, German, and Italian artisans are
under no particular necessity to discuss the processes and results
of their business with one another, science is cosmopolitan, and the
difficulties of the study of Zoology would be prodigiously increased,
if Zoologists of different nationalities used different technical
terms for the same thing. They need a universal language; and it
has been found convenient that the language shall be the Latin in
form, and Latin or Greek in origin. What in English is Crayfish,
is _Écrevisse_ in French; _Flusskrebs_, in German; _Cammaro_, or
_Gambaro_, or _Gammarello_, in Italian: but the Zoologist of each
nationality knows that, in the scientific works of all the rest,
he shall find what he wants to read under the head of _Astacus
fluviatilis_.

But granting the expediency of a technical name for the Crayfish,
why should that name be double? The reply is still, practical
convenience. If there are ten children of one family, we do not
call them all Smith, because such a procedure would not help us to
distinguish one from the other; nor do we call them simply John,
James, Peter, William, and so on, for that would not help us to
identify them as of one family. So we give them all two names,
one indicating their close relation, and the other their separate
individuality—as John Smith, James Smith, Peter Smith, William Smith,
&c. The same thing is done in Zoology; only, in accordance with the
genius of the Latin language, {16} we put the Christian name, so to
speak, after the surname.

There are a number of kinds of Crayfish, so similar to one another
that they bear the common surname of _Astacus_. One kind, by way
of distinction, is called _fluviatile_, another _slender-handed_,
another _Dauric_, from the region in which it lives; and these double
names are rendered by—_Astacus fluviatilis_, _Astacus leptodactylus_,
and _Astacus dauricus_; and thus we have a nomenclature which is
exceedingly simple in principle, and free from confusion in practice.
And I may add that, the less attention is paid to the original
meaning of the substantive and adjective terms of this binomial
nomenclature, and the sooner they are used as proper names, the
better. Very good reasons for using a term may exist when it is first
invented, which lose their validity with the progress of knowledge.
Thus _Astacus fluviatilis_ was a significant name so long as we knew
of only one kind of crayfish; but now that we are acquainted with
a number of kinds, all of which inhabit rivers, it is meaningless.
Nevertheless, as changing it would involve endless confusion, and
the object of nomenclature is simply to have a definite name for a
definite thing, nobody dreams of proposing to alter it.

       *       *       *       *       *

Having learned this much about the origin of the names of the
crayfish, we may next proceed to consider those points which an
observant Naturalist, who did not {17} care to go far beyond the
surface of things, would find to notice in the animal itself.

Probably the most conspicuous peculiarity of the crayfish, to any one
who is familiar only with the higher animals, is the fact that the
hard parts of the body are outside and the soft parts inside; whereas
in ourselves, and in the ordinary domestic animals, the hard parts,
or bones, which constitute the skeleton, are inside, and the soft
parts clothe them. Hence, while our hard framework is said to be an
_endoskeleton_, or internal skeleton; that of the crayfish is termed
an _exoskeleton_, or external skeleton. It is from the circumstance
that the body of the crayfishes is enveloped in this hard crust, that
the name of _Crustacea_ is applied to them, along with the crabs,
shrimps, and other such animals. Insects, spiders, and centipedes
have also a hard exoskeleton, but it is usually not so hard and thick
as in the _Crustacea_.

If a piece of the crayfish’s skeleton is placed in strong vinegar,
abundant bubbles of carbonic acid gas are given off from it, and
it rapidly becomes converted into a soft laminated membrane, while
the solution will be found to contain lime. In fact the exoskeleton
is composed of a peculiar animal matter, so much impregnated with
carbonate and phosphate of lime that it becomes dense and hard.

[Illustration: FIG. 2.—_Astacus fluviatilis._—Dorsal or tergal views
(nat. size). A, male; B, female:—_bcg_, branchio-cardiac groove,
which marks the boundary between the pericardial and the branchial
cavities; _cg_, cervical groove; these letters are placed on the
carapace; _r_, rostrum; _t, t′_, the two divisions of the telson;
_1_, eye-stalks; _2_, antennules; _3_, antennæ; _20_, lateral lobes
of tail-fin; XV–XX, somites of the abdomen.]

It will be observed that the body of the crayfish is naturally marked
out into several distinct regions. There {19} is a firm and solid
front part, covered by a large continuous shield, which is called the
_carapace_; and a jointed hind part, commonly termed the tail (fig.
2). From the perception of a partially real, and partially fanciful,
analogy with the regions into which the body is divided in the higher
animals, the fore part is termed the _cephalo-thorax_, or head
(_cephalon_) and chest (_thorax_) combined, while the hinder part
receives the name of _abdomen_.

Now the exoskeleton is not of the same constitution throughout these
regions. The abdomen, for example, is composed of six complete hard
rings (fig. 2, XV–XX), and a terminal flap, on the under side of
which the vent (fig. 3, _a_) is situated, and which is called the
_telson_ (fig. 2, _t, t′_). All these are freely moveable upon one
another, inasmuch as the exoskeleton which connects them is not
calcified, but is, for the most part, soft and flexible, like the
hard exoskeleton when the lime salts have been removed by acid.
The mechanism of the joints will have to be attentively considered
by-and-by; it is sufficient, at present, to remark that, wherever a
joint, exists, it is produced in the same fashion, by the exoskeleton
remaining soft in certain regions of the jointed part.

The carapace is not jointed; but a transverse groove is observed
about the middle of it, the ends of which run down on the sides
and then turn forwards (figs. 1 and 2, _cg_). This is called the
_cervical groove_, and it marks off {20} the region of the head, in
front, from that of the thorax behind.

The thorax seems at first not to be jointed at all; but if its
under, or what is better called its _sternal_, surface is examined
carefully, it will be found to be divided into as many transverse
bands, or segments, as there are pairs of legs (fig. 3); and,
moreover, the hindermost of these segments is not firmly united with
the rest, but can be moved backwards and forwards through a small
space (fig. 3, B; xiv).

Attached to the sternal side of every ring of the abdomen of the
female there is a pair of limbs, called _swimmerets_. In the five
anterior rings, these are small and slender (fig. 3, B; _15, 19_);
but those of the sixth ring are very large, and each ends in two
broad plates (_20_). These two plates on each side, with the telson
in the middle, constitute the flapper of the crayfish, by the aid
of which it executes its retrograde swimming movements. The small
swimmerets move together with a regular swing, like paddles, and
probably aid in propelling the animal forwards. In the breeding
female (B), the eggs are attached to them; while, in the male, the
two anterior pairs (A; _15, 16_) are converted into the peculiar
styles which distinguish that sex.

[Illustration: FIG. 3.—_Astacus fluviatilis._—Ventral or sternal
views (nat. size). A, male; B, female:—_a_, vent; _gg_, opening
of the green gland; _lb_, labrum; _mt_, metastoma or lower lip;
_od_, opening of the oviduct; _vd_, that of the vas deferens. _1_,
eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _8_, second
maxillipede; _9_, third or external maxillipede; _10_, forceps; _11_,
first leg; _14_, fourth leg; _15, 16, 19, 20_, first, second, fifth,
and sixth abdominal appendages; X., XI., XIV., sterna of the fourth,
fifth, and eighth thoracic somite; XVI., sternum of the second
abdominal somite. In the male, the 9th to the 14th and the 16th to
the 19th appendages are removed on the animal’s left side: in the
female, the antenna (with the exception of its basal joint) and the
5th to the 14th appendages on the animal’s right are removed; the
eggs also are shown attached to the swimmerets of the left side of
the body.]

The four pairs of legs which are employed for walking purposes, are
divided into a number of joints, and the foremost two pairs are
terminated by double claws, arranged so as to form a pincer, whence
they are said to {22} be _chelate_. The two hindermost pairs, on
the other hand, end in simple claws.

In front of these legs, come the great prehensile limbs (_10_), which
are chelate, like those which immediately follow them, but vastly
larger. They often receive the special name of _chelæ_; and the large
terminal joints are called the “hand.” We shall escape confusion if
we call these limbs the _forceps_, and restrict the name of _chela_
to the two terminal joints.

All the limbs hitherto mentioned subserve locomotion and prehension
in various degrees. The crayfish swims by the help of its abdomen,
and the hinder pairs of abdominal limbs; walks by means of the four
hinder pairs of thoracic limbs; lays hold of anything to fix itself,
or to assist in climbing, by the two chelate anterior pairs of these
limbs, which are also employed in tearing the food seized by the
forceps and conveying it to the mouth; while it seizes its prey and
defends itself with the forceps. The part which each of these limbs
plays is termed its _function_ and it is said to be the _organ_ of
that function; so that all these limbs may be said to be organs of
the functions of locomotion, of offence and defence.

In front of the forceps, there is a pair of limbs which have a
different character, and take a different direction from any of the
foregoing (_9_). These limbs, in fact, are turned directly forwards,
parallel with one another, and with the middle line of the body. They
are divided into a number of joints, of which one of those near the
base {23} is longer than the rest, and strongly toothed along the
inner edge, or that which is turned towards its fellow. It is obvious
that these two limbs are well adapted to crush and tear whatever
comes between them, and they are, in fact, _jaws_ or organs of
manducation. At the same time, it will be noticed that they retain a
curiously close general resemblance to the hinder thoracic legs; and
hence, for distinction’s sake, they are called outer _foot-jaws_ or
external _maxillipedes_.

If the head of a stout pin is pushed between these external
maxillipedes, it will be found that it passes without any difficulty
into the interior of the body, through the mouth. In fact, the
mouth is relatively rather a large aperture; but it cannot be
seen without forcing aside, not only these external foot-jaws,
but a number of other limbs, which subserve the same function of
manducation, or chewing and crushing the food. We may pass by the
organs of manducation, for the present, with the remark that there
are altogether three pairs of maxillipedes, followed by two pairs of
somewhat differently formed _maxillæ_, and one pair of very stout
and strong jaws, which are termed the _mandibles_ (_4_). All these
jaws work from side to side, in contradistinction to the jaws of
vertebrated animals, which move up and down. In front of, and above
the mouth, with the jaws which cover it, are seen the long feelers,
which are called the _antennæ_ (_3_); above, and in front of them,
follow the small feelers, or _antennules_ (_2_); and over them,
again, lie {24} the _eye stalks_ (_1_). The antennæ are organs of
touch; the antennules, in addition, contain the organs of hearing;
while, at the ends of the eyestalks, are the organs of vision.

Thus we see that the crayfish has a jointed and segmented body, the
rings of which it is composed being very obvious in the abdomen, but
more obscurely traceable elsewhere; that it has no fewer than twenty
pairs of what may be called by the general name of _appendages_; and
that these appendages are turned to different uses, or are organs of
different functions, in different parts of the body. The crayfish is
obviously a very complicated piece of living machinery. But we have
not yet come to the end of all the organs that may be discovered even
by cursory inspection. Every one who has eaten a boiled crayfish, or
a lobster, knows that the great shield, or carapace, is very easily
separated from the thorax and abdomen, the head and the limbs which
belong to that region coming away with the carapace. The reason of
this is not far to seek. The lower edges of that part of the carapace
which belongs to the thorax approach the bases of the legs pretty
closely, but a cleft-like space is left; and this cleft extends
forwards to the sides of the region of the mouth, and backwards and
upwards, between the hinder margin of the carapace and the sides of
the first ring of the abdomen, which are partly overlapped by, and
partly overlap, that margin. If the blade of a pair of scissors is
{25} carefully introduced into the cleft from behind, as high up as
it will go without tearing anything, and a cut is then made, parallel
with the middle line, as far as the cervical groove, and thence
following the cervical groove to the base of the outer foot-jaws, a
large flap will be removed. This flap of the carapace is called the
_branchiostegite_ (fig. 1, _bg_), because it covers the gills or
_branchiæ_ (fig. 4), which are now exposed. They have the appearance
of a number of delicate plumes, which take a direction from the
bases of the legs upwards and forwards behind, upwards and backwards
in front, their summits converging towards the upper end of the
cavity in which they are placed, and which is called the _branchial
chamber_. These branchiæ are the respiratory organs; and they perform
the same functions as the gills of a fish, to which they present some
similarity.

If the gills are cleared away, it is seen that the branchial cavity
is bounded, on the inner side, by a sloping wall, formed by a
delicate, but more or less calcified layer of the exoskeleton, which
constitutes the proper outer wall of the thorax. At the upper limit
of the branchial cavity, the layer of exoskeleton is very thin, and
turning outwards, is continued into the inner wall or lining of the
branchiostegite, which is also very thin (_see_ fig. 15, p. 70).

[Illustration: FIG. 4.—_Astacus fluviatilis._—In A, the gills,
exposed by the removal of the branchiostegite, are seen in their
natural position; in B, the podobranchiæ (_see_ p. 75) are removed,
and the anterior set of arthrobranchiæ turned downwards (× 2):
_1_, eye-stalk; _2_, antennule; _3_, antenna; _4_, mandible; _6_,
scaphognathite; _7_, first maxillipede, in B the epipodite, to which
the line points, is partly removed; _8_, second maxillipede; _9_,
third maxillipede; _10_, forceps; _14_, fourth ambulatory leg; _15_,
first abdominal appendage; XV., first, and XVI., second abdominal
somite; _arb. 8_, _arb. 9_, _arb. 13_, the posterior arthrobranchiæ
of the second and third maxillipedes and of the third ambulatory
leg; _arb′. 9_, _arb′. 13_, the anterior arthrobranchiæ of the third
maxillipede and of the third ambulatory leg; _pbd. 8_, podobranchiæ
of the second maxillipede; _pbd. 13_, that of the third ambulatory
leg; _plb. 12_, _plb. 13_, the two rudimentary pleurobranchiæ; _plb.
14_, the functional pleurobranchia; _r_, rostrum.]

Thus the branchial chamber is altogether outside the body, to which
it stands in somewhat the same relation as the space between the
flaps of a man’s coat and his waistcoat would do to the part of the
body enclosed by the {27} waistcoat, if we suppose the lining of
the flaps to be made in one piece with the sides of the waistcoat.
Or a closer parallel still would be brought about, if the skin of a
man’s back were loose enough to be pulled out, on each side, into two
broad flaps covering the flanks.

It will be observed that the branchial chamber is open behind, below,
and in front; and, therefore, that the water in which the crayfish
habitually lives has free ingress and egress. Thus the air dissolved
in the water enables breathing to go on, just as it does in fishes.
As is the case with many fishes, the crayfish breathes very well out
of the water, if kept in a situation sufficiently cool and moist to
prevent the gills from drying up; and thus there is no reason why,
in cool and damp weather, the crayfish should not be able to live
very well on land, at any rate among moist herbage, though whether
our common crayfishes do make such terrestrial excursions is perhaps
doubtful. We shall see, by-and-by, that there are some exotic
crayfish which habitually live on land, and perish if they are long
submerged in water.

       *       *       *       *       *

With respect to the internal structure of the crayfish, there are
some points which cannot escape notice, however rough the process of
examination may be.

[Illustration: FIG. 5.—_Astacus fluviatilis._—A male specimen, with
the roof of the carapace and the terga of the abdominal somites
removed to show the viscera (nat. size):—_aa_, antennary artery;
_ag_, anterior gastric muscles; _amm_, adductor muscles of the
mandibles; _cs_, cardiac portion of the stomach; _gg_, green glands;
_h_, heart; _hg_, hind gut, or large intestine; _Lr_, liver; _oa_,
ophthalmic artery; _pg_, posterior gastric muscles; _saa_, superior
abdominal artery; _t_, testis; _vd_, vas deferens.]

Thus, when the carapace is removed in a crayfish which has been
just killed, the heart is seen still pulsating. It is an organ
of considerable relative size (fig. 5, _h_), which is situated
immediately beneath the {29} middle region of that part of the
carapace which lies behind the cervical groove; or, in other words,
in the dorsal region of the thorax. In front of it, and therefore in
the head, is a large rounded sac, the stomach (fig. 5, _cs_; fig. 6,
_cs_, _ps_), from which a very delicate intestine (figs. 5 and 6,
_hg_) passes straight back through the thorax and abdomen to the vent
(fig. 6, _a_).

[Illustration: FIG. 6.—_Astacus fluviatilis._—A longitudinal vertical
section of the alimentary canal, with the outline of the body (nat.
size):—_a_, vent; _ag_, anterior gastric muscle; _bd_, entrance
of left bile duct; _cg_, cervical groove; _cæ_, cæcum; _cpv_,
cardio-pyloric valve; _cs_, cardiac portion of stomach; the circular
area immediately below the end of the line from _cs_ marks the
position of the gastrolith of the left side; _hg_, hind-gut; _lb_,
labrum; _lt_, lateral tooth of stomach; _m_, mouth; _mg_, mid-gut;
_mt_, median tooth; _œ_, œsophagus; _pc_, procephalic process; _pg_,
posterior gastric muscle; _ps_, pyloric portion of stomach; _r_,
annular ridge, marking the commencement of the hind-gut.]

In summer, there are commonly to be found at the sides of the stomach
two lenticular calcareous masses, which are known as “crabs’-eyes,”
or _gastroliths_, and were, in old times, valued in medicine as
sovereign remedies for all sorts of disorders. These bodies (fig. 7)
are smooth and flattened, or concave, on the side which is turned
towards {30} the cavity of the stomach; while the opposite side,
being convex and rough with irregular prominences, is something like
a “brain-stone” coral.

[Illustration: FIG. 7.—_Astacus fluviatilis._—A gastrolith; A, from
above; B, from below; C, from one side (all × 5); D, in vertical
section (× 20).]

Moreover, when the stomach is laid open, three large reddish teeth
are seen to project conspicuously into its interior (fig. 6, _lt_,
_mt_); so that, in addition to its six pairs of jaws, the crayfish
has a supplementary crushing mill in its stomach. On each side of
the stomach, there is a soft yellow or brown mass, commonly known
as the {31} liver (fig. 5, _Lr_); and, in the breeding season, the
ovaries of the females, or organs in which the eggs are formed, are
very conspicuous from the dark-coloured eggs which they contain,
and which, like the exoskeleton, turn red when they are boiled.
The corresponding part in a cooked lobster goes by the name of the
“coral.”

Beside these internal structures, the most noticeable are the large
masses of flesh, or muscle, in the thorax and abdomen, and in the
pincers; which, instead of being red, as in most of the higher
animals, is white. It will further be observed that the blood, which
flows readily when a crayfish is wounded, is a clear fluid, and is
either almost colourless, or of a very pale reddish or neutral tint.
Hence the older Naturalists thought that the crayfish was devoid of
blood, and had merely a sort of ichor in place of it. But the fluid
in question is true blood; and if it is received into a vessel, it
soon forms a soft, but firm, gelatinous clot.

       *       *       *       *       *

The crayfish grows rapidly in youth, but enlarges more and more
slowly as age advances. The young animal which has just left the egg
is of a greyish colour, and about one quarter of an inch long. By
the end of the year, it may have reached nearly an inch and a half
in length. Crayfishes of a year old are, on an average, two inches
long; at two years, two inches and four-fifths; at three years, three
inches and a half; at four years, four inches and a half nearly;
and at five years, five inches. They {32} go on growing till, in
exceptional cases, they may attain between seven inches and eight
inches in length; but at what degree of longevity this unusual
dimension is reached is uncertain. It seems probable, however, that
the life of these animals may be prolonged to as much as fifteen or
twenty years. They appear to reach maturity, so far as the power of
reproduction is concerned, in their fifth or, more usually, their
sixth year. However, I have seen a female, with eggs attached under
the abdomen, only two inches long, and therefore, probably, in her
second year. The males are commonly larger than females of the same
age.

       *       *       *       *       *

The hard skeleton of a crayfish, once formed, is incapable of being
stretched, nor can it increase by interstitial addition to its
substance, as the bone of one of the higher animals grows. Hence
it follows, that the enlargement of the body, which actually takes
place, involves the shedding and reproduction of its investment. This
might be effected by insensible degrees, and in different parts of
the body at different times, as we shed our hair; but, as a matter
of fact, it occurs periodically and universally, somewhat as the
feathers of birds are moulted. The whole of the old coat of the body
is thrown off at once, and suddenly; and the new coat, which has, in
the meanwhile, been formed beneath the old one, remains soft for a
time, and allows of a rapid increase in the dimensions of the body
before it {33} hardens. This sort of moulting is what is technically
termed _ecdysis_, or _exuviation_. It is commonly spoken of as the
“shedding of the skin,” and there is no harm in using this phrase,
if we recollect that the shed coat is not the skin, in the proper
sense of the word, but only what is termed a _cuticular layer_,
which is secreted upon the outer surface of the true integument. The
cuticular skeleton of the crayfish, in fact, is not even so much
a part of the skin as the cast of a snake, or as our own nails.
For these are composed of coherent, formed parts of the epidermis;
while the hard investment of the crayfish contains no such formed
parts, and is developed on the outside of those structures which
answer to the constituents of the epidermis in the higher animals.
Thus the crayfish grows, as it were, by starts; its dimensions
remaining stationary in the intervals of its moults, and then rapidly
increasing for a few days, while the new exoskeleton is in the course
of formation.

The ecdysis of the crayfish was first thoroughly studied a century
and a half ago, by one of the most accurate observers who ever lived,
the famous Réaumur, and the following account of this very curious
process is given nearly in his words.[2]

     [2] See Réaumur’s two Memoirs, “Sur les diverses reproductions
     qui se font dans les écrevisses, les omars, les crabes, etc.,”
     “Histoire de l’Académie royale des Sciences,” année 1712; and
     “Additions aux observations sur la mue des écrevisses données
     dans les Mémoires de 1712.” Ibid. 1718.

A few hours before the process of exuviation {34} commences, the
crayfish rubs its limbs one against the other, and, without changing
its place, moves each separately, throws itself on its back, bends
its tail, and then stretches it out again, at the same time vibrating
its antennæ. By these movements, it gives the various parts a little
play in their loosened sheaths. After these preparatory steps,
the crayfish appears to become distended; in all probability, in
consequence of the commencing retraction of the limbs into the
interior of the exoskeleton of the body. In fact, it has been
remarked, that if, at this period, the extremity of one of the great
claws is broken off, it will be found empty, the contained soft parts
being retracted as far as the second joint. The soft membranous part
of the exoskeleton, which connects the hinder end of the carapace
with the first ring of the abdomen, gives way, and the body, covered
with the new soft integument, protrudes; its dark brown colour
rendering it easily distinguishable from the greenish-brown old
integument.

Having got thus far, the crayfish rests for a while, and then the
agitation of the limbs and body recommences. The carapace is forced
upwards and forwards by the protrusion of the body, and remains
attached only in the region of the mouth. The head is next drawn
backwards, while the eyes and its other appendages are extracted from
their old investment. Next the legs are pulled out, either one at
a time, or those of one, or both, sides together. Sometimes a limb
gives way and is left behind in its sheath. {35} The operation is
facilitated by the splitting of the old integument of the limb along
one side longitudinally.

When the legs are disengaged, the animal draws its head and limbs
completely out of their former covering; and, with a sudden spring
forward, while it extends its abdomen, it extracts the latter, and
leaves its old skeleton behind. The carapace falls back into its
ordinary position, and the longitudinal fissures of the sheaths of
the limbs close up so accurately, that the shed integument has just
the appearance the animal had when the exuviation commenced. The
cast exoskeleton is so like the crayfish itself, when the latter is
at rest, that, except for the brighter colour of the latter, the two
cannot be distinguished.

After exuviation, the owner of the cast skin, exhausted by its
violent struggles, which are not unfrequently fatal, lies in a
prostrate condition. Instead of being covered by a hard shell,
its integument is soft and flabby, like wet paper; though Réaumur
remarks, that if a crayfish is handled immediately after exuviation,
its body feels hard; and he ascribes this to the violent contraction
which its muscles have undergone, leaving them in a state of cramp.
In the absence of the hard skeleton, however, there is nothing to
bring the contracted muscles at once back into position, and it
must be some time before the pressure of the internal fluids is so
distributed as to stretch them out.

When the process of exuviation has proceeded so far {36} that the
carapace is raised, nothing stops the crayfish from continuing its
struggles. If taken out of the water in this condition, they go on
moulting in the hand, and even pressure on their bodies will not
arrest their efforts.

The length of time occupied from the first giving way of the
integuments to the final emergence of the animal, varies with its
vigour, and the conditions under which it is placed, from ten minutes
to several hours. The chitinous lining of the stomach, with its
teeth, and the “crabs’-eyes,” are shed along with the rest of the
cuticular exoskeleton; but they are broken up and dissolved in the
stomach.

The new integuments of the crayfish remain soft for a period which
varies from one to three days; and it is a curious fact, that the
animal appears to be quite aware of its helplessness, and governs
itself accordingly.

An observant naturalist says: “I once had a domesticated crayfish
(_Astacus fluviatilis_), which I kept in a glass pan, in water, not
more than an inch and a half deep, previous experiment having shown
that in deeper water, probably from want of sufficient aëration, this
animal would not live long. By degrees my prisoner became very bold,
and when I held my fingers at the edge of the vessel, he assailed
them with promptness and energy. About a year after I had him, I
perceived, as I thought, a second crayfish with him. On examination,
I found it to be his old coat, which he had left in a most perfect
state. My friend had now lost his heroism, and {37} fluttered about
in the greatest agitation. He was quite soft; and every time I
entered the room during the next two days, he exhibited the wildest
terror. On the third, he appeared to gain confidence, and ventured to
use his nippers, though with some timidity, and he was not yet quite
so hard as he had been. In about a week, however, he became bolder
than ever; his weapons were sharper, and he appeared stronger, and
a nip from him was no joke. He lived in all about two years, during
which time his food was a very few worms at very uncertain times;
perhaps he did not get fifty altogether.”[3]

     [3] The late Mr. Robert Ball, of Dublin, in Bell’s “British
     Crustacea,” p. 239.

It would appear, from the best observations that have yet been made,
that the young crayfish exuviate two or three times in the course
of the first year; and that, afterwards, the process is annual, and
takes place usually about midsummer. There is reason to suppose that
very old crayfish do not exuviate every year.

       *       *       *       *       *

It has been stated that, in the course of its violent efforts to
extract its limbs from the cast-off exoskeleton, the crayfish
sometimes loses one or other of them; the limb giving way, and the
greater part, or the whole, of it remaining in the exuviæ. But it
is not only in this way that crayfishes part with their limbs. At
all times, if the animal is held by one of its pincers, so that it
cannot get away, it is apt to solve the difficulty by casting off
{38} the limb, which remains in the hand of the captor, while the
crayfish escapes. This voluntary amputation is always effected at the
same place; namely, where the limb is slenderest, just beyond the
articulation which unites the basal joint with the next. The other
limbs also readily part at the joints; and it is very common to meet
with crayfish which have undergone such mutilation. But the injury
thus inflicted is not permanent, as these animals possess the power
of reproducing lost parts to a marvellous extent, whether the loss
has been inflicted by artificial amputation, or voluntarily.

Crayfishes, like all the _Crustacea_, bleed very freely when wounded;
and if one of the large joints of a leg is cut through, or if the
animal’s body is injured, it is very likely to die rapidly from the
ensuing hæmorrhage. A crayfish thus wounded, however, commonly throws
off the limb at the next articulation, where the cavity of the limb
is less patent, and its sides more readily fall together; and, as we
have seen, the pincers are usually cast off at their narrowest point.
When such amputation has taken place, a crust, probably formed of
coagulated blood, rapidly forms over the surface of the stump; and,
eventually, it becomes covered with a cuticle. Beneath this, after a
time, a sort of bud grows out from the centre of the surface of the
stump, and gradually takes on the form of as much of the limb as has
been removed. At the next ecdysis, the covering cuticle is thrown off
along with the rest of the exoskeleton; while the {39} rudimentary
limb straightens out, and, though very small, acquires all the
organization appropriate to that limb. At every moult it grows; but,
it is only after a long time that it acquires nearly the size of its
uninjured and older fellow. Hence, it not unfrequently happens, that
crayfish are found with pincers and other limbs, which, though alike
useful and anatomically complete, are very unequal in size.

Injuries inflicted while the crayfish are soft after moulting, are
apt to produce abnormal growths of the part affected; and these
may be perpetuated, and give rise to various monstrosities, in the
pincers and in other parts of the body.

       *       *       *       *       *

In the reproduction of their kind by means of eggs the co-operation
of the males with the females is necessary. On the basal joint of the
hindermost pair of legs of the male a small aperture is to be seen
(fig. 3, A; _vd_). In these, the ducts of the apparatus in which the
fecundating substance is formed terminate. The fecundating material
itself is a thickish fluid, which sets into a white solid after
extrusion. The male deposits this substance on the thorax of the
female, between the bases of the hindermost pairs of thoracic limbs.

The eggs formed in the ovary are conducted to apertures, which are
situated on the bases of the last pair of ambulatory legs but two,
that is, in the hinder of the two pair which are provided with
chelate extremities (fig. 3, B; _od_). {40}

After the female has received the deposit of the spermatic matter
of the male, she retires to a burrow, in the manner already stated,
and then the process of laying the eggs commences. These, as they
leave the apertures of the oviducts, are coated with a viscid matter,
which is readily drawn out into a short thread. The end of the thread
attaches itself to one of the long hairs, with which the swimmerets
are fringed, and as the viscid matter rapidly hardens, the egg thus
becomes attached to the limb by a stalk. The operation is repeated,
until sometimes a couple of hundred eggs are thus glued on to the
swimmerets. Partaking in the movements of the swimmerets, they are
washed backwards and forwards in the water, and thus aërated and kept
free of impurities; while the young crayfish is formed much in the
same way as the chick is formed in a hen’s egg.

The process of development, however, is very slow, as it occupies the
whole winter. In late spring-time, or early summer, the young burst
the thin shell of the egg, and, when they are hatched, present a
general resemblance to their parents. This is very unlike what takes
place in crabs and lobsters, in which the young leave the egg in a
condition very different from the parent, and undergo a remarkable
metamorphosis before they attain their proper form.

For some time after they are hatched, the young hold on to the
swimmerets of the mother, and are carried about, protected by her
abdomen, as in a kind of nursery. {41}

[Illustration: FIG. 8.—_Astacus fluviatilis._—A, two recently hatched
crayfish attached to one of the swimmerets of the mother (× 4). _pr_,
protopodite; _en_, endopodite; and _ex_, exopodite of the swimmeret;
_ec_, ruptured egg-cases. B, chela of a recently hatched crayfish
(× 10).]

That most careful naturalist, Roesel von Rosenhof, says of the young,
when just hatched:—

“At this time they are quite transparent; and when such a crayfish
[a female with young] is brought to table, it looks quite disgusting
to those who do not know {42} what the young are; but if we examine
it more closely, especially with a magnifying-glass, we see with
pleasure that the little crayfish are already perfect, and resemble
the large one in all respects. When the mother of these little
crayfish, after they have begun to be active, is quiet for a while,
they leave her and creep about, a short way off. But, if they spy the
least sign of danger, or there is any unusual movement in the water,
it seems as if the mother recalled them by a signal; for they all at
once swiftly return under her tail, and gather into a cluster, and
the mother hies to a place of safety with them, as quickly as she
can. A few days later, however, they gradually forsake her.”[4]

Fishermen declare that “Hen Lobsters” protect their young in a
similar manner.[5] Jonston,[6] who wrote in the middle of the
seventeenth century, says that the little crayfish are often to be
seen adhering to the tail of the mother. Roesel’s observations imply
the same thing; but he does not describe the exact mode of adherence,
and I can find no observations on the subject in the works of later
writers.

     [4] “Der Monatlich-herausgegeben Insecten Belustigung.” Dritter
     Theil, p. 336. 1755.

     [5] Bell’s “British Crustacea,” p. 249.

     [6] “Joannis Jonstoni Historiæ naturalis de Piscibus et Cetis
     Libri quinque. Tomus IV. ‘De Cammaro seu Astaco fluviatili.’”

It has been seen that the eggs are attached to the swimmerets by a
viscid substance, which is, as it were, smeared over them and the
hairs with which they are {43} fringed, and is continued by longer
or shorter thread-like pedicles into the coat of the same material
which invests each egg. It very soon hardens, and then becomes very
firm and elastic.

When the young crayfish is ready to be hatched, the egg case splits
into two moieties, which remain attached, like a pair of watch
glasses, to the free end of the pedicle of the egg (fig. 8, A; _ec_).
The young animal, though very similar to the parent, does not quite
“resemble it in all respects,” as Roesel says. For not only are the
first and the last pairs of abdominal limbs wanting, while the telson
is very different from that of the adult; but the ends of the great
chelæ are sharply pointed and bent down into abruptly incurved hooks,
which overlap when the chelæ are shut (fig. 8, B). Hence, when the
chelæ have closed upon anything soft enough to allow of the imbedding
of these hooks, it is very difficult, if not impossible, to open them
again.

Immediately the young are set free, they must instinctively bury
the ends of their forceps in the hardened egg-glue which is smeared
over the swimmerets, for they are all found to be holding on in
this manner. They exhibit very little movement, and they bear rough
shaking or handling without becoming detached; in consequence, I
suppose, of the interlocking of the hooked ends of the chelæ imbedded
in the egg-glue.

Even after the female has been plunged into alcohol, the young remain
attached. I have had a female, with young affixed in this manner,
under observation for five {44} days, but none of them showed any
signs of detaching themselves; and I am inclined to think that they
are set free only at the first moult. After this, it would appear
that the adhesion to the parent is only temporary.

The walking legs are also hooked at their extremities, but they play
a less important part in fixing the young to the parent, and seem to
be always capable of loosing their hold.

I find the young of a Mexican crayfish (_Cambarus_) to be attached in
the same manner as those of the English crayfish; but, according to
Mr. Wood-Mason’s recent observations, the young of the New Zealand
crayfishes fix themselves to the swimmerets of the parent by the
hooked ends of their hinder ambulatory limbs.

       *       *       *       *       *

Crayfishes, in every respect similar to those found in our English
rivers, that is to say, of the species _Astacus fluviatilis_, are
met with in Ireland, and on the Continent, as far south as Italy and
northern Greece; as far east as western Russia; and as far north as
the shores of the Baltic. They are not known to occur in Scotland; in
Spain, except about Barcelona, they are either rare, or have remained
unnoticed.

There is, at present, no proof of the occurrence of _Astacus
fluviatilis_ in the fossil state.

       *       *       *       *       *

Curious myths have gathered about crayfishes, as about other animals.
At one time “crabs’-eyes” were {45} collected in vast numbers, and
sold for medicinal purposes as a remedy against the stone, among
other diseases. Their real utility, inasmuch as they consist almost
entirely of carbonate of lime, with a little phosphate of lime and
animal matter, is much the same as that of chalk, or carbonate of
magnesia. It was, formerly, a current belief that crayfishes grow
poor at the time of new moon, and fat at that of full moon; and,
perhaps, there may be some foundation for the notion, considering
the nocturnal habits of the animals. Van Helmont, a great dealer in
wonders, is responsible for the story that, in Brandenburg, where
there is a great abundance of crayfishes, the dealers were obliged to
transport them to market by night, lest a pig should run under the
cart. For if such a misfortune should happen, every crayfish would be
found dead in the morning: “Tam exitialis est porcus cancro.” Another
author improves the story, by declaring that the steam of a pig-stye,
or of a herd of swine, is instantaneously fatal to crayfish. On the
other hand, the smell of putrifying crayfish, which is undoubtedly of
the strongest, was said to drive even moles out of their burrows.

{46}



CHAPTER II.

THE PHYSIOLOGY OF THE CRAYFISH. THE MECHANISM BY WHICH THE PARTS OF
THE LIVING ENGINE ARE SUPPLIED WITH THE MATERIALS NECESSARY FOR THEIR
MAINTENANCE AND GROWTH.


An analysis of such a sketch of the “Natural History of the Crayfish”
as is given in the preceding chapter, shows that it provides brief
and general answers to three questions. First, what is the form and
structure of the animal, not only when adult, but at different stages
of its growth? Secondly, what are the various actions of which it is
capable? Thirdly, where is it found? If we carry our investigations
further, in such a manner as to give the fullest attainable answers
to these questions, the knowledge thus acquired, in the case of the
first question, is termed the _Morphology_ of the crayfish; in the
case of the second question, it constitutes the _Physiology_ of the
animal; while the answer to the third question would represent what
we know of its _Distribution_ or _Chorology_. There remains a fourth
problem, which can hardly be regarded as seriously under discussion,
so long as knowledge has advanced no further than the Natural History
stage; the question, namely, {47} how all these facts comprised
under Morphology, Physiology, and Chorology have come to be what they
are; and the attempt to solve this problem leads us to the crown of
Biological effort, _Ætiology_. When it supplies answers to all the
questions which fall under these four heads, the Zoology of Crayfish
will have said its last word.

       *       *       *       *       *

As it matters little in what order we take the first three questions,
in expanding Natural History into Zoology, we may as well follow
that which accords with the history of science. After men acquired a
rough and general knowledge of the animals about them, the next thing
which engaged their interest was the discovery in these animals of
arrangements by which results, of a kind similar to those which their
own ingenuity effects through mechanical contrivances, are brought
about. They observed that animals perform various actions; and, when
they looked into the disposition and the powers of the parts by which
these actions are performed, they found that these parts presented
the characters of an apparatus, or piece of mechanism, the action of
which could be deduced from the properties and connections of its
constituents, just as the striking of a clock can be deduced from the
properties and connections of its weights and wheels.

Under one aspect, the result of the search after the _rationale_
of animal structure thus set afoot is _Teleology_; or the doctrine
of adaptation to purpose. Under another {48} aspect, it is
_Physiology_; so far as Physiology consists in the elucidation of
complex vital phenomena by deduction from the established truths of
Physics and Chemistry, or from the elementary properties of living
matter.

       *       *       *       *       *

We have seen that the crayfish is a voracious and indiscriminate
feeder; and we shall be safe in assuming that, if duly supplied with
nourishment, a full-grown crayfish will consume several times its own
weight of food in the course of the year. Nevertheless, the increase
of the animal’s weight at the end of that time is, at most, a small
fraction of its total weight; whence it is quite clear, that a very
large proportion of the food taken into the body must, in some shape
or other, leave it again. In the course of the same period, the
crayfish absorbs a very considerable quantity of oxygen, supplied by
the atmosphere to the water which it inhabits; while it gives out,
into that water, a large amount of carbonic acid, and a larger or
smaller quantity of nitrogenous and other excrementitious matters.
From this point of view, the crayfish may be regarded as a kind of
chemical manufactory, supplied with certain alimentary raw materials,
which it works up, transforms, and gives out in other shapes. And the
first physiological problem which offers itself to us is the mode of
operation of the apparatus contained in this factory, and the extent
to which the products of its activity are to be accounted for by
reasoning from known physical and chemical principles. {49}

We have learned that the food of the crayfish is made up of very
diverse substances, both animal and vegetable; but, so far as they
are competent to nourish the animal permanently, these matters all
agree in containing a peculiar nitrogenous body, termed _protein_,
under one of its many forms, such as albumen, fibrin, and the like.
With this may be associated fatty matters, starchy and saccharine
bodies, and various earthy salts. And these, which are the essential
constituents of the food, may be, and usually are, largely mixed up
with other substances, such as wood, in the case of vegetable food,
or skeletal and fibrous parts, in the case of animal prey, which are
of little or no utility to the crayfish.

The first step in the process of feeding, therefore, is to reduce the
food to such a state, that the separation of its nutritive parts,
or those which can be turned to account, from its innutritious, or
useless, constituents, may be facilitated. And this preliminary
operation is the subdivision of the food into morsels of a convenient
size for introduction into that part of the machinery in which the
extraction of the useful products is performed.

The food may be seized by the pincers, or by the anterior chelate
ambulatory limbs; and, in the former case, it is usually, if not
always, transferred to the first, or second, or both of the anterior
pairs of ambulatory limbs. These grasp the food, and, tearing it into
pieces of the proper dimensions, thrust them between the external
maxillipedes, which are, at the same time, {50} worked rapidly to
and fro sideways, so as to bring their toothed edges to bear upon
the morsel. The other five pairs of jaws are no less active, and
they thus crush and divide the food brought to them, as it is passed
between their toothed edges to the opening of the mouth.

As the alimentary canal stretches from the mouth, at one end, to the
vent at the other, and, at each of these limits, is continuous with
the wall of the body, we may conceive the whole crayfish to be a
hollow cylinder, the cavity of which is everywhere closed, though it
is traversed by a tube, open at each end (fig. 6). The shut cavity
between the tube and the walls of the cylinder may be termed the
_perivisceral cavity_; and it is so much filled up by the various
organs, which are interposed between the alimentary canal and the
body wall, that all that is left of it is represented by a system
of irregular channels, which are filled with blood, and are termed
_blood sinuses_. The wall of the cylinder is the outer wall of the
body itself, to which the general name of _integument_ may be given;
and the outermost layer of this, again, is the _cuticle_, which
gives rise to the whole of the exoskeleton. This cuticle, as we have
seen, is extensively impregnated with lime salts; and, moreover, in
consequence of its containing _chitin_, it is often spoken of as the
_chitinous cuticula_.

Having arrived at this general conception of the disposition of the
parts of the factory, we may next proceed to consider the machinery
of alimentation which is {51} contained within it, and which is
represented by the various divisions of the alimentary canal, with
its appendages; by the apparatus for the distribution of nutriment;
and by two apparatuses for getting rid of those products which are
the ultimate result of the working of the whole organism.

And here we must trench somewhat upon the province of _Morphology_,
as some of these pieces of apparatus are complicated; and their
action cannot be comprehended without a certain knowledge of their
anatomy.

The mouth of the crayfish is a longitudinally elongated,
parallel-sided opening, in the integument of the ventral or sternal
aspect of the head. Just outside its lateral boundaries, the
strong mandibles project, one on each side (fig 3, B; _4_); their
broad crushing surfaces, which are turned towards one another, are
therefore completely external to the oral cavity. In front, the
mouth is overlapped by a wide shield-shaped plate termed the upper
lip, or _labrum_ (figs. 3 and 6, _lb_); while, immediately behind
the mandibles, there is, on each side, an elongated fleshy lobe,
joined with its fellow by the posterior boundary of the mouth.
These together constitute the _metastoma_ (fig. 3, B; _mt_), which
is sometimes called the lower lip. A short wide gullet, termed the
œsophagus (fig. 6, _oe_), leads directly upwards into a spacious bag,
the _stomach_, which occupies almost the whole cavity of the head. It
is divided by a constriction into a large anterior chamber (_cs_),
into the under face of which the {52} gullet opens, and a small
posterior chamber (_ps_), from which the intestine (_hg_) proceeds.

In a man’s stomach, the opening by which the gullet communicates
with the stomach is called the _cardia_, while that which places the
stomach in communication with the intestine is named the _pylorus_;
and these terms having been transferred from human anatomy to that
of the lower animals, the larger moiety of the crayfish’s stomach
is called the _cardiac division_, while the smaller is termed the
_pyloric division_ of the organ. It must be recollected, however,
that, in the crayfish, the so-called cardiac division is that which
is actually furthest from the heart, not that which is nearest to it,
as in man.

The gullet is lined by a firm coat which resembles thin parchment.
At the margins of the mouth, this strong lining is easily seen to
be continuous with the cuticular exoskeleton; while, at the cardiac
orifice, it spreads out and forms the inner or cuticular wall of the
whole gastric cavity, as far as the pylorus, where it ends in certain
valvular projections. The chitinous cuticle which forms the outermost
layer of the integument is thus, as it were, turned in, to constitute
the innermost layer of the walls of the stomach; and it confers
upon them so great an amount of stiffness that they do not collapse
when the organ is removed from the body. Furthermore, just as the
cuticle of the integument is calcified to form the hard parts of the
exoskeleton, so is the cuticle of the stomach calcified, or otherwise
hardened, to give rise, in the first {53} place, to the very
remarkable and complicated apparatus which has already been spoken
of, as a sort of _gastric mill_ or _food-crusher_; and, secondly, to
a _filter_ or _strainer_, whereby the nutritive juices are separated
from the innutritious hard parts of the food and passed on into the
intestine.

[Illustration: FIG. 9.—_Astacus fluviatilis._—A, the stomach with
its outer coat removed, seen from the left side; B, the same viewed
from the front, after removal of the anterior wall; C, the ossicles
of the gastric mill separated from one another; D, the prepyloric
ossicle and median tooth, seen from the right side; E, transverse
section of the pyloric region along the line _xy_ in A (all × 2).
_c_, cardiac ossicle; _cpv_, cardio-pyloric valve; _lp_, lateral
pouch; _lt_, lateral tooth, seen through the wall of the stomach
in A; _mg_, mid-gut; _mt_, median tooth, seen through the wall of
the stomach in A; _œs_, œsophagus; _p_, pyloric ossicle; _pc_,
pterocardiac ossicle; _pp_, prepyloric ossicle; _uc_, uro-cardiac
process; _t_, convexities on the free surface of its hinder end;
_v_^1, median pyloric valve; _zc_, zygocardiac ossicle.]

{54}

The gastric mill begins in the hinder half of the cardiac division.
Here, on the upper wall of the stomach, we see a broad transverse
calcified bar (figs. 9–11, _c_) from the middle of the hinder part of
which another bar (_uc_), united to the first by a flexible portion,
is continued backwards in the middle line. The whole has, therefore,
somewhat the shape of a cross-bow. Behind the first-mentioned piece,
the dorsal wall of the stomach is folded in, in such a manner as
to give rise to a kind of pouch; and the second piece, or what we
may call the handle of the crossbow, lies in the front wall of this
pouch. The end of this piece is dense and hard, and its free surface,
which looks into the top of the cardiac chamber, is raised into two
oval, flattened convex surfaces (_t_). Connected by a transverse
joint with the end of the handle of the crossbow, there is another
solid bar, which ascends obliquely forwards in the back wall of the
pouch (_pp_). The end which is articulated with the handle of the
crossbow is produced into a strong reddish conical tooth (_mt_),
curved forwards and bifurcated at the summit; consequently, when the
cavity of the stomach is inspected from the fore part of the cardiac
pouch (fig. 9, B), the two-pointed curved tooth (_mt_) is seen
projecting behind the convex surfaces (_t_), in the middle line, into
the interior of that cavity. The joint which connects the handle of
the crossbow with the hinder middle piece is elastic; hence, if the
two are straightened out, they return to their bent disposition as
soon as they are released. The upper end of {55} the hinder middle
piece (_pp_) is connected with a second flat transverse plate which
lies in the dorsal wall of the pyloric chamber (_p_). The whole
arrangement, thus far, may be therefore compared to a large cross-bow
and a small one, with the ends of their handles fastened together by
a spring joint, in such a manner that the handle of the one makes an
acute angle with the handle of the other; while the middle of each
bow is united with the middle of the other by the bent arm formed
by the two handles. But, in addition to this, the outer ends of the
two bows are also connected together. A small, curved, calcified bar
(_pc_) passes from the outer end of the front crosspiece downwards
and outwards in the wall of the stomach, and its hinder and lower
extremity is articulated with another larger bar (_zc_) which runs
upwards and backwards to the hinder or pyloric crosspiece, with
which it articulates. Internally, this piece projects into the
cardiac cavity of the stomach as a stout elongated reddish elevation
(_lt_), the surface of which is produced into a row of strong sharp,
transverse ridges, which diminish in size from before backwards, and
constitute a crushing surface almost like that of the grinder of an
elephant. Thus, when the front part of the cardiac cavity is cut
away, not only are the median teeth already mentioned seen, but, on
each side of them, there is one of these long lateral teeth.

[Illustration: FIG. 10.—_Astacus fluviatilis._—Longitudinal section
of the stomach (× 4), _c_, cardiac ossicle; _cæ_, cæcum; _c.p.v_,
cardio-pyloric valve; _cs_, cushion-shaped surface; _hg_, hind-gut;
_hp_, aperture of right bile duct; _lp_, lateral pouch; _lt_,
lateral teeth; _mg_, mid-gut; _mt_, median tooth; _œs_, œsophagus;
_p_, pyloric ossicle; _pc_, pterocardiac ossicles; _pp_, prepyloric
ossicle; _uc_, urocardiac process; _v_^1, median pyloric valve;
_v_^2, lateral pyloric valve; _x_, position of gastrolith; _zc_,
zygocardiac ossicle.]

There are two small pointed teeth, one under each of the lateral
teeth, and each of these is supported by {56} a broad plate, hairy
on its inner surface, which enters into the lateral wall of the
cardiac chamber. There are various other smaller skeletal parts, but
the most important are those which have been described; and these,
from what has been said, will be seen to form a sort of hexagonal
frame, with more or less flexible joints at the angles, and having
the anterior and the posterior sides {57} connected by a bent
jointed middle bar. As all these parts are merely modifications of
the hard skeleton, the apparatus is devoid of any power of moving
itself. It is set in motion, however, by the same substance as that
which gives rise to all the other bodily movements of the crayfish,
namely, _muscle_. The chief muscles which move it are four very
strong bundles of fibres. Two of these are attached to the front
crosspiece, and proceed thence, upwards and forwards, to be fixed to
the inner face of the carapace in the front part of the head (figs.
5, 6, and 12, _ag_). The two others, which are fixed into the hinder
crosspiece and hinder lateral pieces, pass upwards and backwards,
to be attached to the inner face of the carapace in the back part
of the head (_pg_). When these muscles shorten, or contract, they
pull the front and back crosspieces further away from one another;
consequently, the angle between the handles becomes more open and the
tooth which is borne on their ends travels downwards and forwards.
But, at the same time, the angle between the side bars becomes
more open and the lateral tooth of each side moves inwards till it
crosses in front of the middle tooth, and strikes against this and
the opposite lateral tooth, which has undergone a corresponding
change of place. The muscles being now relaxed, the elasticity of
the joints suffices to bring the whole apparatus back to its first
position, when a new contraction brings about a new clashing of the
teeth. Thus, by the alternate contraction and relaxation of these
two pair of muscles, the {58} three teeth are made to stir up and
crush whatever is contained in the cardiac chamber. When the stomach
is removed and the front part of the cardiac chamber is cut away,
the front cross-piece may be seized with one pair of forceps and the
hind cross-piece with another. On slightly pulling the two, so as to
imitate the action of the muscles, the three teeth will be found to
come together sharply, exactly in the manner described.

Works on mechanics are full of contrivances for the conversion of
motion; but it would, perhaps, be difficult to discover among these
a prettier solution of the problem; given a straight pull, how to
convert it into three simultaneous convergent movements of as many
points.

What I have called the _filter_ is constructed mainly out of
the chitinous lining of the pyloric chamber. The aperture of
communication between this and the cardiac chamber, already narrow,
on account of the constriction of the walls of the stomach at this
point, is bounded at the sides by two folds; while, from below, a
conical tongue-shaped process (figs. 6, 10, and 11, _cpv_), the
surface of which is covered with hairs, further obstructs the
opening. In the posterior half of the pyloric chamber, its side
walls are, as it were, pushed in; and, above, they so nearly meet in
the middle line, that a mere vertical chink is left between them;
while even this is crossed by hairs set upon the two surfaces. In
its lower half, however, each side wall curves outwards, and forms
a cushion-shaped surface (fig. 10, _cs_) which looks downwards and
inwards. If the {59} floor of the pyloric chamber were flat, a
wide triangular passage would thus be left open in its lower half.
But, in fact, the floor rises into a ridge in the middle, while, at
the sides, it adapts itself to the shape of the two cushion-shaped
surfaces; the result of which is that the whole cavity of the
posterior part of the pyloric division of the stomach is reduced to a
narrow three-rayed fissure. In transverse section, the vertical ray
of this fissure is straight, while the two lateral ones are concave
upwards (fig. 9, _E_). The cushions of the side walls are covered
with short close-set hairs. The corresponding surfaces of the floor
are raised into longitudinal parallel ridges, the edge of each of
which is fringed with very fine hairs. As everything which passes
from the cardiac sac to the intestine must traverse this singular
apparatus, only the most finely divided solid matters can escape
stoppage, so long as its walls are kept together.

Finally, at the opening of the pyloric sac into the intestine, the
chitinous investment terminates in five symmetrically arranged
processes, the disposition of which is such that they must play the
part of valves in preventing any sudden return of the contents of the
intestine to the stomach, while they readily allow of a passage the
other way. One of these valvular processes is placed in the middle
line above (figs. 10 and 11, _v_^1). It is longer than the others and
concave below. The lateral processes (_v_^2,) of which there are two
on each side, are triangular and flat. {60}

[Illustration: FIG. 11.—_Astacus fluviatilis._—View of the roof of
the stomach, the ventral wall of which, and of the mid-gut, is laid
open by a longitudinal incision (× 4). On the right side (the left in
the figure), the lateral tooth is cut away, as well as the floor of
the lateral pouch. The letters have the same signification as in fig.
10.]

The cuticular lining which gives rise to all the complicated
apparatus which has just been described, must not be confounded with
the proper wall of the stomach, which invests it, and to which it
owes it origin, just as the cuticle of the integument is produced
by the soft {61} true skin which lies beneath it. The wall of
the stomach is a soft pale membrane containing variously disposed
muscular fibres; and, beyond the pylorus, it is continued into the
wall of the intestine.

It has already been mentioned that the intestine is a slender
and thin-walled tube, which passes straight through the body
almost without change, except that it becomes a little wider and
thicker-walled near the vent. Immediately behind the pyloric valves,
its surface is quite smooth and soft (figs. 9, 10, and 12, _mg_),
and its floor presents a relatively large aperture, the termination
of the bile duct (fig. 12, _bd_, fig. 10, _hp_), on each side.
The roof is, as it were, pushed out into a short median pouch or
_cæcum_ (_cæ_). Behind this, its character suddenly changes, and six
squarish elevations, covered with a chitinous cuticle, encircle the
cavity of the intestine (_r_). From each of these, a longitudinal
ridge, corresponding with a fold of the wall of the intestine, takes
its rise, and passes, with a slight spiral twist, to its extremity
(_hg_). Each of these ridges is beset with small papillæ, and the
chitinous lining is continued over the whole to the vent, where it
passes into the general cuticle of the integument, just as the lining
of the stomach is continuous with the cuticle of the integument at
the mouth. The alimentary canal may, therefore, be distinguished
into a _fore_ and a _hind-gut_ (_hg_), which have a thick internal
lining of cuticular membrane; and a very short _mid-gut_ (_mg_),
which has no thick cuticular layer. It will be of {63} importance
to recollect this distinction by-and-by, when the development of the
alimentary canal is considered.

[Illustration: FIG. 12.—_Astacus fluviatilis._—A dissection of a
male specimen from the right side (nat. size). _a_, anus; _aa_,
antennary artery, cut short; _ag_, anterior gastric muscles, the
right cut away to its insertion; _bd_, aperture of right bile duct;
_cm_, constrictor muscles of stomach; _cæ_, cæcum; _cpm_, right
cardio-pyloric muscle; _cs_, cardiac portion of stomach; _cm_,
extensor muscles of abdomen; _fm_, flexor muscles of abdomen;
_ga_, gastric artery; _gn._ 1, supraœsophageal ganglion; _gn._
2, sub-œsophageal ganglion; _gn._ 13, last abdominal ganglion;
_h_, heart; _ha_, hepatic artery; _hg_, hind-gut; _iaa_, inferior
abdominal artery; _la_, right lateral aperture of heart; _lr_, left
liver; _mg_, mid-gut; _oa_, ophthalmic artery; _œ_, œsophagus; _pg_,
posterior gastric muscles, the right cut away to its insertion;
_ps_, pyloric portion of stomach; _sa_, sternal artery; _saa_,
superior abdominal artery; _t_ (to the left), telson; _t_ (near the
heart), testis; _vd_, left vas deferens; _vd′_, aperture of left
vas deferens; _2_, right antennule; _4_, left mandible; _9_, left
external maxillipede; _10_, left forceps; _15_, first, _16_, second,
and _20_, sixth abdominal appendages of the left side.]

If the treatment to which the food is subjected in the alimentary
apparatus were of a purely mechanical nature, there would be nothing
more to describe in this part of the crayfish’s mechanism. But,
in order that the nutritive matters may be turned to account, and
undergo the chemical metamorphoses, which eventually change them
into substances of a totally different character, they must pass out
of the alimentary canal into the blood. And they can do this only
by making their way through the walls of the alimentary canal; to
which end they must either be in a state of extremely fine division,
or they must be reduced to the fluid condition. In the case of the
fatty matters, minute subdivision may suffice; but the amylaceous
substances and the insoluble protein compounds, such as the fibrin
of flesh, must be brought into a state of solution. Therefore some
substances must be poured into the alimentary canal, which, when
mixed with the crushed food, will play the part of a chemical agent,
dissolving out the insoluble proteids, changing the amyloids into
soluble sugar, and converting all the proteids into those diffusible
forms of protein matter, which are known as _peptones_.

The details of the processes here indicated, which may be included
under the general name of _digestion_, have only quite recently been
carefully investigated in the crayfish; and we have probably still
much to learn about {64} them; but what has been made out is very
interesting, and proves that considerable differences exist between
crayfishes and the higher animals in this respect.

The physiologist calls those organs, the function of which is to
prepare and discharge substances of a special character, _glands_;
and the matter which they elaborate is termed their _secretion_.
On the one side, glands are in relation with the blood, whence
they derive the materials which they convert into the substances
characteristic of their secretion; on the other side, they have
access, directly or indirectly, to a free surface, on to which they
pour their secretion as it is formed.

Of such glands, the alimentary canal of the crayfish is provided
with a pair, which are not only of very large size, but are further
extremely conspicuous, on account of their yellow or brown colour.
These two glands (figs. 12 and 13, _lr_) are situated beneath, and
on each side of, the stomach and the anterior part of the intestine,
and answer in position to the glands termed liver and pancreas in
the higher animals, inasmuch as they pour their secretion into the
mid-gut. These glands have hitherto always been regarded as the
_liver_, and the name may be retained, though their secretion appears
rather to correspond with the pancreatic fluid than with the bile of
the higher animals.

[Illustration: FIG. 13.—_Astacus fluviatilis._—The alimentary canal
and livers seen from above (nat. size). _bd_, bile-duct; _cæ_,
cæcum; _cs_, cardiac portion of stomach, the line pointing to the
cardiac ossicle; _hg_, hind-gut; _mg_, mid-gut; _pc_, pterocardiac
ossicle; _ps_, pyloric portion of stomach, the line pointing to the
pyloric ossicle; _r_, ridge separating mid-gut from hind-gut; _zc_,
zygocardiac ossicle.]

Each liver consists of an immense number of short tubes, or _cæca_,
which are closed at one end, but open at the other into a general
conduit, which is termed their _duct_. The mass of the liver is
roughly divided into {66} three lobes, one anterior, one lateral,
and one posterior; and each lobe has its main duct, into which all
the tubes composing it open. The three ducts unite together into
a wide common duct (_bd_), which opens, just behind the pyloric
valves, into the floor of the mid-gut. Hence the apertures of the
two _hepatic ducts_ are seen, one on each side, in this part of the
alimentary canal when it is laid open from above. Every cæcum of the
liver has a thin outer wall, lined internally by a layer of cells,
constituting what is termed an _epithelium_; and, at the openings of
the hepatic ducts, this epithelium passes into a layer of somewhat
similar structure, which lines the mid-gut, and is continued through
the rest of the alimentary canal, beneath the cuticula. Hence the
liver may be regarded as a much divided side pouch of the mid-gut.

The epithelium is made up of _nucleated cells_, which are particles
of simple living matter, or _protoplasm_, in the midst of each of
which is a rounded body, which is termed the _nucleus_. It is these
cells which are the seat of the manufacturing process which results
in the formation of the secretion; it is, as it were, their special
business to form that secretion. To this end they are constantly
being newly formed at the summits of the cæca. As they grow, they
pass down towards the duct and, at the same time, separate into their
interior certain special products, among which globules of yellow
fatty matter are very conspicuous. When these products are fully
formed, what remains of the substance of the cells dissolves away,
and {67} the yellow fluid accumulating in the ducts passes into the
mid-gut. The yellow colour is due to the globules of fat. In the
young cells, at the summit of the cæca, these are either absent, or
very small, whence the part appears colourless. But, lower down,
small yellow granules appear in the cells, and these become bigger
and more numerous in the middle and lower parts. In fact, few glands
are better fitted for the study of the manner in which secretion is
effected than the crayfish’s liver.

       *       *       *       *       *

We may now consider the alimentary machinery, the general structure
of which has been explained, in action.

The food, already torn and crushed by the jaws, is passed through
the gullet into the cardiac sac, and there reduced to a still more
pulpy state by the gastric mill. By degrees, such parts as are
sufficiently fluid are drained off into the intestine, through the
pyloric strainer, while the coarser parts of the useless matters
are probably rejected by the mouth, as a hawk or an owl rejects
his casts. There is reason to believe, though it is not certainly
known, that fluids from the intestine mix with the food while it is
undergoing trituration, and effect the transformation of the starchy
and the insoluble protein compounds into a soluble state. At any
rate, as soon as the strained-off fluid passes into the mid-gut it
must be mixed with the secretion of the liver, the action of which is
probably {68} similar to that of the pancreatic juice of the higher
animals.

[Illustration: FIG. 14.—_Astacus fluviatilis._—The corpuscles of
the blood (highly magnified). _1–8_ show the changes undergone by
a single corpuscle during a quarter of an hour; _9_ and _10_ are
corpuscles killed by magenta, and having the nucleus deeply stained
by the colouring matter. _n_, nucleus.]

The mixture thus produced, which answers to the chyle of the higher
animals, passes along the intestine, and the greater part of it,
transuding through the walls of the alimentary canal, enters the
blood, while the rest accumulates as dark coloured fæces in the hind
gut, and is eventually passed out of the body by the vent. The fæcal
matters are small in amount, and the strainer is so efficient that
they rarely contain solid particles of sensible size. Sometimes,
however, there are a good many minute fragments of vegetable tissue.

The blood of which the nutritive elements of the food {69} have thus
become integral parts, is a clear fluid, either colourless, or of a
pale neutral tint or reddish hue, which, to the naked eye, appears
like so much water. But if subjected to microscopic examination,
it is found to contain innumerable pale, solid particles, or
_corpuscles_, which, when examined fresh, undergo constant changes
of form (fig. 14). In fact, they correspond very closely with the
colourless corpuscles which exist in our own blood; and, in its
general characters, the crayfish’s blood is such as ours would be if
it were somewhat diluted and deprived of its red corpuscles. In other
words, it resembles our lymph more than it does our blood. Left to
itself it soon coagulates, giving rise to a pretty firm clot.

The sinuses, or cavities in which the greater part of the blood is
contained, are disposed very irregularly in the intervals between the
internal organs. But there is one of especially large size on the
ventral or sternal side of the thorax (fig. 15, _sc_), into which
all the blood in the body sooner or later makes its way. From this
_sternal sinus_ passages (_av_) lead to the gills, and from these
again six canals (_bcv_), pass up on the inner side of the inner wall
of each branchial chamber to a cavity situated in the dorsal region
of the thorax, termed the _pericardium_ (_p_), into which they open.

[Illustration: FIG. 15.—_Astacus fluviatilis._—A diagrammatic
transverse section of the thorax through the twelfth somite,
showing the course of the circulation of the blood (× 3). _arb.
12_, the anterior or lower, and _arb′. 12_, the posterior or upper
arthrobranchia of the twelfth somite; _av_, afferent branchial
vessel; _bcv_, branchio-cardiac vein; _bg_, branchiostegite; _em_,
extensor muscles of abdomen; _ep_, epimeral wall of thoracic cavity;
_ev_, efferent branchial vessel; _fm_, flexor muscles of abdomen;
_fp_, floor of pericardium; _gn._ 6, fifth thoracic ganglion; _h_,
heart; _hg_, hind-gut; _iaa_, inferior abdominal artery, in cross
section; _la_, lateral valvular apertures of heart; _lr_, liver;
_mp_, indicates the position of the mesophragm by which the sternal
canal is bounded laterally; _p_, pericardial sinus; _pdb. 12_,
podobranchia, and _plb. 12_, pleurobranchia of the twelfth somite;
_sa_, sternal artery; _saa_, superior abdominal artery; _sc_, sternal
canal; _t_, testis; XII., sternum of twelfth somite. The arrows
indicate the direction of the blood flow.]

The blood of the crayfish is kept in a state of constant circulating
motion by a pumping and distributing machinery, composed of the
_heart_ and of the _arteries_, with {71} their larger and smaller
branches, which proceed from it and ramify through the body, to
terminate eventually in the blood sinuses, which represent the veins
of the higher animals.

When the carapace is removed from the middle of the region which lies
behind the cervical groove, that is, when the dorsal or _tergal_
wall of the thorax is taken away, a spacious chamber is laid open
which is full of blood. This is the cavity already mentioned as the
_pericardium_ (fig. 15, _p_), though, as it differs in some respects
from that which is so named in the higher animals, it will be better
to term it the _pericardial sinus_.

The heart (fig. 15, _h_), lies in the midst of this sinus. It is a
thick muscular body (fig. 16), with an irregularly hexagonal contour
when viewed from above, one angle of the hexagon being anterior and
another posterior. The lateral angles of the hexagon are connected
by bands of fibrous tissue (_ac_) with the walls of the pericardial
sinus. Otherwise, the heart is free, except in so far as it is kept
in place by the arteries which leave it and traverse the walls of
the pericardium. One of these arteries (figs. 5, 12, and 16, _saa_),
starting from the hinder part of the heart, of which it is a sort of
continuation, runs along the middle line of the abdomen above the
intestine, to which it gives off many branches. A second large artery
starts from a dilatation, which is common to it with the foregoing,
but passing directly downwards (figs. 12 and 15, _sa_, and fig. 16,
_st.a_), either on the right or on the left side of the intestine,
{72} traverses the nervous cord (figs. 12 and 15), and divides into
an anterior (fig. 12, _sa_) and a posterior (_iaa_) branch, both of
which run beneath and parallel with that cord. A third artery runs,
from the front part of the heart, forwards in the middle line, over
the stomach, to the eyes and fore part of the head (figs. 5, 12, and
16, _oa_); and two others diverge one on each side of this, and sweep
{73} round the stomach to the antennæ (_aa_). Behind these, yet two
other arteries are given off from the under side of the heart, and
supply the liver (_ha_). All these arteries branch out and eventually
terminate in fine, so-called _capillary_, ramifications.

[Illustration: FIG. 16.—_Astacus fluviatilis._—The heart (× 4). A,
from above; B, from below; C, from the left side. _aa_, antennary
artery; _ac_, alæ cordis, or fibrous bands connecting the heart with
the walls of the pericardial sinus; _b_, bulbous dilatation at the
origin of the sternal artery; _ha_, hepatic artery; _la_, lateral
valvular apertures; _oa_, ophthalmic artery; _s.a_, superior valvular
apertures; _s.a.a_, superior abdominal artery; _st.a_, sternal
artery, in B cut off close to its origin.]

In the dorsal wall of the heart two small oval apertures are visible,
provided with valvular lips (fig. 16, _sa_), which open inwards, or
towards the internal cavity of the heart. There is a similar aperture
in each of the lateral faces of the heart (_la_), and two others in
its inferior face (_ia_), making six in all. These apertures readily
admit fluid into the heart, but oppose its exit. On the other hand,
at the origins of the arteries, there are small valvular folds,
directed in such a manner as to permit the exit of fluid from the
heart, while they prevent its entrance.

The walls of the heart are muscular, and, during life, they contract
at intervals with a regular rhythm, in such a manner as to diminish
the capacity of the internal cavity of the organ. The result is,
that the blood which it contains is driven into the arteries, and
necessarily forces into their smaller ramifications an equivalent
amount of the blood which they already contained; whence, in the long
run, the same amount of blood passes out of the ultimate capillaries
into the blood sinuses. From the disposition of the blood sinuses,
the impulse thus given to the blood which they contain is finally
conveyed to the blood in the branchiæ, and a proportional quantity
of that {74} blood leaves the branchiæ and passes into the sinuses
which connect them with the pericardial sinus (fig. 15, _bcv_), and
thence into that cavity. At the end of the contraction, or _systole_,
of the heart, its volume is of course diminished by the volume of the
blood forced out, and the space between the walls of the heart and
those of the pericardial sinus is increased to the same extent. This
space, however, is at once occupied by the blood from the branchiæ,
and perhaps by some blood which has not passed through the branchiæ,
though this is doubtful. When the systole is over, the _diastole_
follows; that is to say, the elasticity of the walls of the heart
and that of the various parts which connect it with the walls of the
pericardium, bring it back to its former size, and the blood in the
pericardial sinus flows into its cavity by the six apertures. With
a new systole the same process is repeated, and thus the blood is
driven in a circular course through all parts of the body.

       *       *       *       *       *

It will be observed that the branchiæ are placed in the course of the
current of blood which is returning to the heart; which is the exact
contrary of what happens in fishes, in which the blood is sent from
the heart to the branchiæ, on its way to the body. It follows, from
this arrangement, that the blood which goes to the branchiæ is blood
in which the quantity of oxygen has undergone a diminution, and that
of carbonic acid an increase, as compared with the blood in the heart
itself. For the {75} activity of all the organs, and especially of
the muscles, is inseparably connected with the absorption of oxygen
and the evolution of carbonic acid; and the only source from which
the one can be derived, and the only receptacle into which the other
can be poured, is the blood which bathes and permeates the whole
fabric to which it is distributed by the arteries.

The blood, therefore, which reaches the branchiæ has lost oxygen and
gained carbonic acid; and these organs constitute the apparatus for
the elimination of the injurious gas from the economy on the one
hand, and, on the other, for the taking in of a new supply of the
needful “vital air,” as the old chemists called it. It is thus that
the branchiæ subserve the respiratory function.

The crayfish has eighteen perfect and two rudimentary branchiæ in
each branchial chamber, the boundaries of which have been already
described.

Of the eighteen perfect branchiæ, six (_podobranchiæ_) are attached
to the basal joints of the thoracic limbs, from the last but one to
the second (second maxillipede) inclusively (fig. 4, p. 26, _pdb_,
and fig. 17, A, B); and eleven (_arthrobranchiæ_) are fixed to the
flexible interarticular membranes, which connect these basal joints
with the parts of the thorax to which they are articulated (fig.
4, _arb_, _arb′_, fig. 17, C). Of these eleven branchiæ, two are
attached to the interarticular membranes of all the ambulatory legs
but the last, (=6) and to those of the pincers and of the external
maxillipedes, (=4) and one to that of the {77} second maxillipede.
The first maxillipede and the last ambulatory limb have none.
Moreover, where there are two arthrobranchiæ, one is more or less in
front of and external to the other.

[Illustration: FIG. 17.—_Astacus fluviatilis._—A, one of the
podobranchiæ from the outer side; B, the same from the inner side;
C, one of the arthrobranchiæ; D, a part of one of the coxopoditic
setæ; E, extremity of the same seta; F, extremity of a seta from the
base of the podobranchia; G, hooked seta of the lamina; (A–C, × 3;
D–G, highly magnified). _b_, base of podobranchia; _cs_, coxopoditic
setæ; _cxp_, coxopodite; _l_, lamina, _pl_, plume, and _st_, stem of
podobranchia; _t_, tubercle on the coxopodite, to which the setæ are
attached.]

These eleven arthrobranchiæ are all very similar in structure (fig.
17, C). Each consists of a stem which contains two canals, one
external and one internal, separated by a longitudinal partition. The
stem is beset with a great number of delicate _branchial filaments_,
so that it looks like a plume tapering from its base to its summit.
Each filament is traversed by large vascular channels, which break
up into a net-work immediately beneath the surface. The blood driven
into the external canals of the stem (fig. 15, _av_) is eventually
poured into the inner canal (_ev_), which again communicates with
the channels (_bcv_) which lead to the pericardial sinus (_p_).
In its course, the blood traverses the branchial filaments, the
outer investment of each of which is an excessively thin chitinous
membrane, so that the blood contained in them is practically
separated by a mere film from the aërated water in which the gills
float. Hence, an exchange of gaseous constituents readily takes
place, and as much oxygen is taken in as carbonic acid is given out.

The six podobranchiæ, or gills which are attached to the basal
joints of the legs, play the same part, but differ a good deal in
the details of their structure from those which are fixed to the
interarticular membranes. Each consists of a broad _base_ (fig. 17, A
and B; _b_) beset with many {78} fine straight hairs, or _setæ_ (F),
whence a narrow _stem_ (_st_) proceeds. At its upper end this stem
divides into two parts, that in front, the _plume_ (_pl_), resembling
the free end of one of the gills just described, while that behind,
the _lamina_ (_l_), is a broad thin plate, bent upon itself
longitudinally in such a manner that its folded edge lies forwards,
and covered with minute hooked setæ (G). The gill which follows is
received into the space included between the two lobes or halves
of the folded lamina (fig. 4, p. 26). Each lobe is longitudinally
plaited into about a dozen folds. The whole front and outer face of
the stem is beset with branchial filaments; hence, we may compare one
of these branchiæ to one of the preceding kind, in which the stem
has become modified and has given off a large folded lamina from its
inner and posterior face.

The branchiæ now described are arranged in sets of three for each
of the thoracic limbs, from the third maxillipede to the last but
one ambulatory limb, and two for the second maxillipede, thus making
seventeen in all (3 × 5 + 2 = 17); and, between every two there is
found a bundle of long twisted hairs (fig. 17, A, _cx.s_; D and E),
which are attached to a small elevation (_t_) on the basal joint of
each limb. These _coxopoditic setæ_, no doubt, serve to prevent the
intrusion of parasites and other foreign matters into the branchial
chamber. From the mode of attachment of the six branchiæ it is
obvious that they must share in the movements of the basal joints of
the {79} legs; and that, when the crayfish walks, they must be more
or less agitated in the branchial chamber.

The eighteenth branchia resembles one of the eleven arthrobranchiæ in
structure; but it is larger, and it is attached neither to the basal
joint of the hindermost ambulatory limb, nor to its interarticular
membrane, but to the sides of the thorax, above the joint. From
this mode of attachment it is distinguished from the others as
_pleurobranchia_ (fig. 4, _plb. 14_).

Finally, in front of this, fixed also to the walls of the thorax,
above each of the two preceding pairs of ambulatory limbs, there is a
delicate filament, about a sixteenth of an inch long, which has the
structure of a branchial filament, and is, in fact, a rudimentary
pleurobranchia (fig. 4, _plb. 12_, _plb. 13_).

The quantity of water which occupies the space left in the branchial
chamber by the gills is but small, and as the respiratory surface
offered by the gills is relatively very large, the air contained
in this water must be rapidly exhausted, even when the crayfish
is quiescent; while, when any muscular exertion takes place, the
quantity of carbonic acid formed, and the demand for fresh oxygen,
is at once greatly increased. For the efficient performance of the
function of respiration, therefore, the water in the branchial
chamber must be rapidly renewed, and there must be some arrangement
by which the supply of fresh water may be proportioned to the demand.
In many animals, the respiratory surface is {80} covered with
rapidly vibrating filaments, or _cilia_, by means of which a current
of water is kept continually flowing over the gills, but there are
none of these in the crayfish. The same object is attained, however,
in another way. The anterior boundary of the branchial chamber
corresponds with the cervical groove, which, as has been seen, curves
downwards and then forwards, until it terminates at the sides of the
space occupied by the jaws. If the branchiostegite is cut away along
the groove, it will be found that it is attached to the sides of the
head, which project a little beyond the anterior part of the thorax,
so that there is a depression behind the sides of the head—just as
there is a depression, behind a man’s jaw, at the sides of the neck.
Between this depression in front, the walls of the thorax internally,
the branchiostegite externally, and the bases of the forceps and
external foot-jaws below, a curved canal is included, by which the
branchial cavity opens forwards as by a funnel. Attached to the base
of the second maxilla there is a wide curved plate (fig, 4, _6_)
which fits against the projection of the head, as a shirt collar
might do, to carry out our previous comparison; and this scoop-shaped
plate (termed the _scaphognathite_), which is concave forwards and
convex backwards, can be readily moved backwards and forwards.

If a living crayfish is taken out of the water, it will be found
that, as the water drains away from the branchial cavity, bubbles
of air are forced out of its anterior opening. {81} Again, if,
when a crayfish is resting quietly in the water, a little coloured
fluid is allowed to run down towards the posterior opening of the
branchial chamber, it will very soon be driven out from the anterior
aperture, with considerable force, in a long stream. In fact, as
the scaphognathite vibrates not less than three or four times in
a second, the water in the funnel-shaped front passage of the
branchial cavity is incessantly baled out; and, as fresh water flows
in from behind to make up the loss, a current is kept constantly
flowing over the gills. The rapidity of this current naturally
depends on the more or less quick repetition of the strokes of the
scaphognathite; and hence, the activity of the respiratory function
can be accurately adjusted to the wants of the economy. Slow working
of the scaphognathite answers to ordinary breathing in ourselves,
quick working to panting.

A farther self-adjustment of the respiratory apparatus is gained by
the attachment of the six gills to the basal joints of the legs.
For, when the animal exerts its muscles in walking, these gills are
agitated, and thus not only bring their own surfaces more largely in
contact with the water, but produce the same effect upon the other
gills.

       *       *       *       *       *

The constant oxidation which goes on in all parts of the body not
only gives rise to carbonic acid; but, so far as it affects the
proteinaceous constituents, it produces {82} compounds which contain
nitrogen, and these, like other waste products, must be eliminated.
In the higher animals, such waste products take the form of urea,
uric acid, hippuric acid, and the like; and are got rid of by the
kidneys. We may, therefore, expect to find some organ which plays the
part of a kidney in the crayfish; but the position of the structure,
which there is much reason for regarding as the representative of the
kidney, is so singular that very different interpretations have been
put upon it.

On the basal joint of each antenna it is easy to see a small conical
eminence with an opening on the inner side of its summit (fig. 18).
The aperture (_x_) leads by a short canal into a spacious sac, with
extremely delicate walls (_s_), which is lodged in the front part of
the head, in front of and below the cardiac division of the stomach
(_cs_). Beneath this, in a sort of recess, which corresponds with
the epistoma, and with the base of the antenna, there is a discoidal
body of a dull green colour, in shape somewhat like one of the fruits
of the mallow, which is known as the _green gland_ (_gg_) The sac
narrows below like a wide funnel, and the edges of its small end
are continuous with the walls of the green gland; they surround an
aperture which leads into the interior of the latter structure,
and conveys its products into the sac, whence they are excreted by
the opening in the antennary papilla. The green gland is said to
contain a substance termed _guanin_ (so named because it is found in
the _guano_ which is the accumulated {83} excrement of birds), a
nitrogenous body analogous in some respects to uric acid, but less
highly oxidated; and if this be the case, there can be little doubt
that the green gland represents the kidney, and its secretion {84}
the urinary fluid, while the sac is a sort of urinary bladder.

[Illustration: FIG. 18.—_Astacus fluviatilis._—A, the anterior part
of the body, with the dorsal portion of the carapace removed to show
the position of the green glands; B, the same, with the left side
of the carapace removed; C, the green gland removed from the body
(all × 2). _ag_, left anterior gastric muscle; _c_, circumœsophageal
commissures; _cs_, cardiac portion of stomach; _gg_, green gland,
exposed in A on the left side by the removal of its sac; _ima_,
intermaxillary or cephalic apodeme; _œs_, œsophagus seen in
transverse section in A, the stomach being removed; _s_, sac of green
gland; _x_, bristle passed from the aperture in the basal joint of
the antenna into the sac.]

       *       *       *       *       *

Restricting our attention to the phenomena which have now been
described, and to a short period in the life of the crayfish, the
body of the animal may be regarded as a factory, provided with
various pieces of machinery, by means of which certain nitrogenous
and other matters are extracted from the animal and vegetable
substances which serve for food, are oxidated, and are then delivered
out of the factory in the shape of carbonic acid gas, guanin,
and probably some other products, with which we are at present
unacquainted. And there is no doubt, that if the total amount of
products given out could be accurately weighed against the total
amount of materials taken in, the weight of the two would be found
to be identical. To put the matter in its most general shape, the
body of the crayfish is a sort of focus to which certain material
particles converge, in which they move for a time, and from which
they are afterwards expelled in new combinations. The parallel
between a whirlpool in a stream and a living being, which has often
been drawn, is as just as it is striking. The whirlpool is permanent,
but the particles of water which constitute it are incessantly
changing. Those which enter it, on the one side, are whirled around
and temporarily constitute a part of its individuality; and as they
leave it on the other side, their places are made good by new comers.
{85}

Those who have seen the wonderful whirlpool, three miles below the
Falls of Niagara, will not have forgotten the heaped-up wave which
tumbles and tosses, a very embodiment of restless energy, where the
swift stream hurrying from the Falls is compelled to make a sudden
turn towards Lake Ontario. However changeful in the contour of its
crest, this wave has been visible, approximately in the same place,
and with the same general form, for centuries past. Seen from a mile
off, it would appear to be a stationary hillock of water. Viewed
closely, it is a typical expression of the conflicting impulses
generated by a swift rush of material particles.

Now, with all our appliances, we cannot get within a good many miles,
so to speak, of the crayfish. If we could, we should see that it
was nothing but the constant form of a similar turmoil of material
molecules which are constantly flowing into the animal on the one
side, and streaming out on the other.

The chemical changes which take place in the body of the crayfish,
are doubtless, like other chemical changes, accompanied by the
evolution of heat. But the amount of heat thus generated is so small
and, in consequence of the conditions under which the crayfish lives,
it is so easily carried away, that it is practically insensible. The
crayfish has approximately the temperature of the surrounding medium,
and it is, therefore, reckoned among the cold-blooded animals.

If our investigation of the results of the process of {86}
alimentation in a well-fed Crayfish were extended over a longer
time, say a year or two, we should find that the products given out
were no longer equal to the materials taken in, and the balance would
be found in the increase of the animal’s weight. If we inquired how
the balance was distributed, we should find it partly in store,
chiefly in the shape of fat; while, in part, it had been spent in
increasing the plant and in enlarging the factory. That is to say, it
would have supplied the material for the animal’s growth. And this
is one of the most remarkable respects in which the living factory
differs from those which we construct. It not only enlarges itself,
but, as we have seen, it is capable of executing its own repairs to a
very considerable extent.

{87}



CHAPTER III.

THE PHYSIOLOGY OF THE CRAYFISH—THE MECHANISM BY WHICH THE LIVING
ORGANISM ADJUSTS ITSELF TO SURROUNDING CONDITIONS AND REPRODUCES
ITSELF.


If the hand is brought near a vigorous crayfish, free to move in a
large vessel of water, it will generally give a vigorous flap with
its tail, and dart backwards out of reach; but if a piece of meat is
gently lowered into the vessel, the crayfish will sooner or later
approach and devour it.

If we ask why the crayfish behaves in this fashion, every one has an
answer ready. In the first case, it is said that the animal is aware
of danger, and therefore hastens away; in the second, that it knows
that meat is good to eat, and therefore walks towards it and makes a
meal. And nothing can seem to be simpler or more satisfactory than
these replies, until we attempt to conceive clearly what they mean;
and, then, the explanation, however simple it may be admitted to be,
hardly retains its satisfactory character.

For example, when we say that the crayfish is “aware of danger,” or
“knows that meat is good to eat,” what {88} do we mean by “being
aware” and “knowing”? Certainly it cannot be meant that the crayfish
says to himself, as we do, “This is dangerous,” “That is nice;” for
the crayfish, being devoid of language, has nothing to say either to
himself or any one else. And if the crayfish has not language enough
to construct a proposition, it is obviously out of the question that
his actions should be guided by a logical reasoning process, such
as that by which a man would justify similar actions. The crayfish
assuredly does not first frame the syllogism, “Dangerous things
are to be avoided; that hand is dangerous; therefore it is to be
avoided;” and then act upon the conclusion thus logically drawn.

But it may be said that children, before they acquire the use of
language, and we ourselves, long after we are familiar with conscious
reasoning, perform a great variety of perfectly rational acts
unconsciously. A child grasps at a sweetmeat, or cowers before a
threatening gesture, before it can speak; and any one of us would
start back from a chasm opening at our feet, or stoop to pick up a
jewel from the ground, “without thinking about it.” And, no doubt, if
the crayfish has any mind at all, his mental operations must more or
less resemble those which the human mind performs without giving them
a spoken or unspoken verbal embodiment.

If we analyse these, we shall find that, in many cases, distinctly
felt sensations are followed by a distinct desire to perform some
act, which act is accordingly performed; {89} while, in other cases,
the act follows the sensation without one being aware of any other
mental process; and, in yet others, there is no consciousness even of
the sensation. As I wrote these last words, for example, I had not
the slightest consciousness of any sensation of holding or guiding
the pen, although my fingers were causing that instrument to perform
exceedingly complicated movements. Moreover, experiments upon animals
have proved that consciousness is wholly unnecessary to the carrying
out of many of those combined movements by which the body is adjusted
to varying external conditions.

Under these circumstances, it is really quite an open question
whether a crayfish has a mind or not; moreover, the problem is
an absolutely insoluble one, inasmuch as nothing short of being
a crayfish would give us positive assurance that such an animal
possesses consciousness; and, finally, supposing the crayfish has a
mind, that fact does not explain its acts, but only shows that, in
the course of their accomplishment, they are accompanied by phenomena
similar to those of which we are aware in ourselves, under like
circumstances.

So we may as well leave this question of the crayfish’s mind on one
side for the present, and turn to a more profitable investigation,
namely, that of the order and connexion of the physical phenomena
which intervene between something which happens in the neighbourhood
of the animal and that other something which responds to it, as an
act of the crayfish. {90}

Whatever else it may be, this animal, so far as it is acted upon
by bodies around it and reacts on them, is a piece of mechanism,
the internal works of which give rise to certain movements when it
is affected by particular external conditions; and they do this in
virtue of their physical properties and connexions.

Every movement of the body, or of any organ of the body, is an effect
of one and the same cause, namely, muscular contraction. Whether the
crayfish swims or walks, or moves its antennæ, or seizes its prey,
the immediate cause of the movements of the parts which bring about,
or constitute, these bodily motions is to be sought in a change which
takes place in the flesh, or muscle, which is attached to them.
The change of place which constitutes any movement is an effect of
a previous change in the disposition of the molecules of one or
more muscles; while the direction of that movement depends on the
connexions of the parts of the skeleton with one another, and of the
muscles with them.

The muscle of the crayfish is a dense, white substance; and if a
small portion of it is subjected to examination it will be found to
be very easily broken up into more or less parallel bundles of fine
fibres. Each of these fibres is generally found to be ensheathed in a
fine transparent membrane, which is called the _sarcolemma_, within
which is contained the proper substance of the muscle. When quite
fresh and living, this substance is soft and {91} semi-fluid, but it
hardens and becomes solid immediately after death.

[Illustration: FIG. 19.—_Astacus fluviatilis._—A, a single muscular
fibre; transverse diameter 1‐110th of an inch; B, a portion of
the same more highly magnified; C, a smaller portion still more
highly magnified; D and E, the splitting up of a part of fibre into
fibrillæ; F, the connexion of a nervous with a muscular fibre which
has been treated with acetic acid. _a_, darker, and _b_, clearer
portions of the fibrillæ; _n_, nucleus of sarcolemma; _nv_, nerve
fibre; _s_, sarcolemma; _t_, tendon; 1–5, successive dark bands
answering to the darker portions, _a_, of each fibrilla.]

Examined, with high magnifying powers, in this {92} condition,
the muscle-substance appears marked by very regular transverse
bands, which are alternately opaque and transparent; and it is
characteristic of the group of animals to which the crayfish belongs
that their muscle-substance has this striped character in all parts
of the body.

A greater or less number of these fibres, united into one or more
bundles, constitutes a muscle; and, except when these muscles
surround a cavity, they are fixed at each end to the hard parts
of the skeleton. The attachment is frequently effected by the
intermediation of a dense, fibrous, often chitinous, substance, which
constitutes the _tendon_ (fig. 19, A; _t_) of the muscle.

The property of the living muscle, which enables it to be the cause
of motion, is this: Every muscular fibre is capable of suddenly
changing its dimensions, in such a manner that it shortens and
becomes proportionately thicker. Hence the absolute bulk of the fibre
remains practically unchanged. From this circumstance, muscular
_contraction_, as the change of form of a muscle is called, is
radically different from the process which commonly goes by the same
name in other things, and which involves a diminution of bulk.

The contraction of muscle takes place with great force, and, of
course, if the parts to which its ends are fixed are both free to
move, they are brought nearer at the moment of contraction: if one
only is free to move that is approximated to the fixed part; and if
the muscular {93} fibre surrounds a cavity, the cavity is lessened
when the muscle contracts. This is the whole source of motor power in
the crayfish machine. The results produced by the exertion of that
power depend upon the manner in which the parts to which the muscles
are attached are connected with one another.

[Illustration: FIG. 20.—_Astacus fluviatilis._—The chela of the
forceps, with one side cut away to show, in A, the muscles, in
B, the tendons (× 2). _cp_, carpopodite; _prp_, propodite; _dp_,
dactylopodite; _m_, adductor muscle; _m′_, abductor muscle; _t_,
tendon of adductor muscle; _t′_, tendon of abductor muscle; _x_,
hinge.]

One example of this has already been given in the curious mechanism
of the gastric mill. Another may be found in the chela which
terminates the forceps. If the {94} articulation of the last joint
(fig. 20, _dp_) with the one which precedes it (_prp_) is examined,
it will be found that the base of the terminal segment (_dp_) turns
on two hinges (_x_), formed by the hard exoskeleton and situated
at opposite points of the diameter of the base, on the penultimate
segment; and these hinges are so disposed that the last joint can
be moved only in one plane, to or from the produced angle of the
penultimate segment (_prp_), which forms the fixed claw of the chela.
Between the hinges, on both the inner and the outer sides of the
articulation, the exoskeleton is soft and flexible, and allows the
terminal segment to play easily through a certain arc. It is by this
arrangement that the direction and the extent of the motion of the
free claw of the chela are determined. The source of the motion lies
in the muscles which occupy the interior of the enlarged penultimate
segment of the limb. Two muscles, one of very great size (_m_), the
other smaller (_m′_), are fastened by one end to the exoskeleton of
this segment. The fibres of the larger muscle converge to be fixed
into the two sides of a long flat process of the chitinous cuticula,
on the inner side of the base of the terminal segment, which serves
as a tendon (_t_); while those of the smaller muscle are similarly
attached to a like process which proceeds from the outer side of the
base of the terminal segment (_t′_). It is obvious that, when the
latter muscle shortens it must move the apex of the terminal segment
(_dp_) away from the end of the fixed claw; while, {95} when the
former contracts, the end of the terminal segment is brought towards
that of the fixed claw.

A living crayfish is able to perform very varied movements with its
pincers. When it swims backwards, these limbs are stretched straight
out, parallel with one another, in front of the head; when it walks,
they are usually carried like arms bent at the elbow, the “forearm”
partly resting on the ground; on being irritated, the crayfish sweeps
the pincers round in any direction to grasp the offending body; when
prey is seized, it is at once conveyed, with a circular motion,
towards the region of the mouth. Nevertheless, these very varied
actions are all brought about by a combination of simple flexions and
extensions, each of which is effected in the exact order, and to the
exact extent, needful to bring the chela into the position required.

The skeleton of the stem of the limb which bears the chela is, in
fact, divided into four moveable segments; and each of these is
articulated with the segments on each side of it by a hinge of just
the same character as that which connects the moveable claw of the
chela with the penultimate segment, while the basal segment is
similarly articulated with the thorax.

If the axes of all these articulations[7] were parallel, it is
obvious that, though the limb might be moved as a whole through a
considerable arc, and might be bent in various {96} degrees, yet all
its movements would be limited to one plane. But, in fact, the axes
of the successive articulations are nearly at right angles to one
another; so that, if the segments are successively either extended
or flexed, the chela describes a very complicated curve; and by
varying the extent of flexion or extension of each segment, this
curve is susceptible of endless variation. It would probably puzzle
a good mathematician to say exactly what position should be given to
each segment, in order to bring the chela from any given position
into any other; but if a lively crayfish is incautiously seized,
the experimenter will find, to his cost, that the animal solves the
problem both rapidly and accurately.

     [7] By axis of the articulation is meant a line drawn through
     the pair of hinges which constitute it.

The mechanism by which the retrograde swimming of the crayfish is
effected, is no less easily analysed. The apparatus of motion is, as
we have seen, the abdomen, with its terminal five-pointed flapper.
The rings of the abdomen are articulated together by joints (fig. 21,
×) situated a little below the middle of the height of the rings, at
opposite ends of transverse lines, at right angles to the long axis
of the abdomen.

Each ring consists of a dorsal, arched portion, called the _tergum_
(fig. 21; fig. 36, p. 142, _t. XIX_), and a nearly flat ventral
portion, which is the _sternum_ (fig. 36, _st. XIX_). Where these
two join, a broad plate is sent down on each side, which overlaps
the bases of the abdominal appendages, and is known as the _pleuron_
(fig. 36, _pl. XIX_). {97} The sterna are all very narrow, and are
connected together by wide spaces of flexible exoskeleton.

[Illustration: FIG. 21.—_Astacus fluviatilis._—Two of the abdominal
somites, in vertical section, seen from the inner side, to show ×, ×,
the hinges by which they are articulated with one another (× 3). The
anterior of the two somites is that to the right of the figure.]

When the abdomen is made straight, it will be found that these
_intersternal_ membranes are stretched as far as they will yield. On
the other hand, when the abdomen is bent up as far as it will go, the
sterna come close together, and the intersternal membranes are folded.

The terga are very broad; so broad, in fact, that each overlaps its
successor, when the abdomen is straightened or extended, for nearly
half its length in the middle line; and the overlapped surface is
smooth, convex, and {98} marked off by a transverse groove from the
rest of the tergum as an _articular facet_. The front edge of the
articular facet is continued into a sheet of flexible cuticula, which
turns back, and passes as a loose fold to the hinder edge of the
overlapping tergum (fig. 21). This tergal _interarticular membrane_
allows the terga to move as far as they can go in flexion; whilst,
in extreme extension, they are but slightly stretched. But, even
if the intersternal membranes presented no obstacle to excessive
extension of the abdomen, the posterior free edge of each tergum fits
into the groove behind the facet in the next in such a manner, that
the abdomen cannot be made more than very slightly concave upwards
without breaking.

Thus the limits of motion of the abdomen, in the vertical direction,
are from the position in which it is straight, or has even a very
slight upward concavity, to that in which it is completely bent
upon itself, the telson being brought under the bases of the
hinder thoracic limbs. No lateral movement between the somites of
the abdomen is possible in any of its positions. For, when it is
straight, lateral movement is hindered not only by the extensive
overlapping of the terga, but also by the manner in which the hinder
edges of the pleura of each of the four middle somites overlap the
front edges of their successors. The pleura of the second somite are
much larger than any of the others, and their front edges overlap the
small pleura of the first abdominal somite; and when the abdomen is
much flexed, these pleura even {99} ride over the posterior edges of
the branchiostegites. In the position of extension, the overlap of
the terga is great, while that of the pleura of the middle somites is
small. As the abdomen passes from extension to flexion, the overlap
of the terga of course diminishes; but any decrease of resistance
to lateral strains which may thus arise, is compensated by the
increasing overlap of the pleura, which reaches its maximum when the
abdomen is completely flexed.

It is obvious that longitudinal muscular fibres fixed into the
exoskeleton, above the axes of the joints, must bring the centres
of the terga of the somites closer together, when they contract;
while muscular fibres attached below the axes of the joints must
approximate the sterna. Hence, the former will give rise to
extension, and the latter to flexion, of the abdomen as a whole.

Now there are two pairs of very considerable muscles disposed in this
manner. The dorsal pair, or the _extensors_ of the abdomen (fig. 22,
_e.m_), are attached in front to the side walls of the thorax, thence
pass backwards into the abdomen, and divide into bundles, which are
fixed to the inner surfaces of the terga of all the somites. The
other pair, or the _flexors_ of the abdomen (_f.m_) constitute a
very much larger mass of muscle, the fibres of which are curiously
twisted, like the strands of a rope. The front end of this double
rope is fixed to a series of processes of the exoskeleton of the
thorax, called _apodemata_, some of which roof over the sternal
blood-sinuses {100} and the thoracic part of the nervous system;
while, in the abdomen, its strands are attached to the sternal
exoskeleton of all the somites and extend, on each side of the
rectum, to the telson.

[Illustration: FIG. 22.—_Astacus fluviatilis._—A longitudinal section
of the body to show the principal muscles and their relations to
the exoskeleton (nat. size). _a_, the vent; _add.m_, adductor
muscle of mandible; _e.m_, extensor, and, _f.m_, flexor muscle of
abdomen; _œs_, œsophagus; _pcp_, procephalic process; _t,t′_, the two
segments of the telson; _XV–XX_, the abdominal somites; _1–20_, the
appendages; ×, ×, hinges between the successive abdominal somites.]

When the exoskeleton is cleaned by maceration, the abdomen has a
slight curve, dependent upon the form and the degree of elasticity
possessed by its different parts; and, in a living crayfish at rest,
it will be observed that the curvature of the abdomen is still more
marked. Hence it is ready either for extension or for flexion.

A sudden contraction of the flexor muscles instantly increases the
ventral curvature of the abdomen, and {101} throws the tail fin, the
two side lobes of which are spread out, forwards; while the body is
propelled backwards by the reaction of the water against the stroke.
Then the flexor muscles being relaxed, the extensor muscles come into
play; the abdomen is straightened, but less violently and with a far
weaker stroke on the water, in consequence of the less strength of
the extensors and of the folding up of the lateral plates of the fin,
until it comes into the position requisite to give full force to a
new downward and forward stroke. The tendency of the extension of
the abdomen is to drive the body forward; but from the comparative
weakness and the obliquity of its stroke, its practical effect is
little more than to check the backward motion conferred upon the body
by the flexion of the abdomen.

       *       *       *       *       *

Thus, every action of the crayfish, which involves motion, means the
contraction of one or more muscles. But what sets muscle contracting?
A muscle freshly removed from the body may be made to contract in
various ways, as by mechanical or chemical irritation, or by an
electrical shock; but, under natural conditions, there is only one
cause of muscular contraction, and that is the activity of a nerve.
Every muscle is supplied with one or more nerves. These are delicate
threads which, on microscopic examination, prove to be bundles of
fine tubular filaments, filled with an apparently structureless
substance of gelatinous consistency, the _nerve fibres_ {102} (fig.
23). The nerve bundle which passes to a muscle breaks up into smaller
bundles and, finally, into separate fibres, each of which ultimately
terminates by becoming continuous with the substance of a muscular
fibre (fig. 19, F). Now the peculiarity of a muscle nerve, or _motor_
nerve, as it is called, is that irritation of the nerve fibre at any
part of its length, however distant from the muscle, brings about
muscular contraction, just as if the muscle itself were irritated.
A change is produced in the molecular condition of the nerve at the
point of irritation; and this change is propagated along the nerve,
until it reaches the muscle, in which it gives rise to that change in
the arrangement of its molecules, the most obvious effect of which is
the sudden alteration of form which we call muscular contraction.

[Illustration: FIG. 23.—_Astacus fluviatilis._—Three nerve fibres,
with the connective tissue in which they are imbedded. (Magnified
about 250 diameters.) _n_, nuclei.]

[Illustration: FIG. 24.—_Astacus fluviatilis._—A, one of the (double)
abdominal ganglia, with the nerves connected with it (× 25); B,
a nerve cell or ganglionic corpuscle (× 250). _a_, sheath of the
nerves; _c_, sheath of the ganglion; _co, co′_, commissural cords
connecting the ganglia with those in front, and those behind them.
_gl.c._ points to the ganglionic corpuscles of the ganglia; _n_,
nerve fibres.]

If we follow the course of the motor nerves in a {103} direction
away from the muscles to which they are distributed, they will
be found, sooner or later, to terminate in _ganglia_ (fig. 24 A,
_gl.c_; fig. 25, _gn. 1–13_). A ganglion is a body which is in great
measure composed of nerve fibres; but, interspersed among these,
or disposed around them, there are peculiar structures, which are
termed _ganglionic corpuscles_, or _nerve cells_ (fig. 24, B).
These are nucleated cells, not unlike the epithelial cells which
have been already mentioned, but which are larger {105} and often
give off one or more processes. These processes, under favourable
circumstances, can be traced into continuity with nerve fibres.

[Illustration: FIG. 25.—_Astacus fluviatilis._—The central nervous
system seen from above (nat. size). _a_, vent; _an_, antennary nerve;
_a′n_, antennulary nerve; _c_, circumœsophageal commissures; _gn._
1, supraœsophageal ganglion; _gn._ 2, infraœsophageal ganglion;
_gn._ 6, fifth thoracic ganglion; _gn._ 7, last thoracic ganglion;
_gn._ 13, last abdominal ganglion; _œs_, œsophagus in cross section;
_on_, optic nerve; _sa_, sternal artery in cross section; _sgn_,
stomatogastric nerve.]

The chief ganglia of the crayfish are disposed in a longitudinal
series in the middle line of the ventral aspect of the body close to
the integument (fig. 25). In the abdomen, for example, six ganglionic
masses are readily observed, one lying over the sternum of each
somite, connected by longitudinal bands of nerve fibres, and giving
off branches to the muscles. On careful examination, the longitudinal
connecting bands, or _commissures_ (fig. 24, _co_), are seen to be
double, and each mass appears slightly bilobed. In the thorax, there
are six, larger, double ganglionic masses, likewise connected by
double commissures; and the most anterior of these, which is the
largest (fig. 25, _gn._ 2), is marked at the sides by notches, as
if it were made up of several pairs of ganglia, run together into
one continuous whole. In front of this, two commissures (_c_) pass
forwards, separating widely, to give room for the gullet (_œs_),
which passes between them; while in front of the gullet, just behind
the eyes, they unite with a transversely elongated mass of ganglionic
substance (_gn._ 1), termed the _brain_, or _cerebral ganglion_.

All the motor nerves, as has been said, are traceable, directly or
indirectly, to one or other of these thirteen sets of ganglia; but
other nerves are given off from the ganglia, which cannot be followed
into any muscle. In {106} fact, these nerves go either to the
integument or to the organs of sense, and they are termed _sensory
nerves_.

When a muscle is connected by its motor nerve with a ganglion,
irritation of that ganglion will bring about the contraction of the
muscle, as well as if the motor nerve itself were irritated. Not
only so; but if a sensory nerve, which is in connexion with the
ganglion, is irritated, the same effect is produced; moreover, the
sensory nerve itself need not be excited, but the same result will
take place, if the organ to which it is distributed is stimulated.
Thus the nervous system is fundamentally an apparatus by which two
separate, and it may be distant, parts of the body, are brought into
relation with one another; and this relation is of such a nature,
that a change of state arising in the one part is followed by the
propagation of changes along the sensory nerve to the ganglion, and
from the ganglion to the other part; where, if that part happens
to be muscle, it produces contraction. If one end of a rod of
wood, twenty feet long, is applied to a sounding-board, the sound
of a tuning-fork held against the opposite extremity will be very
plainly heard. Nothing can be seen to happen in the wood, and yet
its molecules are certainly set vibrating, at the same rate as the
tuning-fork vibrates; and when, after travelling rapidly along the
wood, these vibrations affect the sounding-board, they give rise
to vibrations of the molecules of the air, which reaching the ear,
are converted into an audible note. So in the nerve tract: {107}
no apparent change is effected in it by the irritation at one end;
but the rate at which the molecular change produced travels can
be measured; and, when it reaches the muscle, its effect becomes
visible in the change of form of the muscle. The molecular change
would take place just as much if there were no muscle connected with
the nerve, but it would be no more apparent to ordinary observation
than the sound of the tuning-fork is audible in the absence of the
sounding-board.

If the nervous system were a mere bundle of nerve fibres extending
between sensory organs and muscles, every muscular contraction would
require the stimulation of that special point of the surface on which
the appropriate sensory nerve ended. The contraction of several
muscles at the same time, that is, the combination of movements
towards one end, would be possible only if the appropriate nerves
were severally stimulated in the proper order, and every movement
would be the direct result of external changes. The organism would
be like a piano, which may be made to give out the most complicated
harmonies, but is dependent for their production on the depression
of a separate key for every note that is sounded. But it is obvious
that the crayfish needs no such separate impulses for the performance
of highly complicated actions. The simple impression made on the
organs of sensation in the two examples with which we started, gives
rise to a train of complicated and accurately co-ordinated muscular
contractions. To carry the analogy {108} of the musical instrument
further, striking a single key gives rise, not to a single note,
but to a more or less elaborate tune; as if the hammer struck not a
single string, but pressed down the stop of a musical box.

It is in the ganglia that we must look for the analogue of the
musical box. A single impulse conveyed by a sensory nerve to a
ganglion, may give rise to a single muscular contraction, but more
commonly it originates a series of such, combined to a definite end.

The effect which results from the propagation of an impulse along
a nerve fibre to a ganglionic centre, whence it is, as it were,
reflected along another nerve fibre to a muscle, is what is termed a
_reflex action_. As it is by no means necessary that sensation should
be a concomitant of the first impulse, it is better to term the nerve
fibre which carries it _afferent_ rather than sensory; and, as other
phenomena besides those of molar motion may be the ultimate result
of the reflex action, it is better to term the nerve fibre which
transmits the reflected impulse _efferent_ rather than motor.

If the nervous commissures between the last thoracic and the first
abdominal ganglia are cut, or if the thoracic ganglia are destroyed,
the crayfish is no longer able to control the movements of the
abdomen. If the forepart of the body is irritated, for example, the
animal makes no effort to escape by swimming backwards. Nevertheless,
the abdomen is not paralysed, for, if it be irritated, it will flap
vigorously. This is a case of pure {109} reflex action. The stimulus
is conveyed to the abdominal ganglia through afferent nerves, and is
reflected from them, by efferent nerves, to the abdominal muscles.

But this is not all. Under these circumstances it will be seen that
the abdominal limbs all swing backwards and forwards, simultaneously,
with an even stroke; while the vent opens and shuts with a regular
rhythm. Of course, these movements imply correspondingly regular
alternate contractions and relaxations of certain sets of muscles;
and these, again, imply regularly recurring efferent impulses from
the abdominal ganglia. The fact that these impulses proceed from the
abdominal ganglia, may be shown in two ways: first, by destroying
these ganglia in one somite after another, when the movements in
each somite at once permanently cease; and, secondly, by irritating
the surface of the abdomen, when the movements are temporarily
inhibited by the stimulation of the afferent nerves. Whether these
movements are properly reflex, that is, arise from incessant new
afferent impulses of unknown origin, or whether they depend on the
periodical accumulation and discharge of nervous energy in the
ganglia themselves, or upon periodical exhaustion and restoration of
the irritability of the muscles, is unknown. It is sufficient for
the present purpose to use the facts as evidence of the peculiar
co-ordinative function of ganglia.

The crayfish, as we have seen, avoids light; and the slightest
touch of one of its antennæ gives rise to active motions of the
whole body. In fact, the animal’s {110} position and movements are
largely determined by the influences received through the feelers
and the eyes. These receive their nerves from the cerebral ganglia;
and, as might be expected, when these ganglia are extirpated, the
crayfish exhibits no tendency to get away from the light, and the
feelers may not only be touched, but sharply pinched, without effect.
Clearly, therefore, the cerebral ganglia serve as a ganglionic
centre, by which the afferent impulses derived from the feelers and
the eyes are transmuted into efferent impulses. Another very curious
result follows upon the extirpation of the cerebral ganglia. If an
uninjured crayfish is placed upon its back, it makes unceasing and
well-directed efforts to turn over; and if everything else fails,
it will give a powerful flap with the abdomen, and trust to the
chapter of accidents to turn over as it darts back. But the brainless
crayfish behaves in a very different way. Its limbs are in incessant
motion, but they are “all abroad;” and if it turns over on one side,
it does not seem able to steady itself, but rolls on to its back
again.

If anything is put between the chelæ of an uninjured crayfish, while
on its back, it either rejects the object at once, or tries to make
use of it for leverage to turn over. In the brainless crayfish a
similar operation gives rise to a very curious spectacle.[8] If the
object, whatever it {111} be—a bit of metal, or wood, or paper, or
one of the animal’s own antennæ—is placed between the chelæ of the
forceps, it is at once seized by them, and carried backwards; the
chelate ambulatory limbs are at the same time advanced, the object
seized is transferred to them, and they at once tuck it between the
external maxillipedes, which, with the other jaws, begin vigorously
to masticate it. Sometimes the morsel is swallowed; sometimes
it passes out between the anterior jaws, as if deglutition were
difficult. It is very singular to observe that, if the morsel which
is being conveyed to the mouth by one of the forceps is pulled back,
the forceps and the chelate ambulatory limbs of the other side are at
once brought forward to secure it. The movements of the limbs are, in
short, adjusted to meet the increased resistance.

     [8] My attention was first drawn to these phenomena by my friend
     Dr. M. Foster, F.R.S., to whom I had suggested the desirableness
     of an experimental study of the nerve physiology of the crayfish.

All these phenomena cease at once, if the thoracic ganglia are
destroyed. It is in these, therefore, that the simple stimulus set
up by the contact of a body with, for example, one of the forceps,
is translated into all the surprisingly complex and accurately
co-ordinated movements, which have been described. Thus the nervous
system of the crayfish may be regarded as a system of co-ordinating
mechanisms, each of which produces a certain action, or set of
actions, on the receipt of an appropriate stimulus.

When the crayfish comes into the world, it possesses in its
neuro-muscular apparatus certain innate {112} potentialities of
action, and will exhibit the corresponding acts, under the influence
of the appropriate stimuli. A large proportion of these stimuli come
from without through the organs of the senses. The greater or less
readiness of each sense organ to receive impulses, of the nerves to
transmit them, and of the ganglia to give rise to combined impulses,
is dependent at any moment upon the physical condition of these
parts; and this, again, is largely modified by the amount and the
condition of the blood supplied. On the other hand, a certain number
of these stimuli are doubtless originated by changes within the
various organs which compose the body, including the nerve centres
themselves.

When an action arises from conditions developed in the interior of
an animal’s body, inasmuch as we cannot perceive the antecedent
phenomena, we call such an action “spontaneous;” or, when in
ourselves we are aware that it is accompanied by the idea of the
action, and the desire to perform it, we term the act “voluntary.”
But, by the use of this language, no rational person intends to
express the belief that such acts are uncaused or cause themselves.
“Self-causation” is a contradiction in terms; and the notion that
any phenomenon comes into existence without a cause, is equivalent
to a belief in chance, which one may hope is, by this time, finally
exploded.

In the crayfish, at any rate, there is not the slightest reason to
doubt that every action has its definite physical {113} cause, and
that what it does at any moment would be as clearly intelligible, if
we only knew all the internal and external conditions of the case, as
the striking of a clock is to any one who understands clockwork.

       *       *       *       *       *

The adjustment of the body to varying external conditions, which is
one of the chief results of the working of the nervous mechanism,
would be far less important from a physiological point of view than
it is, if only those external bodies which come into direct contact
with the organism[9] could affect it; though very delicate influences
of this kind take effect on the nervous apparatus through the
integument.

     [9] It may be said that, strictly speaking, only those external
     bodies which are in direct contact with the organism do affect
     it—as the vibrating ether, in the case of luminous bodies; the
     vibrating air or water, in the case of sonorous bodies; odorous
     particles, in the case of odorous bodies: but I have preferred
     the ordinary phraseology to a pedantically accurate periphrasis.

It is probable that the _setæ_, or hairs, which are so generally
scattered over the body and the appendages, are delicate tactile
organs. They are hollow processes of the chitinous cuticle, and their
cavities are continuous with narrow canals, which traverse the whole
thickness of the cuticle, and are filled by a prolongation of the
subjacent proper integument. As this is supplied with nerves, it is
likely that fine nerve fibres reach the bases of the hairs, and are
affected by anything which stirs these delicately poised levers.
{114}

[Illustration: FIG. 26.—_Astacus fluviatilis._—A, the right antennule
seen from the inner side (× 5); B, a portion of the exopodite
enlarged; C, olfactory appendage of the exopodite; _a_, front view;
_b_, side view (× 300); _a_, olfactory appendages; _au_, auditory
sac, supposed to be seen through the wall of the basal joint of the
antennule; _b_, setæ; _en_, endopodite; _ex_, exopodite; _sp_, spine
of the basal joint.]

There is much reason to believe that odorous bodies affect crayfish;
but it is very difficult to obtain experimental evidence of the
fact. However, there is a good deal of analogical ground for the
supposition that some peculiar structures, which are evidently of
a sensory {115} nature, developed on the under side of the outer
branch of the antennule, play the part of an olfactory apparatus.

Both the outer (fig. 26 A. _ex_) and the inner (_en_) branches of the
antennule are made up of a number of delicate ring-like segments,
which bear fine setæ (_b_) of the ordinary character.

The inner branch, which is the shorter of the two, possesses only
these setæ; but the under surface of each of the joints of the outer
branch, from about the seventh or eighth to the last but one, is
provided with two bundles of very curious appendages (fig. 27, A,
B, C, _a_), one in front and one behind. These are rather more than
1‐200th of an inch long, very delicate, and shaped like a spatula,
with a rounded handle and a flattened somewhat curved blade, the
end of which is sometimes truncated, sometimes has the form of a
prominent papilla. There is a sort of joint between the handle and
the blade, such as is found between the basal and the terminal parts
of the ordinary setæ, with which, in fact, these processes entirely
correspond in their essential structure. A soft granular tissue fills
the interior of each of these problematical structures, to which
Leydig, their discoverer, ascribes an olfactory function.

It is probable that the crayfish possesses something analogous to
taste, and a very likely seat for the organ of this function is in
the upper lip and the metastoma; but if the organ exists it possesses
no structural peculiarities by which it can be identified. {116}

There is no doubt, however, as to the special recipients of sonorous
and luminous vibrations; and these are of particular importance,
as they enable the nervous machinery to be affected by bodies
indefinitely remote from it, and to change the place of the organism
in relation to such bodies.

       *       *       *       *       *

Sonorous vibrations are enabled to act as the stimulants of a special
nerve (fig. 25, _a′n_) connected with the brain, by means of the very
curious _auditory sacs_ (fig. 26, A, _au_) which are lodged in the
basal joints of the antennules.

Each of these joints is trihedral, the outer face being convex;
the inner, applied to its fellow, flat; and the upper, on which
the eyestalk rests, concave. On this upper face there is a narrow
elongated oval aperture, the outer lip of which is beset with a
flat brush of long close-set setæ, which lie horizontally over the
aperture, and effectually close it. The aperture leads into a small
sac (_au_) with delicate walls formed by a chitinous continuation of
the general cuticula. The inferior and posterior wall of the sac is
raised up along a curved line into a ridge which projects into its
interior (fig. 27, A, _r_). Each side of this ridge is beset with a
series of delicate setæ (_as_), the longest of which measures about
1‐50th of an inch; they thus form a longitudinal band bent upon
itself. These _auditory setæ_ project into the fluid contents of the
sac, and their apices are for the most part imbedded in a gelatinous
mass, which contains irregular particles of sand {117} and sometimes
of other foreign matter. A nerve (_n n′_,) is distributed to the sac,
and its fibres enter the bases of the hairs, and may be traced to
their apices, where they end in peculiar elongated rod-like bodies
(fig. 27, C). Here is an auditory organ of the simplest description.
It retains, in fact, throughout life, the condition of a simple sac
or involution of the integument, such as is that of the vertebrate
ear in its earliest stage.

[Illustration: FIG. 27.—_Astacus fluviatilis._ A, the auditory sac
detached and seen from the outside (× 15); B, auditory hair (× 100);
C, the distal extremity of the same more highly magnified. _a_,
aperture of sac; _as_, auditory setæ; _b_, its inner or posterior
extremity; _n n′_, nerves; _r_, ridge.]

{118}

The sonorous vibrations transmitted through the water in which the
crayfish lives to the fluid and solid contents of the auditory sac
are taken up by the delicate hairs of the ridge, and give rise to
molecular changes which traverse the auditory nerves and reach the
cerebral ganglia.

       *       *       *       *       *

The vibrations of the luminiferous ether are brought to bear upon
the free ends of two large bundles of nerve fibres, termed the optic
nerves (fig. 25, _on_), which proceed directly from the brain, by
means of a highly complex _eye_. This is an apparatus, which, in
part, sorts out the rays of light into as many very small pencils as
there are separate endings of the fibres of the optic nerve, and,
in part, serves as the medium by which the luminous vibrations are
converted into molecular nerve changes.

The free extremity of the eyestalk presents a convex, soft, and
transparent surface, limited by an oval contour. The cuticle in
this region, which is termed the _cornea_, (fig. 28, _a_), is, in
fact, somewhat thinner and less distinctly laminated than in the
rest of the eyestalk, and it contains no calcareous matter. But
it is directly continuous with the rest of the exoskeleton of the
eyestalk, to which it stands in somewhat the same relation as the
soft integument of an articulation does to the adjacent hard parts.

[Illustration: FIG. 28.—_Astacus fluviatilis._—A, a vertical section
of the eye-stalk (× 6); B, a small portion of the same, showing the
visual apparatus more highly magnified; _a_, cornea; _b_, outer dark
zone; _c_, outer white zone; _d_, middle dark zone; _e_, inner white
zone; _f_, inner dark zone; _cr_, crystalline cones; _g_, optic
ganglion; _op_, optic nerve; _sp_, striated spindles.]

The _cornea_ is divided into a great number of minute, usually square
facets, by faint lines, which cross it from side {119} to side
nearly at right angles with one another. A longitudinal section shows
that both the horizontal and the vertical contours of the cornea
are very nearly semicircular, and that the lines which mark off the
facets merely arise from a slight modification of its substance
between the facets. The outer contour of each facet forms part of
the general curvature of the outer face of the cornea; the inner
contour sometimes exhibits a slight deviation {120} from the general
curvature of the inner face, but usually nearly coincides with it.

When a longitudinal or a transverse section is taken through the
whole eyestalk, the optic nerve (fig. 28, A, _op_) is seen to
traverse its centre. At first narrow and cylindrical, it expands
towards its extremity into a sort of bulb (B, _g_), the outer surface
of which is curved in correspondence with the inner surface of the
cornea. The terminal half of the bulb contains a great quantity of
dark colouring matter or pigment, and, in section, appears as what
may be termed the _inner dark zone_ (_f_). Outside this, and in
connection with it, follows a white line, the _inner white zone_
(_e_), then comes a _middle dark zone_ (_d_); outside this an outer
pale band, which may be called the _outer white zone_ (_c_), and
between this and the cornea (_a_) is another broad band of dark
pigment, the _outer dark zone_ (_b_).

When viewed under a low power, by reflected light, this outer dark
zone is seen to be traversed by nearly parallel straight lines,
each of which starts from the boundary between two facets, and can
be followed inwards through the outer white zone to the middle dark
zone. Thus the whole substance of the eye between the outer surface
of the bulb of the optic nerve and the inner surface of the cornea is
marked out into as many segments as the cornea has facets; and each
segment has the form of a wedge or slender pyramid, the base of which
is four-sided, and is applied against the inner surface of {121} one
of the facets of the cornea, while its summit lies in the middle dark
zone. Each of these _visual pyramids_ consists of an axial structure,
the _visual rod_, invested by a sheath. The latter extends inwards
from the margin of each facet of the cornea, and contains pigment
in two regions of its length, the intermediate space being devoid
of pigment. As the position of the pigmented regions in relation to
the length of the pyramid is always the same, the pigmented regions
necessarily take the form of two consecutive zones when the pyramids
are in their natural position.

The visual rod consists of two parts, an external _crystalline cone_
(fig. 28, B, _cr_), and an internal _striated spindle_ (_sp_).
The _crystalline cone_ consists of a transparent glassy-looking
substance, which may be made to split up longitudinally into four
segments. Its inner end narrows into a filament which traverses
the outer white zone, and, in the middle dark zone, thickens
into a four-sided spindle-shaped transparent body, which appears
transversely striated. The inner end of this _striated spindle_
narrows again, and becomes continuous with nerve fibres which proceed
from the surface of the optic bulb.

The exact mode of connection of the nerve-fibres with the visual rods
is not certainly made out, but it is probable that there is direct
continuity of substance, and that each rod is really the termination
of a nerve fibre.

Eyes having essentially the same structure as that of {122} the
crayfish are very widely met with among _Crustacea_ and _Insecta_,
and are commonly known as _compound eyes_. In many of these animals,
in fact, when the cornea is removed, each facet is found to act as
a separate lens; and when proper arrangements are made, as many
distinct pictures of external objects are found behind it as there
are facets. Hence the notion suggested itself that each visual
pyramid is a separate eye, similar in principle of construction to
the human eye, and forming a picture of so much of the external world
as comes within the range of its lens, upon a retina supposed to
be spread out on the surface of the crystalline cone, as the human
retina is spread over the surface of the vitreous humour.

But, in the first place, there is no evidence, nor any probability,
that there is anything corresponding to a retina on the outer face of
the crystalline cone; and secondly, if there were, it is incredible
that, with such an arrangement of the refractive media as exists in
the cornea and crystalline cones, rays proceeding from points in
the external world should be brought to a focus in correspondingly
related points of the surface of the supposed retina. But without
this no picture could be formed, and no distinct vision could take
place. It is very probable, therefore, that the visual pyramids do
not play the part of the simple eyes of the _Vertebrata_, and the
only alternative appears to be the adoption of a modification of the
theory of _mosaic vision_, propounded many years by Johannes Müller.
{123}

Each visual pyramid, isolated from its fellows by its coat of
pigment, may be supposed, in fact, to play the part of a very narrow
straight tube, with blackened walls, one end of which is turned
towards the external world, while the other incloses the extremity of
one of the nerve fibres. The only light which can reach the latter,
under these circumstances, is such as proceeds from points which lie
in the direction of a straight line represented by the produced axis
of the tubes.

[Illustration: FIG. 29.—Diagram showing the course of rays of light
from three points _x_, _y_, _z_, through the nine visual rods
(supposed to be empty tubes) A–I of a compound eye; _a–i_, the nerve
fibres connected with the visual rods.]

Suppose A–I to be nine such tubes, _a–i_ the corresponding nerve
fibres, and _x_ _y_ _z_ three points from which light proceeds. Then
it will be obvious that the only light {124} from _x_ which will
excite sensation, will be the ray which traverses B and reaches the
nerve-fibre _b_, while that from _y_ will affect only _e_, and that
from _x_ only _h_. The result, translated into sensation, will be
three points of light on a dark ground, each of which answers to one
of the luminous points, and indicates its direction in reference to
the eye and its angular distance from the other two.[10]

The only modification needed in the original form of the theory of
mosaic vision, is the supposition that part, or the whole, of the
visual rod, is not merely a passive transmitter of light to the
nerve-fibre, but is, itself, in someway concerned in transmuting
the mode of motion, light, into that other mode of motion which we
call nervous energy. The visual rod is, in fact, to be regarded as
the physiological end of the nerve, and the instrument by which the
conversion of the one form of motion into the other takes place; just
as the auditory hairs are instruments by which the sonorous waves are
converted into molecular movements of the substance of the auditory
nerves.

     [10] Since the visual rods are strongly refracting solids, and
     not empty tubes, the diagram given in fig. 29 does not represent
     the true course of the rays, indicated by dotted lines, which
     fall obliquely on any cornea of a crayfish’s eye. Such rays
     will be more or less bent towards the axis of the visual rod
     of that cornea; but whether they reach its apex and so affect
     the nerve or not will depend on the curvature of the cornea;
     its refractive index and that of the crystalline cone; and the
     relation between the length and the thickness of the latter.

It is wonderfully interesting to observe that, when the so-called
compound eye is interpreted in this manner, {125} the apparent
wide difference between it and the vertebrate eye gives place to a
fundamental resemblance. The rods and cones of the retina of the
vertebrate eye are extraordinarily similar in their form and their
relations to the fibres of the optic nerve, to the visual rods of
the arthropod eye. And the morphological discrepancy, which is at
first so striking, and which arises from the fact that the free ends
of the visual rods are turned towards the light, while those of the
rods and cones of the vertebrate eye are turned from it, becomes a
confirmation of the parallel between the two when the development of
the vertebrate eye is taken into account. For it is demonstrable that
the deep surface of the retina in which the rods and cones lie, is
really a part of the outer surface of the body turned inwards, in the
course of the singular developmental changes which give rise to the
brain and the eye of vertebrate animals.

       *       *       *       *       *

Thus the crayfish has, at any rate, two of the higher sense organs,
the ear and the eye, which we possess ourselves; and it may seem a
superfluous, not to say a frivolous, question, if any one should ask
whether it can hear and see.

But, in truth, the inquiry, if properly limited, is a very pertinent
one. That the crayfish is led by the use of its eyes and ears to
approach some objects and avoid others, is beyond all doubt; and, in
this sense, most indubitably it can both hear and see. But if the
question {126} means, do luminous vibrations give it the sensations
of light and darkness, of colour and form and distance, which they
give to us? and do sonorous vibrations produce the feelings of noise
and tone, of melody and of harmony, as in us?—it is by no means to
be answered hastily, perhaps cannot be answered at all, except in a
tentative, probable way.

The phenomena to which we give the names of sound and colour are
not physical things, but are states of consciousness, dependent,
there is every reason to believe, on the functional activity of
certain parts of our brains. Melody and harmony are names for states
of consciousness which arise when at least two sensations of sound
have been produced. All these are manufactured articles, products
of the human brain; and it would be exceedingly hazardous to affirm
that organs capable of giving rise to the same products exist in the
vastly simpler nervous system of the crustacean. It would be the
height of absurdity to expect from a meat-jack the sort of work which
is performed by a Jacquard loom; and it appears to me to be little
less preposterous to look for the production of anything analogous to
the more subtle phenomena of the human mind in something so minute
and rude in comparison to the human brain, as the insignificant
cerebral ganglia of the crayfish.

At the most, one may be justified in supposing the existence of
something approaching dull feeling in ourselves; and, to return to
the problem stated in the {127} beginning of this chapter, so far
as such obscure consciousness accompanies the molecular changes of
its nervous substance, it will be right to speak of the mind of a
crayfish. But it will be obvious that it is merely putting the cart
before the horse, to speak of such a mind as a factor in the work
done by the organism, when it is merely a dim symbol of a part of
such work in the doing.

Whether the crayfish possesses consciousness or not, however, does
not affect the question of its being an engine, the actions of which
at any moment depend, on the one hand, upon the series of molecular
changes excited, either by internal or by external causes, in its
neuro-muscular machinery; and, on the other, upon the disposition
and the properties of the parts of that machinery. And such a
self-adjusting machine, containing the immediate conditions of its
action within itself, is what is properly understood by an automaton.

       *       *       *       *       *

Crayfishes, as we have seen, may attain a considerable age; and there
is no means of knowing how long they might live, if protected from
the innumerable destructive influences to which they are at all ages
liable.

It is a widely received notion that the energies of living matter
have a natural tendency to decline, and finally disappear; and that
the death of the body, as a whole, is the necessary correlate of
its life. That all living things sooner or later perish needs no
demonstration, but it would be difficult to find satisfactory grounds
{128} for the belief that they must needs do so. The analogy of a
machine that, sooner or later, must be brought to a standstill by the
wear and tear of its parts, does not hold, inasmuch as the animal
mechanism is continually renewed and repaired; and, though it is
true that individual components of the body are constantly dying,
yet their places are taken by vigorous successors. A city remains,
notwithstanding the constant death-rate of its inhabitants; and such
an organism as a crayfish is only a corporate unity, made up of
innumerable partially independent individualities.

Whatever might be the longevity of crayfishes under imaginable
perfect conditions, the fact that, notwithstanding the great number
of eggs they produce, their number remains pretty much the same in a
given district, if we take the average of a period of years, shows
that about as many die as are born; and that, without the process of
reproduction, the species would soon come to an end.

There are many examples among members of the group of _Crustacea_
to which the crayfish belongs, of animals which produce young from
internally developed germs, as some plants throw off bulbs which
are capable of reproducing the parent stock; such is the case, for
example, with the common water flea (_Daphnia_). But nothing of this
kind has been observed in the crayfish; in which, as in the higher
animals, the reproduction of the species is dependent upon the
combination of two kinds of living {129} matter, which are developed
in different individuals, termed _males_ and _females_.

These two kinds of living matter are _ova_ and _spermatozoa_, and
they are developed in special organs, the _ovary_ and the _testis_.
The ovary is lodged in the female; the testis, in the male.

The _ovary_ (fig. 30, _ov_) is a body of a trefoil form, which is
situated immediately beneath, or in front of, the heart, between the
floor of the pericardial sinus and the alimentary canal. From the
ventral face of this organ two short and wide canals, the _oviducts_
(_od_), lead down to the bases of the second pair of walking limbs,
and terminate in the apertures (_od′_) already noticed there.

[Illustration: FIG. 30.—_Astacus fluviatilis._—The female
reproductive organs (× 2); _ov_, ovary; _od_, oviduct; _od′_,
aperture of oviduct.]

[Illustration: FIG. 31.—_Astacus fluviatilis._—The male reproductive
organs (× 2); _t_, testis; _vd_, vas deferens; _vd′_, aperture of vas
deferens.]

The _testis_ (fig. 31, _t_) is somewhat similar in form to the ovary,
but, the three divisions are much narrower {130} and more elongated:
the hinder median division lies under the heart; the anterior
divisions are situated between the heart behind, and the stomach and
the liver in front (figs. 5 and 12, _t_). From the point at which
the three divisions join, proceed two ducts, which are termed the
_vasa deferentia_ (fig. 31, _vd_). These are very narrow, long, and
make many coils before they reach the apertures upon the bases of the
hindermost pair of walking limbs, by which they open externally (fig.
31, _vd′_, and fig. 35, _vd_). Both the ovary and the testis are very
much larger {131} during the breeding season than at other times;
the large brownish-yellow eggs become conspicuous in the ovary, and
the testis assumes a milk-white colour, at this period.

[Illustration: FIG. 32.—_Astacus fluviatilis._—A, a two-thirds grown
egg contained in its ovisac (× 50); B, an egg removed from the ovisac
(× 10); C, a portion of the wall of an ovisac with the adjacent
portion of the contained egg, highly magnified; _ep_, epithelium of
ovisac; _gs_, germinal spots; _gv_, germinal vesicle; _m_, membrana
propria; _v_, vitellus; _vm_, vitelline membrane; _w_, stalk of
ovisac.]

[Illustration: FIG. 33.—_Astacus fluviatilis._—A, a lobule of
the testis, showing _a_, acini, springing from _b_, the ultimate
termination of a duct (× 50). B, spermatic cells; _a_, with an
ordinary globular nucleus _n_; _b_, with a spindle-shaped nucleus;
_c_, with two similar nuclei; and _d_, with a nucleus undergoing
division (× 600).]

The walls of the ovary are lined internally by a layer of {132}
nucleated cells, separated from the cavity of the organ by a delicate
structureless membrane. The growth of these cells gives rise to
papillary elevations which project into the cavity of the ovary,
and eventually become globular bodies attached by short stalks, and
invested by the structureless membrane as a _membrana propria_ (fig.
32, _m_). These are the _ovisacs_. In the mass of cells which becomes
the ovisac, one rapidly increases in size and occupies the centre
of the ovisac, while the others {133} surround it as a peripheral
coat (_ep._). This central cell is the _ovum_. Its nucleus enlarges,
and becomes what is called the _germinal vesicle_ (_g.v._). At the
same time numerous small corpuscles, flattened externally and convex
internally, appear in it and are the _germinal spots_ (_g.s._). The
protoplasm of the cell, as it enlarges, becomes granular and opaque,
assumes a deep brownish-yellow colour, and is thus converted into
the _yelk_ or _vitellus_ (_v._). As the egg grows, a structureless
_vitelline membrane_ is formed between the vitellus and the cells
which line the ovisac, and incloses the egg, as in a bag. Finally,
the ovisac bursts, and the egg, falling into the cavity of the ovary,
makes its way down the oviduct, and sooner or later passes out by
its aperture. When they leave the oviduct, the ova are invested by a
viscous, transparent substance, which attaches them to the swimmerets
of the female, and then sets; thus each egg, inclosed in a tough
case, is firmly suspended by a stalk, which, on the one side, is
continued into the substance of the case, while, on the other, it is
fixed to the swimmeret. The swimmerets are kept constantly in motion,
so that the eggs are well supplied with aërated water. {134}

[Illustration: FIG. 34.—_Astacus fluviatilis._—A–D, different stages
in the development of a spermatozoon from a seminal cell; E, a mature
spermatozoon seen from the side; F, the same viewed _on face_ (all
× 850); G, a diagrammatic vertical section of the same.]

The testis consists of an immense number of minute spheroidal
vesicles (fig. 33, A, _a_), attached like grapes to the ends of
short stalks (_b_), formed by the ultimate ramifications of the vasa
deferentia. The vesicles may, in fact, be regarded as dilatations of
the ends and sides {135} of the finest branches of the ducts of the
testis. The cavity of each vesicle is filled by the large nucleated
cells which line its walls (fig. 33, B), and, as the breeding season
approaches, these cells multiply by division. Finally, they undergo
some very singular changes of form and internal structure (fig. 34,
A–D), each becoming converted into a flattened spheroidal body, about
1‐1700th of an inch diameter, provided with a number of slender
curved rays, which stand out from its sides (fig. 34, E–G). These are
the _spermatozoa_.

The spermatozoa accumulate in the testicular vesicles, and give rise
to a milky-looking substance, which traverses the smaller ducts,
and eventually fills the vasa deferentia. This substance, however,
consists, in addition to the spermatozoa, of a viscid material,
secreted by the walls of the vasa deferentia, which envelopes the
spermatozoa, and gives the secretion of the testis the form and the
consistency of threads of vermicelli.

The ripening and detachment of both the ova and the spermatozoa
take place immediately after the completion of ecdysis in the
early autumn; and at this time, which is the breeding season, the
males seek the females with great avidity, in order to deposit the
fertilizing matter contained in the vasa deferentia on the sterna
of their hinder thoracic and anterior abdominal somites. There it
adheres as a whitish, chalky-looking mass; but the manner in which
the contained spermatozoa reach and enter the ova is unknown. The
analogy {136} of what occurs in other animals, however, leaves
no doubt that an actual mixture of the male and female elements
takes place and constitutes the essential part of the process of
impregnation.

Ova to which spermatozoa have had no access, give rise to no progeny;
but, in the impregnated ovum, the young crayfish takes its origin in
a manner to be described below, when the question of development is
dealt with.

[Illustration: FIG. 35.—_Astacus fluviatilis._—The last thoracic
sternum, seen from behind, with the proximal ends of the appendages,
A, in the male, B, in the female, (× 3). _am_, articular membrane;
_cxp_, coxopodite; _st XIV_, last thoracic sternum; _vd_, aperture of
vas deferens.]

{137}



CHAPTER IV.

THE MORPHOLOGY OF THE COMMON CRAYFISH: THE STRUCTURE AND THE
DEVELOPMENT OF THE INDIVIDUAL.


In the two preceding chapters the crayfish has been studied from
the point of view of the physiologist, who, regarding an animal as
a mechanism, endeavours to discover how it does that which it does.
And, practically, this way of looking at the matter is the same as
that of the teleologist. For, if all that we know concerning the
purpose of a mechanism is derived from observation of the manner in
which it acts, it is all one, whether we say that the properties and
the connexions of its parts account for its actions, or that its
structure is adapted to the performance of those actions.

Hence it necessarily follows that physiological phenomena can be
expressed in the language of teleology. On the assumption that the
preservation of the individual, and the continuance of the species,
are the final causes of the organization of an animal, the existence
of that organization is, in a certain sense, explained, when it is
shown that it is fitted for the attainment of those ends; although,
perhaps, the importance of {138} demonstrating the proposition that
a thing is fitted to do that which it does, is not very great.

But whatever may be the value of teleological explanations, there
is a large series of facts, which have as yet been passed over, or
touched only incidentally, of which they take no account. These
constitute the subject matter of _Morphology_, which is related to
physiology much as, in the not-living world, crystallography is
related to the study of the chemical and physical properties of
minerals.

Carbonate of lime, for example, is a definite compound of calcium,
carbon, and oxygen, and it has a great variety of physical and
chemical properties. But it may be studied under another aspect, as
a substance capable of assuming crystalline forms, which, though
extraordinarily various, may all be reduced to certain geometrical
types. It is the business of the crystallographer to work out the
relations of these forms; and, in so doing, he takes no note of the
other properties of carbonate of lime.

In like manner, the morphologist directs his attention to the
relations of form between different parts of the same animal, and
between different animals; and these relations would be unchanged
if animals were mere dead matter, devoid of all physiological
properties—a kind of mineral capable of a peculiar mode of growth.

A familiar exemplification of the difference between teleology and
morphology may be found in such works of human art as houses. {139}

A house is certainly, to a great extent, an illustration of
adaptation to purpose, and its structure is, to that extent,
explicable by teleological reasonings. The roof and the walls are
intended to keep out the weather; the foundation is meant to afford
support and to exclude damp; one room is contrived for the purpose
of a kitchen; another for that of a coal-cellar; a third for that
of a dining-room; others are constructed to serve as sleeping
rooms, and so on; doors, chimneys, windows, drains, are all more or
less elaborate contrivances directed towards one end, the comfort
and health of the dwellers in the house. What is sometimes called
sanitary architecture, now-a-days, is based upon considerations
of house teleology. But though all houses are, to begin with and
essentially, means adapted to the ends of shelter and comfort, they
may be, and too often are, dealt with from a point of view, in which
adaptation to purpose is largely disregarded, and the chief attention
of the architect is given to the form of the house. A house may
be built in the Gothic, the Italian, or the Queen Anne style; and
a house in any one of these styles of architecture may be just as
convenient or inconvenient, just as well or as ill adapted to the
wants of the resident therein, as any of the others. Yet the three
are exceedingly different.

To apply all this to the crayfish. It is, in a sense a house with
a great variety of rooms and offices, in which the work of the
indwelling life in feeding, breathing, moving, and reproducing
itself, is done. But the {140} same may be said of the crayfish’s
neighbours, the perch and the water-snail; and they do all these
things neither better nor worse, in relation to the conditions
of their existence, than the crayfish does. Yet the most cursory
inspection is sufficient to show that the “styles of architecture”
of the three are even more widely different than are those of the
Gothic, Italian, and Queen Anne houses.

That which Architecture, as an art conversant with pure form, is to
buildings, Morphology, as a science conversant with pure form, is
to animals and plants. And we may now proceed to occupy ourselves
exclusively with the morphological aspect of the crayfish.

       *       *       *       *       *

As I have already mentioned, when dealing with the physiology of the
crayfish, the entire body of the animal, when reduced to its simplest
morphological expression, may be represented as a cylinder, closed
at each end, except so far as it is perforated by the alimentary
apertures (fig. 6); or we may say that it is a tube, inclosing
another tube, the edges of the two being continuous at their
extremities. The outer tube has a chitinous outer coat or cuticle,
which is continued on to the inner face of the inner tube. Neglecting
this for the present, the outermost part of the wall of the outer
tube, which answers to the _epidermis_ of the higher animals, and
the innermost part of the wall of the inner tube, which is an
_epithelium_, are formed by a layer of nucleated cells. A continuous
layer of cells, therefore, is everywhere to {141} be found on both
the external and the internal free surfaces of the body. So far as
these cells belong to the proper external wall of the body, they
constitute the _ectoderm_, and so far as they belong to its proper
internal wall, they compose the _endoderm_. Between these two layers
of nucleated cells lie all the other parts of the body, composed of
connective tissue, muscles, vessels, and nerves; and all these (with
the exception of the ganglionic chain, which we shall see properly
belongs to the ectoderm) may be regarded as a single thick stratum,
which, as it lies between the ectoderm and the endoderm, is called
the _mesoderm_.

If the intestine were closed posteriorly instead of opening by the
vent, the crayfish would virtually be an elongated sac, with one
opening, the mouth, affording an entrance into the alimentary cavity:
and, round this cavity, the three layers just referred to—endoderm,
mesoderm, and ectoderm—would be disposed concentrically.

       *       *       *       *       *

We have seen that the body of the crayfish thus composed is obviously
separable into three regions—the _cephalon_ or head, the _thorax_,
and the _abdomen_. The latter is at once distinguished by the size
and the mobility of its segments: while the thoracic region is marked
off from that of the head, outwardly, only by the cervical groove.
But, when the carapace is removed, the lateral depression already
mentioned, in which the {142} scaphognathite lies, clearly indicates
the natural boundary between the head and the thorax. It has further
been observed that there are, in all, twenty pairs of appendages, the
six hindermost of which are attached to the abdomen. If the other
fourteen pairs are carefully removed, it will be found that the six
anterior belong to the head, and the eight posterior to the thorax.

The abdominal region may now be studied in further detail. Each of
its seven movable segments, except the telson, represents a sort of
morphological unit, the repetition of which makes up the whole fabric
of the body.

[Illustration: FIG. 36.—_Astacus fluviatilis._—A transverse section
through the nineteenth (fifth abdominal) somite (× 2). _e.m._,
extensor muscles; _f.m._, flexor muscles; _gn._ 12, the fifth
abdominal ganglion; _h.g._, hind-gut; _i.a.a._, inferior abdominal
artery; _s.a.a._, superior abdominal artery; _pl. XIX_, pleura of the
somite; _st. XIX_, its sternum; _t. XIX_, its tergum; _ep. XIX_, its
epimera; _19_, its appendages.]

If the abdomen is divided transversely between the {143} fourth and
fifth, and the fifth and sixth segments, the fifth will be isolated,
and can be studied apart. It constitutes what is called a _metamere_;
in which are distinguishable a central part termed the _somite_, and
two _appendages_ (fig. 36).

In the exoskeleton of the somites of the abdomen several regions
have already been distinguished; and although they constitute one
continuous whole, it will be convenient to speak of the _sternum_
(fig. 36, _st. XIX_), the _tergum_ (_t. XIX_), and, the _pleura_
(_pl. XIX_), as if they were separate parts, and to distinguish that
portion of the sternal region, which lies between the articulation of
the appendage and the pleuron, on each side, as the _epimeron_ (_ep.
XIX_). Adopting this nomenclature, it may be said of the fifth somite
of the abdomen, that it consists of a segment of the exoskeleton,
divisible into tergum, pleura, epimera, and sternum, with which two
appendages are articulated; that it contains a double ganglion (_gn._
12), a section of the flexor (_fm_) and extensor (_em_) muscles, and
of the alimentary (_hg_) and vascular (_s.a.a_, _i.a.a_) systems.

[Illustration: FIG. 37.—_Astacus fluviatilis._—Appendages of the
left side of the abdomen (× 3). A, the posterior face of the first
appendage of the male; B, the same of the female; C, posterior,
and C′, anterior faces of the second appendage of the male; D, the
third appendage of the male; E, the same of the female; F, the sixth
appendage. _a_, the rolled plate of the endopodite; _b_, the jointed
extremity of the same; _bp._, basipodite; _cx.p._, coxopodite;
_en.p._, endopodite; _ex.p._, exopodite.]

The appendage (fig. 36, _19_), which is attached to an articular
cavity situated between the sternum and the epimeron, is seen to
consist of a stalk or stem, which is made up of a very short basal
joint, the _coxopodite_ (fig. 37, D and E, _cx.p_), followed by a
long cylindrical second joint, the _basipodite_ (_b.p_), and receives
the name of _protopodite_. At its free end, it bears two flattened
narrow {145} plates, of which one is attached to the inner side of
the extremity of the protopodite, and is called the _endopodite_
(_en.p_), while the other is fixed a little higher up to the outer
side of that extremity, and is the _exopodite_ (_ex.p_). The
exopodite is shorter than the endopodite. The endopodite is broad
and is undivided for about half its length, from the attached end;
the other half is narrower and is divided into a number of small
segments, which, however, are not united by definite articulations,
but are merely marked off from one another by slight constrictions
of the exoskeleton. The exopodite has a similar structure, but its
undivided portion is shorter and narrower. The edges of both the
exopodite and the endopodite are fringed with long setæ.

In the female crayfish, the appendages of this and of the fourth and
third somites are larger than in the male (compare _D_ and _E_, fig.
37).

The fourth and fifth somites, with their appendages, may be described
in the same terms as the third, and in the sixth there is no
difficulty in recognising the corresponding parts of the somite; but
the appendages (fig. 37, _F_), which constitute the lateral portions
of the caudal fin, at first sight appear very different. In their
size, no less than in their appearance, they depart widely from the
appendages of the preceding somites. Nevertheless, each will be found
to consist of a basal stalk, answering to the protopodite (_cx.p_),
which however is very broad and thick, and is not divided into two
{146} joints; and of two terminal oval plates, which represent the
endopodite (_en.p_) and the exopodite (_ex.p_). The latter is divided
by a transverse suture into two pieces; and the edge of the larger or
basal moiety is beset with short spines, of which two, at the outer
end of the series, are larger than the rest.

The second somite is longer than the first (fig. 1); it has very
broad pleura, while those of the first somite are small and hidden by
the overlapping front margins of the pleura of the second somite.

In the female, the appendages of the second somite of the abdomen
are similar to those of the third, fourth, and fifth somites; but in
those of the first somite (fig. 37, _B_), there is a considerable
variation. Sometimes, in fact, the appendages of this somite are
altogether wanting; sometimes one is present, and not the other; and
sometimes both are found. But, when they exist, these appendages are
always small; and the protopodite is followed by only one imperfectly
jointed filament, which appears to represent the endopodite of the
other appendages.

In the male, the appendages of the first and second somites of the
abdomen are not only of relatively large size, but they are widely
different from the rest, those of the first somite departing from
the general type further than those of the second. In the latter
(_C, C′_) there is a protopodite (_cx.p_, _bp_) with the ordinary
structure, and it is followed by an endopodite (_en.p_) and an
exopodite {147} (_ex.p_); but the former is singularly modified.
The undivided basal part is large, and is produced on the inner side
into a lamella (_a_), which extends slightly beyond the end of the
terminal jointed portion (_b_). The inner half of this lamella is
rolled upon itself, in such a manner as to give rise to a hollow
cone, something like an extinguisher (_C′_, _a_).

The appendage of the first somite (_A_) is an unjointed styliform
body, which appears to represent the protopodite, together with
the basal part and the inner prolongation of the endopodite of the
preceding appendage. The terminal half of the appendage is really a
broad plate, slightly bifid at the summit, but the sides of the plate
are rolled in, in such a manner that the anterior half bends round
and partially incloses the posterior half. They thus give rise to a
canal, which is open at each end, and only partially closed behind.

These two pairs of curiously modified appendages are ordinarily
turned forwards and applied against the sterna of the posterior
part of the thorax, in the interval between the bases of the hinder
thoracic limbs (see fig. 3, _A_). They serve as conduits by which the
spermatic matter of the male is conveyed from the openings of the
ducts of the testes to its destination.

If we confine our attention to the third, fourth, and fifth metameres
of the abdomen of the crayfish, it is obvious that the several
somites and their appendages, and the various regions or parts into
which they are {148} divisible, correspond with one another, not
only in form, but in their relations to the general plan of the
whole abdomen. Or, in other words, a diagrammatic plan of one somite
will serve for all the three somites, with insignificant variations
in detail. The assertion that these somites are constructed upon
the same plan, involves no more hypothesis than the statement of an
architect, that three houses are built upon the same plan, though the
façades and the internal decorations may differ more or less.

In the language of morphology, such conformity in the plan of
organisation is termed _homology_. Hence, the several metameres in
question and their appendages, are _homologous_ with one another;
while the regions of the somites, and the parts of their appendages,
are also _homologues_.

When the comparison is extended to the sixth metamere, the homology
of the different parts with those of the other metameres, is
undeniable, notwithstanding the great differences which they present.
To recur to a previous comparison, the ground plan of the building
is the same, though the proportions are varied. So with regard to
the first and second metameres. In the second pair of appendages
of the male, the difference from the ordinary type of appendage is
comparable to that produced by adding a portico or a turret to the
building; while, in the first pair of appendages of the female, it is
as if one wing of the edifice were left unbuilt; {149} and, in those
of the male, as if all the rooms were run into one.

It is further to be remarked, that, just as of a row of houses
built upon the same plan, one may be arranged so as to serve as a
dwelling-house, another as a warehouse, and another as a lecture
hall, so the homologous appendages of the crayfish are made to
subserve various functions. And as the fitness of the dwelling-house,
the warehouse, and the lecture-hall for their several purposes would
not in the least help us to understand why they should all be built
upon the same general plan; so, the adaptation of the appendages
of the abdomen of the crayfish to the discharge of their several
functions does not explain why those parts are homologous. On the
contrary, it would seem simpler that each part should have been
constructed in such a manner as to perform its allotted function
in the best possible manner, without reference to the rest. The
proceedings of an architect, who insisted on constructing every
building in a town on the plan of a Gothic cathedral, would not be
explicable by considerations of fitness or convenience.

       *       *       *       *       *

In the cephalothorax, the division into somites is not at first
obvious, for, as we have seen, the dorsal or tergal surface is
covered over by a continuous shield, distinguished into thoracic and
cephalic regions only by the cervical groove. Even here, however,
when a transverse section of the thorax is compared with that of the
{150} abdomen (figs. 15 and 36), it will be obvious that the tergal
and the sternal regions of the two answer to one another; while the
branchiostegites correspond with greatly developed pleura; and the
inner wall of the branchial chamber, which extends from the bases of
the appendages to the attachment of the branchiostegite, represents
an immensely enlarged epimeral region.

On examination of the sternal aspect of the cephalothorax the signs
of division into somites become plain (figs. 3 and 39, _A_). Between
the last two ambulatory limbs there is an easily recognisable sternum
(_XIV._), though it is considerably narrower than any of the sterna
of the abdominal somites, and differs from them in shape.

The deep transverse fold which separates this hindermost thoracic
sternum from the rest of the sternal wall of the cephalothorax, is
continued upwards on the inner or epimeral wall of the branchial
cavity; and thus the sternal and the epimeral portions of the
posterior thoracic somite are naturally marked off from those of the
more anterior somites.

[Illustration: FIG. 38.—_Astacus fluviatilis._—The mode of connexion
between the last thoracic and the first abdominal somites (× 3). _a_,
L-shaped bar; _cpe_, carapace; _cxp. 14_, coxopodite of the last
ambulatory leg; _plb._, place of attachment of the pleurobranchia;
_st. XV_, sternum, and _t. XV_, tergum of the first abdominal somite.]

The epimeral region of this somite presents a very curious structure
(fig. 38). Immediately above the articular cavities for the
appendages there is a shield-shaped plate, the posterior, convex edge
of which is sharp, prominent, and setose. Close to its upper boundary
the plate exhibits a round perforation (_plb._), to the margins of
which the stem of the hindermost {151} pleurobranchia (fig. 4, _plb.
14_) is attached; and in front of this, it is connected, by a narrow
neck, with an elongated triangular piece, which takes a vertical
direction, and lies in the fold which separates the posterior
thoracic somite from the next in front. The base of this piece unites
with the epimeron of the penultimate somite. Its apex is connected
with the anterior end of the horizontal arm of an L-shaped calcified
bar (fig. 38, _a_), the upper end of the vertical arm of which is
firmly, but moveably, connected with the anterior and lateral edge of
the tergum of the first abdominal somite (_t. XV._). The tendon of
one {152} of the large extensor muscles of the abdomen is attached
close to it.

The sternum and the shield-shaped epimeral plates constitute a
solid, continuously calcified, ventral element of the skeleton, to
which the posterior pair of legs is attached; and as this structure
is united with the somites in front of and behind it only by soft
cuticle, except where the shield-shaped plate is connected, by the
intermediation of the triangular piece, with the epimeron which lies
in front of it, it is freely movable backwards and forwards on the
imperfect hinge thus constituted.

In the same way, the first somite of the abdomen, and, consequently,
the abdomen as a whole, moves upon the hinges formed by the union of
the L-shaped pieces with the triangular pieces.

In the rest of the thorax, the sternal and the epimeral regions of
the several somites are all firmly united together. Nevertheless,
shallow grooves answering to folds of the cuticle, which run from the
intervals between the articular cavities for the limbs towards the
tergal end of the inner wall of the branchial chamber, mark off the
epimeral portions of as many somites as there are sterna, from one
another.

[Illustration: FIG. 39.—_Astacus fluviatilis._—The cephalothoracic
sterna and the endophragmal system (× 2). _A_, from beneath; _B_,
from above. _a_, _a′_, arthrophragms or partitions between the
articular cavities for the limbs; _c.ap_, cephalic apodeme; _cf_,
cervical fold; _epn. 1_, epimeron of the antennulary somite; _h_,
anterior, and _h′_, posterior horizontal process of endopleurite;
_lb_, labrum; _m_, mesophragm; _mt_, metastoma; _p_, paraphragm;
_I–XIV_, cephalothoracic sterna; _1–14_, articular cavities of the
cephalothoracic appendages. (The anterior cephalic sterna are bent
downwards in A so as to bring them into the same plane with the
remaining cephalothoracic sterna; in B these sterna are not shown.)]

A short distance above the articular cavities a transverse groove
separates a nearly square area of the lower part of the epimeron from
the rest. Towards the anterior and upper angle of this area, in the
two somites {153} which lie immediately in front of the hindermost,
there is a small round aperture for the attachment of the {154}
rudimentary branchia. These areæ of the epimera, in fact, correspond
with the shield-shaped plate of the hindermost somite. In the next
most anterior somite (that which bears the first pair of ambulatory
legs) there is only a small elevation in the place of the rudimentary
branchia; and in the anterior four thoracic somites nothing of the
kind is visible.

On the sternal aspect of the thorax (figs. 3 and 39, A) a triangular
space is interposed between the basal joints or coxopodites of the
penultimate and the ante-penultimate pairs of ambulatory legs, while
the coxopodites of the more anterior limbs are closely approximated.
The triangular area in question is occupied by two sterna (fig. 39,
A, _XII_, _XIII_), the lateral margins of which are raised into
flange-like ridges. The next two sterna (_X_, _XI_) are longer,
especially that which lies between the forceps (_X_), but they
are very narrow; while the lateral processes are reduced to mere
tubercles at the posterior ends of the sterna. Between the three
pairs of maxillipedes, the sterna (_VII_, _VIII_, _IX_) are yet
narrower, and become gradually shorter; but traces of the tubercles
at their posterior ends are still discernible. The most anterior of
these sternal rods passes into a transversely elongated plate, shaped
like a broad arrow (_V_, _VI_), which is constituted by the conjoined
sterna of the two posterior somites of the head.

Anteriorly to this, and between it and the posterior end of the
elongated oral aperture, the sternal region is {155} occupied only
by soft or imperfectly calcified cuticle, which, on each side of
the hinder part of the mouth, passes into one of the lobes of the
metastoma (_mt_). At the base of each of these lobes there is a
calcified plate, united by an oblique suture with another, which
occupies the whole length of the lobe and gives it firmness. The
soft narrow lip which constitutes the lateral boundary of the oral
aperture, and lies between it and the mandible, passes, in front,
into the posterior face of the labrum (_lb_).

In front of the mouth, the sternal region which appertains, in part,
to the antennæ, and, in part, to the mandibles, is obvious as a
broad plate (_III_), termed the _epistoma_. The middle third of the
posterior edge of the epistoma gives rise to a thickened transverse
ridge, with rounded ends, slightly excavated behind, and is then
continued into the labrum (_lb_), which is strengthened by three
pairs of calcifications, arranged in a longitudinal series. The
sides of the front edge of the epistoma are excavated, and bound the
articular cavities for the basal joints of the antennæ (_3_); but, in
the middle line, the epistoma is continued forwards into a spear-head
shaped process (figs. 39 and 40, _II_), to which the posterior end
of the antennulary sternum contributes. The antennulary sternum is
very narrow, and its anterior or upper end runs into a small but
distinct conical median spine (fig. 40, _t._). Upon this follows
an uncalcified plate, bent into the form of a half cylinder (_I_),
which lies between the inner ends of {156} the eye-stalks and is
united with adjacent parts only by flexible cuticle, so that it is
freely movable. This represents the whole of the sternal region, and
probably more, of the ophthalmic somite.

[Illustration: FIG. 40.—_Astacus fluviatilis._—The ophthalmic and
antennulary somites (× 3). _I_, ophthalmic, and _II_, antennulary
sternum; _1_, articular surface for eyestalk; _2_, for antennule;
_epm_, epimeral plate; _pcp_, procephalic process; _r_, base of
rostrum; _t_, tubercle.]

The sterna of fourteen somites are thus identifiable in the
cephalothorax. The corresponding epimera are represented, in the
thorax, by the thin inner walls of the branchial chamber; the pleura,
by the branchiostegites; and the terga, by so much of the median
region of the carapace as lies behind the cervical groove. That part
of the carapace which is situated in front of this groove occupies
the place of the terga of the head; while the low ridge, skirting
the oral and præ-oral region, in which it terminates laterally,
represents the pleura of the cephalic somites.

The epimera of the head are, for the most part, very narrow; but
those of the antennulary somite are broad plates (fig. 40, _epm._),
which constitute the posterior {157} wall of the orbits. I am
inclined to think that a transverse ridge, which unites these under
the base of the rostrum, represents the tergum of the antennulary
somite, and that the rostrum itself belongs to the next or antennary
somite.[11]

     [11] There are some singular marine crustacea, the _Squillidæ_,
     in which both the ophthalmic and the antennary somites are free
     and movable, while the rostrum is articulated with the tergum of
     the antennary somite.

The sharp convex ventral edge of the rostrum (fig. 41) is produced
into a single, or sometimes two divergent spines, which descend,
in front of the ophthalmic somite, towards the conical tubercle
mentioned above: it thus gives rise to an imperfect partition between
the orbits.

[Illustration: FIG. 41.—_Astacus fluviatilis._—The rostrum, seen from
the left side.]

The internal face of the sternal wall of the whole of the thorax and
of the post-oral part of the head, presents a complicated arrangement
of hard parts, which is known as the _endophragmal system_ (figs.
39, _B_, 42, and 43), and which performs the office of an internal
skeleton by affording attachment to muscles, and serving to protect
important viscera, while at the same time it ties the somites
together, and unites them into a solid whole. In reality, however,
the curious pillars and bulkheads which enter into the composition
of the endophragmal system are all {158} mere infoldings of the
cuticle, or _apodemes_; and, as such, they are shed along with the
other cuticular structures during the process of ecdysis.

Without entering into unnecessary details, the general principle
of the construction of the endophragmal skeleton may be stated as
follows. Four apodemes are developed between every two somites,
and as every apodeme is a fold of the cuticle, it follows that
the anterior wall of each belongs to the somite in front, and
the posterior wall to the somite behind. All four apodemes lie
in the ventral half of the somite and form a single transverse
series; consequently there are two nearer the middle line, which
are termed the _endosternites_, and two further off, which are the
_endopleurites_. The former lie at the inner, and the latter at the
outer ends of the partitions or _arthrophragms_ (fig. 39, A, _a, a′_,
fig. 42, _aph_), between the articular cavities for the basal joints
of the limbs, and they spring partly from the latter and partly from
the sternum and the epimera respectively.

The endosternite (fig. 42, _ens._) ascends vertically, with a slight
inclination forwards, and its summit narrows and assumes the form
of a pillar, with a flat, transversely elongated capital. The inner
prolongation of the capital is called the _mesophragm_ (_mph._), the
outer _paraphragm_ (_pph._). The mesophragms of the two endosternites
of a somite usually unite by a median suture, and thus form a
complete arch over the sternal canal (_s.c._), which lies between the
endosternites. {159}

The endopleurites (_en.pl._) are also vertical plates, but they
are relatively shorter, and their inner angles give off two nearly
horizontal processes, one of which passes obliquely forwards (fig.
39, B, _h_, fig. 42, _h.p._) and unites with the paraphragm of
the endosternite of the somite in front, while the other, passing
obliquely backwards (fig. 39, _h′_), becomes similarly connected with
the endosternite of the somite behind.

[Illustration: FIG. 42.—_Astacus fluviatilis._—A segment of the
endophragmal system (× 3). _aph_, arthrophragm; _arth_, arthrodial or
articular cavity; _cxp_, coxopodite of the ambulatory leg; _enpl_,
endopleurite; _ens_, endosternite; _epm_, epimeron; _hp_, horizontal
process of endopleurite; _mph_, mesophragm; _pph_, paraphragm; _s_,
sternum of somite; _sc_, sternal canal.]

The endopleurites of the last thoracic somite are rudimentary, and
its endosternites are small. On the other hand, the mesophragmal
processes of the endosternites of the two posterior somites of
the head (fig. 39, B, _c.ap_), by which the endophragmal system
terminates in front, are particularly strong and closely united
together. They thus, with their endopleurites, form a solid partition
between the stomach, which lies upon them, and the mass of {160}
coalesced anterior thoracic and posterior cephalic ganglia situated
beneath them. Strong processes are given off from their anterior and
outer angles, which curve round the tendons of the adductor muscles
of the mandibles, and give attachment to the abductors.

In front of the mouth there is no such endophragmal system as that
which lies behind it. But the anterior gastric muscles are attached
to two flat calcified plates, which appear to lie in the interior
of the head (though they are really situated in its upper and front
wall) on each side of the base of the rostrum, and are called the
_procephalic processes_ (figs. 40, 43, _p.cp_). Each of these plates
constitutes the posterior wall of a narrow cavity which opens
externally into the roof of the orbit, and has been regarded (though,
as it appears to me, without sufficient reason) as an olfactory
organ. I am disposed to think, though I have not been able to obtain
complete evidence of the fact, that the procephalic processes are
the representatives of the “procephalic lobes” which terminate the
anterior end of the body in the embryo crayfish. At any rate, they
occupy the same position relatively to the eyes and to the carapace;
and the hidden position of these processes, in the adult, appears to
arise from the extension of the carapace at the base of the rostrum
over the fore part of the originally free sternal surface of the
head. It has thus covered over the procephalic processes, in which
the sternal wall of the body terminated; and the cavities which lie
in front of them are {161} simply the interspaces left between
the inferior or posterior wall of the prolongation of the carapace
and the originally exposed external faces of these regions of the
cephalic integument.

       *       *       *       *       *

Fourteen somites having thus been distinguished in the cephalothorax,
and six being obvious in the abdomen, it is clear that there is a
somite for every pair of appendages. And, if we suppose the carapace
divided into segments answering to these sterna, the whole body will
be made up of twenty somites, each having a pair of appendages.
As the carapace, however, is not actually divided into terga in
correspondence with the sterna which it covers, all we can safely
conclude from the anatomical facts is that it represents the tergal
region of the somites, not that it is formed by the coalescence of
primarily distinct terga. In the head, and in the greater part of
the thorax, the somites are, as it were, run together, but the last
thoracic somite is partly free and to a slight extent moveable,
while the abdominal somites are all free, and moveably articulated
together. At the anterior end of the body, and, apparently, from the
antennary somite, the tergal region gives rise to the rostrum, which
projects between and beyond the eyes. At the opposite extremity,
the telson is a corresponding median outgrowth of the last somite,
which has become moveably articulated therewith. The narrowing of the
sternal moieties of the anterior thoracic somites, {162} together
with the sudden widening of the same parts in the posterior cephalic
somites, gives rise to the lateral depression (fig. 39, _cf_) in
which the scaphognathite lies. The limit thus indicated corresponds
with that marked by the cervical groove upon the surface of the
carapace, and separates the head from the thorax. The three pair
of maxillipedes (_7, 8, 9_), the forceps (_10_), the ambulatory
{163} limbs (_11–14_), and the eight somites of which they are the
appendages (_VII–XIV_), lie behind this boundary and belong to the
thorax. The two pairs of maxillæ (_5, 6_) the mandibles (_4_), the
antennæ (_3_), the antennules (_2_), the eyestalks (_1_), and the
six somites to which they are attached (_I–VI_), lie in front of the
boundary and compose the head.

[Illustration: FIG. 43.—_Astacus fluviatilis._—Longitudinal section
of the anterior part of the cephalothorax (× 3). _I–IX_, sterna of
first nine cephalothoracic somites; _1_, eyestalk; _2_, basal joint
of antennule; _3_, basal joint of antenna; _4_, mandible; _a_, inner
division of the masticatory surface of the mandible; _a′_, apophysis
of the mandible for muscular attachment; _cp_, free edge of carapace;
_e_, endosternite; _enpl_, endopleurite; _epm_, epimeral plate; _l_,
labrum; _m_, muscular fibres connecting epimera with interior of
carapace; _mt_, metastoma; _pcp_, procephalic process.]

Another important point to be noticed is that, in front of the mouth,
the sternum of the antennary somite (fig. 43, _III_) is inclined at
an angle of 60° or 70° to the direction of the sterna behind the
mouth. The sternum of the antennulary somite (_II_) is at right
angles to the latter; and that of the eyes (_I_) looks upwards as
well as forwards. Hence, the front of the head beneath the rostrum,
though it looks forwards, or even upwards, is homologous with the
sternal aspect of the other somites. It is for this reason that the
feelers and the eyestalks take a direction so different from that of
the other appendages. The change of aspect of the sternal surface in
front of the mouth, thus effected, is what is termed the _cephalic
flexure_.

       *       *       *       *       *

Since the skeleton which invests the trunk of the crayfish is made
up of a twenty-fold repetition of somites, homologous with those of
the abdomen, we may expect to find that the appendages of the thorax
and of the head, however unlike they may seem to be to those of the
abdomen, are nevertheless reducible to the same fundamental plan.
{164}

The third maxillipede is one of the most complete of these
appendages, and may be advantageously made the starting point of the
study of the whole series.

[Illustration: FIG. 44.—_Astacus fluviatilis._—The third or external
maxillipede of the left side (× 3). _e_, lamina, and _br_, branchial
filaments of the podobranchia; _cxp_, coxopodite; _cxs_, coxopoditic
setæ; _bp_, basipodite; _ex_, exopodite; _ip_, ischiopodite; _mp_,
meropodite; _cp_, carpopodite; _pp_, propodite; _dp_, dactylopodite.]

Neglecting details for the moment, it may be said that the appendage
consists of a basal portion (fig. 44, _cxp, bp_), {165} with two
terminal divisions (_ip_ to _dp_, and _ex_), which are directed
forwards, below the mouth, and a third, lateral appendage (_e, br_),
which runs up, beneath the carapace, into the branchial chamber.
The latter is the gill, or podobranchia, attached to this limb,
and it is something not represented in the abdominal limbs. But,
with regard to the rest of the maxillipede, it is obvious that the
basal portion (_cxp, bp_) represents the protopodite, and the two
terminal divisions the endopodite and the exopodite respectively. It
has been observed that, in the abdominal appendages, the extent to
which segmentation occurs in homologous parts varies indefinitely;
an endopodite, for example, may be a continuous plate, or may be
subdivided into many joints. In the maxillipede, the basal portion
is divided into two joints; and, as in the abdominal limb, the
first, or that which articulates with the thorax, is termed the
_coxopodite_ (_cxp_), while the second is the _basipodite_ (_bp_).
The stout, leg-like endopodite appears to be the direct continuation
of the basipodite; while the much more narrow and slender exopodite
articulates with its outer side. The exopodite (_ex_) is by no means
unlike one of the exopodites of the abdominal limbs, consisting as it
does of an undivided base and a many-jointed terminal filament. The
endopodite, on the contrary, is strong and massive, and is divided
into five joints, named, from that nearest to the base onwards,
_ischiopodite_ (_ip_), _meropodite_ (_mp_), _carpopodite_ (_cp_),
_propodite_ (_pp_), and _dactylopodite_ (_dp_). {166}

[Illustration: FIG. 45.—_Astacus fluviatilis._—A, the first; B, the
second maxillipede of the left side (× 3). _cxp_, coxopodite; _bp_,
basipodite; _e, br_, podobranchia; _ep_, epipodite; _en_, endopodite;
_ex_, exopodite; _ip_, ischiopodite; _mp_, meropodite; _cp_,
carpopodite; _pp_, propodite; _dp_, dactylopodite.]

The second maxillipede (fig. 45, B) has essentially the same
composition as the first, but the exopodite (_ex_) is relatively
larger, the endopodite (_ip–dp_) smaller and softer; and, while the
ischiopodite (_ip_) is the longest joint in the third maxillipede, it
is the meropodite (_mp_) which is longest in the second. In the first
maxillipede (fig. 45, A) a great modification has taken place. The
coxopodite (_cxp_) and the basipodite (_bp_) are broad thin plates
with setose cutting edges, while the endopodite (_en_) is short and
only two-jointed, and the undivided portion of the exopodite (_ex_)
is very long. The place of {167} the podobranchia is taken by a
broad soft membranous plate entirely devoid of branchial filaments
(_ep_). Thus, in the series of the thoracic limbs, on passing
forwards from the third maxillipede, we find that though the plan
of the appendages remains the same; (1) the protopodite increases
in relative size; (2) the endopodite diminishes; (3) the exopodite
increases; (4) the podobranchia finally takes the form of a broad
membranous plate and loses its branchial filaments.

Writers on descriptive Zoology usually refer to the parts of the
maxillipedes under different names from those which are employed
here. The protopodite and the endopodite taken together are commonly
called the _stem_ of the maxillipede, while the exopodite is the
_palp_, and the metamorphosed podobranchia, the real nature of which
is not recognised, is termed the _flagellum_.

When the comparison of the maxillipedes with the abdominal members,
however, had shown the fundamental uniformity of composition of the
two, it became desirable to invent a nomenclature of the homologous
parts which should be capable of a general application. The names
of protopodite, endopodite, exopodite, which I have adopted as
the equivalents of the “stem” and the “palp,” were proposed by
Milne-Edwards, who at the same time suggested _epipodite_ for the
“flagellum.” And the lamellar process of the first maxillipede is now
very generally termed an epipodite; while the podobranchiæ, which
have exactly the same relations to the following {168} limbs, are
spoken of as if they were totally different structures, under the
name of branchiæ or gills.

The flagellum or epipodite of the first maxillipede, however, is
nothing but the slightly modified stem of a podobranchia, which
has lost its branchial filaments; but the term “epipodite” may be
conveniently used for podobranchiæ thus modified. Unfortunately, the
same term is applied to certain lamelliform portions of the branchiæ
of other crustacea, which answer to the laminæ of the crayfishes’
branchiæ; and this ambiguity must be borne in mind, though it is of
no great moment.

On examining an appendage from that part of the thorax which lies
behind the third maxillipede, say, for example, the sixth thoracic
limb (the second walking leg) (fig. 46), the two joints of the
protopodite and the five joints of the endopodite are at once
identifiable, and so is the podobranchia; but the exopodite has
vanished altogether. In the eighth, or last, thoracic limb, the
podobranchia has also disappeared. The fifth and sixth limbs also
differ from the seventh and eighth, in being chelate; that is to
say, one angle of the distal end of the propodite is prolonged and
forms the fixed leg of the pincer. The produced angle is that which
is turned downwards when the limb is fully extended (fig. 46). In
the forceps, the great chela is formed in just the same way; the
only important difference lies in the fact that, as in the external
maxillipede, the basipodite and the ischiopodite are immoveably
united. Thus, {170} the limbs of the thorax are all reducible to
the same type as those of the abdomen, if we suppose that, in the
posterior five pair, the exopodites are suppressed; and that, in all
but the last, podobranchiæ are superadded.

[Illustration: FIG. 46.—_Astacus fluviatilis._—The second ambulatory
leg of the left side (× 3). _cxp_, coxopodite; _bp_, basipodite;
_br_, gill; _cxs_, coxopoditic setæ; _e_, lamina of gill or
epipodite; _ip_, ischiopodite; _mp_, meropodite; _cp_, carpopodite;
_pp_, propodite; _dp_, dactylopodite.]

Turning to the appendages of the head, the second maxilla (fig.
47, C) presents a further modification of the disposition of the
parts seen in the first maxillipede. The coxopodite (_cxp_) and
the basipodite (_bp_) are still thinner and more lamellar, and are
subdivided by deep fissures which extend from their inner edges.
The endopodite (_en_) is very small and undivided. In the place of
the exopodite and the epipodite there is only one great plate, the
scaphognathite (_sg_) which either is such an epipodite as that of
the first maxillipede with its anterior basal process much enlarged,
or represents both the exopodite and the epipodite. In the first
maxilla (B), the exopodite and the epipodite have disappeared,
and the endopodite (_en_) is insignificant and unjointed. In the
mandibles (A), the representative of the protopodite is strong
and transversely elongated. Its broad inner or oral end presents
a semicircular masticatory surface divided by a deep longitudinal
groove into two toothed ridges. The one of these follows the convex
anterior or inferior contour of the masticatory surface, projects
far beyond the other, and is provided with a sharp serrated edge;
the other (fig. 43, _a_) gives rise to the straight posterior or
superior contour of the masticatory surface, and is more obtusely
tuberculated. In front, the inner {171} ridge is continued into a
process by which the mandible articulates with the epistoma (fig. 47,
A, _ar_). The endopodite is represented by the three-jointed _palp_
(_p_), the terminal joint of which is oval and beset with numerous
strong setæ, which are especially abundant along its anterior edge.

[Illustration: FIG. 47.—_Astacus fluviatilis._—A, mandible; B, first
maxilla; C, second maxilla of the left side (× 3). _ar_, internal,
and _ar′_, external articular process of the mandible; _bp_,
basipodite; _cxp_, coxopodite; _en_, endopodite; _p_, palp of the
mandible; _sg_, scaphognathite; _x_, internal process of the first
maxilla.]

{172}

In the antenna (fig. 48, C) the protopodite is two-jointed. The
basal segment is small, and its ventral face presents the conical
prominence on the posterior aspect of which is the aperture of the
duct of the renal gland (_gg_). The terminal segment is larger and
is subdivided by deep longitudinal folds, one upon the dorsal and
one upon the ventral face, into two moieties which are more or less
moveable upon one another. In front and externally it bears the broad
flat _squame_ (_exp_) of the antenna, as an exopodite. Internally,
the long annulated “feeler” which represents the endopodite, is
connected with it by two stout basal segments.

[Illustration: FIG. 48.—_Astacus fluviatilis._—A, eye-stalk; B,
antennule; C, antenna of the left side (× 3). _a_, spine of the basal
joint of the antennule; _c_, corneal surface of the eye; _exp_,
exopodite or squame of the antenna; _gg_, aperture of the duct of the
green gland.]

{173}

The antennule (fig. 48, B) has a three-jointed stem and two terminal
annulated filaments, the outer of which is thicker and longer than
the inner, and lies rather above as well as external to the latter.
The peculiar form of the basal segment of the stem of the antennule
has already been adverted to (p. 116). It is longer than the other
two segments put together, and near the anterior end its sternal
edge is produced into a single strong spine (_a_). The stem of the
antennule answers to the protopodite of the other limbs, though its
division into three joints is unusual; the two terminal annulated
filaments represent the endopodite and the exopodite.

Finally, the eyestalk (A) has just the same structure as the
protopodite of an abdominal limb, having a short basal and a long
cylindrical terminal joint.

       *       *       *       *       *

From this brief statement of the characters of the appendages, it
is clear that, in whatever sense it is allowable to say that the
appendages of the abdomen are constructed upon one plan, which is
modified in execution by the excess of development of one part over
another, or by the suppression of parts, or by the coalescence of one
part with another, it is allowable to say that all the appendages are
constructed on the same plan, and are modified on similar principles.
Given a general type of appendage consisting of a protopodite,
bearing a podobranchia, an endopodite and an exopodite, all the
actual appendages are readily derivable from that type. {174}

In addition, therefore, to their adaptation to the purposes which
they subserve, the parts of the skeleton of the crayfish show a
unity in diversity, such as, if the animal were a piece of human
workmanship, would lead us to suppose that the artificer was under
an obligation not merely to make a machine capable of doing certain
kinds of work, but to subordinate the nature and arrangement of the
mechanism to certain fixed architectural conditions.

The lesson thus taught by the skeletal organs is reiterated and
enforced by the study of the nervous and the muscular systems.
As the skeleton of the whole body is capable of resolution into
the skeletons of twenty separate metameres, variously modified
and combined; so is the entire ganglionic chain resolvable into
twenty pairs of ganglia various in size, distant in this region
and approximated in that; and so is the muscular system of the
trunk conceivable as the sum of twenty _myotomes_ or segments of
the muscular system appropriate to a metamere, variously modified
according to the degree of mobility of the different regions of the
organism.

       *       *       *       *       *

The building up of the body by the repetition and the modification
of a few similar parts, which is so obvious from the study of the
general form of the somites and of their appendages, is still more
remarkably illustrated, if we pursue our investigations further, and
trace {175} out the more intimate structure of these parts. The
tough, outer coat, which has been termed the _cuticula_, except so
far as it presents different degrees of hardness, from the presence
or absence of calcareous salts, is obviously everywhere of the same
nature; and, by macerating a crayfish in caustic alkali, which
destroys all its other components of the body, it will be readily
enough seen that a continuation of the cuticular layer passes in at
the mouth and the vent, and lines the alimentary canal; furthermore,
that processes of the cuticle covering various parts of the trunk
and limbs extend inwards, and afford surfaces of attachment to the
muscles, as the _apodemata_ and _tendons_. In technical language, the
cuticular substance which thus enters so largely into the composition
of the bodily fabric of the crayfish is called a _tissue_.

The flesh, or _muscle_, is another kind of tissue, which is readily
enough distinguished from cuticular tissue by the naked eye; but, for
a complete discrimination of all the different tissues, recourse must
be had to the microscope, the application of which to the study of
the ultimate optical characters of the morphological constituents of
the body has given rise to that branch of morphology which is known
as _Histology_.

[Illustration: FIG. 49.—_Astacus fluviatilis._—The corpuscles of
the blood, highly magnified. _1–8_, show the changes undergone by a
single corpuscle during a quarter of an hour; _n_, the nucleus; _9_
and _10_ are corpuscles killed by magenta, and having the nucleus
deeply stained by the colouring matter.]

If we count every formed element of the body, which is separable from
the rest by definite characters, as a tissue, there are no more than
eight kinds of such tissues in the crayfish; that is to say, every
solid constituent {176} of the body consists of one or more of the
following eight histological groups:—

1. Blood corpuscles; 2. Epithelium; 3. Connective tissue; 4. Muscle;
5. Nerve; 6. Ova; 7. Spermatozoa; 8. Cuticle.

1. A drop of freshly-drawn blood of the crayfish contains multitudes
of small particles, the _blood corpuscles_, which rarely exceed
1‐700th, and usually are about 1‐1000th, of an inch in diameter (fig.
49). They are sometimes pale and delicate, but generally more or
less dark, from containing a number of minute strongly refracting
granules, and they are ordinarily exceedingly irregular in form. If
one of them is watched {177} continuously for two or three minutes,
its shape will be seen to undergo the constant but slow changes to
which passing reference has already been made (p. 69). One or other
of the irregular prolongations will be drawn in, and another thrown
out elsewhere. The corpuscle, in fact, has an inherent contractility,
like one of those low organisms, known as an _Amœba_, whence its
motions are frequently called _amœbiform_. In its interior, an
ill-marked oval contour may be seen, indicating the presence of a
spheroidal body, about 1‐2000th of an inch in diameter, which is the
nucleus of the corpuscle (_n_). The addition of some re-agents, such
as dilute acetic acid, causes the corpuscles at once to assume a
spherical shape, and renders the nucleus very conspicuous (fig. 49,
_9_ and _10_). The blood corpuscle is, in fact, a simple nucleated
cell, composed of a contractile protoplasmic mass, investing a
nucleus; it is suspended freely in the blood; and, though as much
a part of the crayfish organism as any other of its histological
elements, leads a quasi-independent existence in that fluid.

2. Under the general name of _epithelium_, may be included a form
of tissue, which everywhere underlies the exoskeleton (where it
corresponds with the epidermis of the higher animals), and the
cuticular lining of the alimentary canal, extending thence into the
hepatic cæca. It is further met with in the generative organs, and
in the green gland. Where it forms the subcuticular layer of the
integument and of the alimentary canal, it is found to {178} consist
of a protoplasmic substance (fig. 50), in which close set nuclei
(_n_) are imbedded. If a number of blood corpuscles could be supposed
to be closely aggregated together into a continuous sheet, they would
give rise to such a structure as this; and there can be no doubt
that it really is an aggregate of nucleated cells, though the limits
between the individual cells are rarely visible in the fresh state.
In the liver, however, the cells grow, and become detached from one
another in the wider and lower parts of the cæca, and their essential
nature is thus obvious.

[Illustration: FIG. 50.—_Astacus fluviatilis._—Epithelium, from the
epidermic layer subjacent to the cuticle, highly magnified. _A_, in
vertical section; _B_, from the surface. _n_, nuclei.]

3. Immediately beneath the epithelial layer follows a tissue,
disposed in bands or sheets, which extend to the subjacent parts,
invest them, and connect one with another. Hence this is called
_connective tissue_.

[Illustration: FIG. 51.—_Astacus fluviatilis._—Connective tissue;
_A_, second form; _B_, third form. _a_, cavities; _n_, nuclei. Highly
magnified.]

The connective tissue presents itself under three forms. In the
first there is a transparent homogeneous-looking matrix, or ground
substance, through which are scattered many nuclei. In fact,
this form of connective tissue {179} very closely resembles the
epithelial tissue, except that the intervals between the nuclei are
wider, and that the substance in which they are imbedded cannot be
broken up into a separate cell-body for each nucleus. In the second
form (fig. 51, _A_) the matrix exhibits fine wavy parallel lines, as
if it were marked out into imperfect fibres. In this form, as in the
next to be described, more or less spherical cavities, which contain
a clear fluid, are excavated in the matrix; and the number of {180}
these is sometimes so great, that the matrix is proportionally
very much reduced, and the structure acquires a close superficial
similarity to that of the parenchyma of plants. This is still more
the case with a third form, in which the matrix itself is marked off
into elongated or rounded masses, each of which has a nucleus in its
interior (fig. 51, _B_). Under one form or another, the connective
tissue extends throughout the body, ensheathing the various organs,
and forming the walls of the blood sinuses.

The third form is particularly abundant in the outer investment
of the heart, the arteries, the alimentary canal, and the nervous
centres. About the cerebral and anterior thoracic ganglia, and on
the exterior of the heart, it usually contains more or less fatty
matter. In these regions, many of the nuclei, in fact, are hidden by
the accumulation round them of granules of various sizes, some of
which are composed of fat, while others consist of a proteinaceous
material. These aggregates of granules are usually spheroidal; and,
with the matrix in which they are imbedded and the nucleus which
they surround, they are often readily detached when a portion of the
connective tissue is teased out, and are then known as _fat cells_.
From what has been said respecting the distribution of the connective
tissue, it is obvious that if all the other tissues could be removed,
this tissue would form a continuous whole, and represent a sort of
model, or cast, of the whole body of the crayfish. {181}

[Illustration: FIG. 52.—_Astacus fluviatilis._—A, a single muscular
fibre, transverse diameter 1‐110th of an inch; B, a portion of
the same more highly magnified; C, a smaller portion treated with
alcohol and acetic acid still more highly magnified; D and E, the
splitting up of a part of a fibre, treated with picro-carmine, into
fibrillæ; F, the connection of a nervous with a muscular fibre which
has been treated with alcohol and acetic acid. _a_, darker, and _b_,
clearer portions of the fibrillæ; _n_, nuclei; _nv_, nerve fibre;
_s_, sarcolemma; _t_, tendon; _1–5_, successive dark granular striæ
answering to the granular portions, _a_, of each fibrilla.]

4. The _muscular tissue_ of the crayfish always has the form of
bands or fibres, of very various thickness, marked, when viewed by
transmitted light, by alternate darker and {182} lighter striæ,
transversely to the axis of the fibres (fig. 52 A). The distance of
the transverse striæ from one another varies with the condition of
the muscle, from 1‐4,000th of an inch in the quiescent state to as
little as 1‐30,000th of an inch in that of extreme contraction. The
more delicate muscular fibres, like those of the heart and those of
the intestine, are imbedded in the connective tissue of the organ,
but have no special sheaths. The fibres which make up the more
conspicuous muscles of the trunk and limbs, on the other hand, are
much larger, and are invested by a thin, transparent, structureless
sheath, which is termed the _sarcolemma_. Nuclei are scattered, at
intervals, through the striated substance of the muscle; and, in the
larger muscular fibres, a layer of nucleated protoplasm lies between
the sarcolemma and the striated muscle substance.

[Illustration: FIG. 53.—_Astacus fluviatilis._—A, living muscular
fibres very highly magnified; _B_, a fibrilla treated with solution
of sodium chloride; _C_, a fibrilla treated with strong nitric acid,
_s_, septal lines; _sz_, septal zones; _is_, interseptal zones; _a_,
transverse line in the interseptal zone.]

{183}

This much is readily seen in a specimen of muscular fibre taken from
any part of the body, and whether alive or dead. But the results
of the ultimate optical analysis of these appearances, and the
conclusions respecting the normal structure of striped muscle which
may be legitimately drawn from them, have been the subjects of much
controversy.

Quiescent muscular fibres from the chela of the forceps of a
crayfish, examined while still living, without the addition of any
extraneous fluid, and with magnifying powers of not less than seven
or eight hundred diameters, exhibit the following appearance. At
intervals of about 1‐4000th of an inch, very delicate but dark and
well-defined transverse lines are visible; and these, on careful
focussing, appear beaded, as if they were made of a series of
close-set minute granules not more than 1‐20,000th to 1‐30,000th of
an inch in diameter. These may be termed the _septal lines_ (fig. 52,
D and E, _a_; C, _1–5_; fig. 53, _s_). On each side of every septal
line there is a very narrow perfectly transparent band, which may be
distinguished as the _septal zone_ (fig. 53, _sz_). Upon this follows
a relatively broad band of a substance which has a semi-transparent
aspect, like very finely ground glass, and hence appears somewhat
dark relatively to the septal zone. Upon this _inter-septal zone_
(_i s_) follows another septal zone, then a septal line, another
septal zone, an inter-septal zone, and so on throughout the whole
length of the fibre. {184}

In the perfectly unaltered state of the muscle no other transverse
markings than these are discernible. But it is always possible to
observe certain longitudinal markings; and these are of three kinds.
In the first place, the nuclei which, in the perfectly fresh muscle,
are delicate transparent oval bodies, are lodged in spaces which
taper off at each end into narrow longitudinal clefts (fig. 52, A,
B). Prolongations of the protoplasmic sheath of the fibre extend
inwards and fill these clefts. Secondly, there are similar clefts
interposed between these, but narrow and merely linear throughout.
Sometimes these clefts contain fine granules. Thirdly, even in
the perfectly fresh muscle, extremely faint parallel longitudinal
striæ 1‐7,000th of an inch, or thereabouts, apart, traverse the
several zones, so that longer or shorter segments of the successive
septal lines are inclosed between them. A transverse section of
the muscle appears divided into rounded or polygonal areæ of the
same diameter, separated from one another here and there by minute
interstices. Moreover, on examination of perfectly fresh muscle with
high magnifying powers, the septal lines are hardly ever straight
for any distance, but are broken up into short lengths, which answer
to one or more of the longitudinal divisions, and stand at slightly
different heights.

The only conclusion to be drawn from these appearances seems to
me to be that the substance of the muscle is composed of distinct
_fibrils_; and that the longitudinal {185} striæ and the rounded
areæ of the transverse section are simply the optical expressions of
the boundaries of these fibrils. In the perfectly unaltered state of
the tissue, however, the fibrils are so closely packed that their
boundaries are scarcely discernible.

Thus each muscular _fibre_ may be regarded as composed of larger and
smaller bundles of _fibrils_ imbedded in a nucleated protoplasmic
framework which ensheaths the whole and is itself invested by the
sarcolemma.

As the fibre dies, the nuclei acquire hard, dark contours and their
contents become granular, while at the same time the fibrils acquire
sharp and well-defined boundaries. In fact, the fibre may now be
readily teased out with needles, and the fibrils isolated.

In muscle which has been treated with various reagents, such as
alcohol, nitric acid, or solution of common salt, the fibrils
themselves may be split up into filaments of extreme tenuity, each of
which appears to answer to one of the granules of the septal lines.
Such an isolated _muscle filament_ looks like a very fine thread
carrying minute beads at regular intervals.

The septal lines resist most reagents, and remain visible in muscular
fibres which have been subjected to various modes of treatment;
but they may have the appearance of continuous bars, or be more
or less completely resolved into separate granules, according to
circumstances. On the other hand, what is to be seen in {186} the
interspace between every two septal lines depends upon the reagent
employed. With dilute acids and strong solutions of salt, the
inter-septal substance swells up and becomes transparent, so that it
ceases to be distinguishable from the septal zone. At the same time
a distinct but faint transverse line may appear in the middle of its
length. Strong nitric acid, on the contrary, renders the inter-septal
substance more opaque, and the septal zones consequently appear very
well defined.

In living and recently dead muscle, as well as in muscles which
have been preserved in spirit or hardened with nitric acid, the
inter-septal zones polarize light; and hence, in the dark field of
the polarizing microscope, the fibre appears crossed by bright bands,
which correspond with the inter-septal zones, or at any rate, with
the middle parts of them. The substance which forms the septal zones,
on the contrary, produces no such effect, and consequently remains
dark; while the septal lines again have the same property as the
inter-septal substance, though in a less degree.

In fibres which have been acted upon by solution of salt, or dilute
acids, the inter-septal zones have lost their polarizing property.
As we know that the reagents in question dissolve the peculiar
constituent of muscle, _myosin_, it is to be concluded that the
inter-septal substance is chiefly composed of myosin.

Thus a fibril may be considered to be made up of {187}
segments of different material arranged in regular order;
S–sz–IS–sz–S–sz–IS–sz–S: S representing the septal line; sz, the
septal zone; IS, the inter-septal zone. Of these, IS is the chief if
not the only seat of the myosin; what the composition of sz and of S
may be is uncertain, but the supposition, that, in the living muscle,
sz is a mere fluid, appears to me to be wholly inadmissible.

When living muscle contracts, the inter-septal zones become shorter
and wider and their margins darker, while the septal zones and the
septal lines tend to become effaced—as it appears to me simply
in consequence of the approximation of the lateral margins of
the inter-septal zones. It is probable that the substance of the
intermediate zone is the chief, if not the only, seat of the activity
of the muscle during contraction.

5. The elements of the _nervous tissue_ are of two kinds,
_nerve-cells_, and _nerve fibres_; the former are found in the
ganglia, and they vary very much in size (fig. 54, B). Each
ganglionic corpuscle consists of a cell body produced into one or
more processes which sometimes, if not always, end in nerve fibres.
A large, clear spherical nucleus is seen in the interior of the
nerve-cell; and in the centre of this is a well defined, small round
particle, the _nucleolus_. The corpuscle, when isolated, is often
surrounded by a sort of sheath of small nucleated cells. {188}

[Illustration: FIG. 54.—_Astacus fluviatilis._—A, one of the (double)
abdominal ganglia, with the nerves connected with it (× 25); B,
a nerve cell or ganglionic corpuscle (× 250). _a_, sheath of the
nerves; _c_, sheath of the ganglion; _co, co′_, commissural cords
connecting the ganglia with those in front, and those behind them.
_gl.c_, points to the ganglionic corpuscles of the ganglia; _n_,
nerve fibres.]

The nerve fibres (fig. 55) of the crayfish are remarkable for
the large size which some of them attain. In the central nervous
system a few reach as much as 1‐200th of an inch in diameter; and
fibres of 1‐300th or 1‐400th of an inch in diameter are not rare
in the main branches. Each fibre is a tube, formed of a strong and
elastic, sometimes fibrillated, sheath, in which nuclei are imbedded
at irregular intervals; and, when the nerve trunk gives {189}
off a branch, more or fewer of these tubes divide, sending off a
prolongation into each branch.

[Illustration: FIG. 55.—_Astacus fluviatilis._—Three nerve fibres,
with the connective tissue in which they are imbedded (magnified
about 250 diameters); _n_, nuclei.]

When quite fresh, the contents of the tubes are perfectly pellucid,
and without the least indication of structure; and, from the manner
in which the contents exude from the cut ends of the tubes, it is
evident that they consist of a fluid of gelatinous consistency. As
the fibre dies, and under the influence of water and of many chemical
re-agents, the contents break up into globules or become turbid and
finely granular.

Where motor nerve fibres terminate in the muscles to which they are
distributed, the sheath of each fibre becomes continuous with the
sarcolemma of the muscle, and the subjacent protoplasm is commonly
raised into a small prominence which contains several nuclei (fig.
52, F). These are called the _terminal_ or _motor plates_. {190}

6, 7. The _ova_ and the _spermatozoa_ have already been described
(pp. 132–135).

       *       *       *       *       *

It will be observed that the blood corpuscles, the epithelial
tissues, the ganglionic corpuscles, the ova and the spermatozoa, are
all demonstrably nucleated cells, more or less modified. The first
form of connective tissue is so similar to epithelial tissue, that
it may obviously be regarded as an aggregate of as many cells as it
presents nuclei, the matrix representing the more or less modified
and confluent bodies of the cells, or products of these. But if this
be so, then the second and third forms have a similar composition,
except so far as the matrix of the cells has become fibrillated,
or vacuolated, or marked off into masses corresponding with the
several nuclei. By a parity of reasoning, muscular tissue may also be
considered a cell aggregate, in which the inter-nuclear substance has
become converted into striated muscle; while, in the nerve fibres,
a like process of metamorphosis may have given rise to the pellucid
gelatinous nerve substance. But, if we accept the conclusions thus
suggested by the comparison of the various tissues with one another,
it follows that every histological element, which has now been
mentioned, is either a simple nucleated cell, a modified nucleated
cell, or a more or less modified cell aggregate. In other words,
every tissue is resolvable into nucleated cells. {191}

[Illustration: FIG. 56.—_Astacus fluviatilis._—The structure of the
cuticle. _A_, transverse section of a joint of the forceps (× 4);
_s_, setæ; _B_, a portion of the same (× 30); _C_, a portion of B
more highly magnified. _a_, epiostracum; _b_, ectostracum; _c_,
endostracum; _d_, canal of seta; _e_, canals filled with air; _s_,
seta. _D_, section of an intersternal membrane of the abdomen, the
portion to the right in the natural condition, the remainder pulled
apart with needles (× 20); _E_, small portion of the same, highly
magnified; _a_, intermediate substance; _b_, laminæ. _F_, a seta,
highly magnified; _a_ and _b_, joints.]

{192}

A notable exception to this generalisation, however, obtains in the
case of the _cuticular structures_, in which no cellular components
are discoverable. In its simplest form, such as that presented by
the lining of the intestine, the cuticle is a delicate, transparent
membrane, thrown off from the surface of the subjacent cells, either
by a process of exudation, or by the chemical transformation of
their superficial layer. No pores are discernible in this membrane,
but scattered over its surface there are oval patches of extremely
minute, sharp conical processes, which are rarely more than 1‐5,000th
of an inch long. Where the cuticle is thicker, as in the stomach and
in the exoskeleton, it presents a stratified appearance, as if it
were composed of a number of laminæ, of varying thickness, which had
been successively thrown off from the subjacent cells.

Where the cuticular layer of the integument is uncalcified, for
example, between the sterna of the abdominal somites, it presents an
external, thin, dense, wrinkled lamina, the _epiostracum_, followed
by a soft substance, which, on vertical section, presents numerous
alternately more transparent and more opaque bands, which run
parallel with one another and with the free surfaces of the slice
(fig. 56, D). These bands are very close-set, often not more than
1‐5000th of an inch apart near the outer and the inner surfaces, but
in the middle of the section they are more distant.

If a thin vertical slice of the soft cuticle is gently {193} pulled
with needles in the direction of its depth, it stretches to eight or
ten times its previous diameter, the clear intervals between the dark
bands becoming proportionally enlarged, especially in the middle of
the slice, while the dark bands themselves become apparently thinner,
and more sharply defined. The dark bands may then be readily drawn
to a distance of as much as 1‐300th of an inch from one another; but
if the slice is stretched further, it splits along, or close to, one
of the dark lines. The whole of the cuticular layer is stained by
such colouring matters as hæmatoxylin; and, as the dark bands become
more deeply coloured than the intermediate transparent substance, the
transverse stratification is made very manifest by this treatment.

Examined with a high magnifying power, the transparent substance is
seen to be traversed by close-set, faint, vertical lines, while the
dark bands are shown to be produced by the cut edges of delicate
laminæ, having a finely striated appearance, as if they were composed
of delicate parallel wavy fibrillæ.

In the calcified parts of the exoskeleton a thin, tough, wrinkled
epiostracum (fig. 56, B, _a_), and, subjacent to this, a number of
alternately lighter and darker strata are similarly discernible:
though all but the innermost laminæ are hardened by a deposit of
calcareous salts, which are generally evenly diffused, but sometimes
take the shape of rounded masses with irregular contours.

Immediately beneath the epiostracum, there is a zone {194} which
may occupy a sixth or a seventh of the thickness of the whole, which
is more transparent than the rest, and often presents hardly any
trace of horizontal or vertical striation. When it appears laminated,
the strata are very thin. This zone may be distinguished as the
_ectostracum_ (_b_), from the _endostracum_ (_c_), which makes up the
rest of the exoskeleton. In the outer part of the endostracum, the
strata are distinct, and may be as much as 1‐500th of an inch thick,
but in the inner part they become very thin, and the lines which
separate them may be not more than 1‐8000th of an inch apart. Fine,
parallel, close-set, vertical striæ (_e_) traverse all the strata of
the endostracum, and may usually be traced through the ectostracum,
though they are always faint, and sometimes hardly discernible, in
this region. When a high magnifying power is employed, it is seen
that these striæ, which are about 1‐7000th of an inch apart, are
not straight, but that they present regular short undulations, the
alternate convexities and concavities of which correspond with the
light and the dark bands respectively.

If the hard exoskeleton has been allowed to become partially or
wholly dry before the section is made, the latter will look white
by reflected and black by transmitted light, in consequence of the
places of the striæ being taken by threads of air of such extreme
tenuity, that they may measure not more than 1‐30,000th of an inch in
diameter. It is to be concluded, therefore, that {195} the striæ are
the optical indications of parallel undulating canals which traverse
the successive strata of the cuticle, and are ordinarily occupied by
a fluid. When this dries up, the surrounding air enters, and more or
less completely fills the tubes. And that this is really the case
may be proved by making very thin sections parallel with the face of
the exoskeleton, for these exhibit innumerable minute perforations,
set at regular distances from one another, which correspond with the
intervals between the striæ in the vertical section; and sometimes
the contours of the areæ which separate the apertures are so well
defined as to suggest a pavement of minute angular blocks, the
corners of which do not quite meet.

When a portion of the hard exoskeleton is decalcified, a chitinous
substance remains, which presents the same structure as that just
described, except that the epiostracum is more distinct; while the
ectostracum appears made up of very thin laminæ, and the tubes are
represented by delicate striæ, which appear coarser in the region of
the dark zones. As in the naturally soft parts of the exoskeleton,
the decalcified cuticle may be split into flakes, and the pores are
then seen to be disposed in distinct areæ circumscribed by clear
polygonal borders. These perforated areæ appear to correspond with
individual cells of the ectoderm, and the canals thus answer to the
so-called “pore-canals,” which are common in cuticular structures and
in the walls of many cells which bound free surfaces. {196}

The whole exoskeleton of the crayfish is, in fact, produced by the
cells which underlie it, either by the exudation of a chitinous
substance, which subsequently hardens, from them; or, as is more
probable, by the chemical metamorphosis of a superficial zone of the
bodies of the cells into chitin. However this may be, the cuticular
products of adjacent cells at first form a simple, continuous, thin
pellicle. A continuation of the process by which it was originated
increases the thickness of the cuticle; but the material thus added
to the inner surface of the latter is not always of the same nature,
but is alternately denser and softer. The denser material gives rise
to the tough laminæ, the softer to the intermediate transparent
substance. But the quantity of the latter is at first very small,
whence the more external laminæ are in close apposition. Subsequently
the quantity of the intermediate substance increases, and gives rise
to the thick stratification of the middle region, while it remains
insignificant in the inner region of the exoskeleton.

The cuticular structures of the crayfish differ from the nails,
hairs, hoofs, and similar hard parts of the higher animals, insomuch
as the latter consist of aggregations of cells, the bodies of which
have been metamorphosed into horny matter. The cuticle, with all its
dependencies, on the contrary, though no less dependent on cells
for its existence, is a derivative product, the formation of which
does not involve the complete {197} metamorphosis and consequent
destruction of the cells to which it owes its origin.

The calcareous salts by which the calcified exoskeleton is hardened
can only be supplied by the infiltration of a fluid in which they are
dissolved from the blood; while the distinctive structural characters
of the epiostracum, the ectostracum, and the endostracum, are the
results of a process of metamorphosis which goes on _pari passu_ with
this infiltration. To what extent this metamorphosis is a properly
vital process; and to what extent it is explicable by the ordinary
physical and chemical properties of the animal membrane on the one
hand, and the mineral salts on the other, is a curious, and at
present, unsolved problem.

The outer surface of the cuticle is rarely smooth. Generally it is
more or less obviously ridged or tuberculated; and, in addition,
presents coarser or finer hair-like processes which exhibit every
gradation from a fine microscopic down to stout spines. As these
processes, though so similar to hairs in general appearance, are
essentially different from the structures known as hairs in the
higher animals, it is better to speak of them as _setæ_.

These setæ (fig. 56, F) are sometimes short, slender, conical
filaments, the surface of which is quite smooth; sometimes
the surface is produced into minute serrations, or scale-like
prominences, disposed in two or more series; in other setæ, the
axis gives off slender lateral {198} branches; and in the most
complicated form the branches are ornamented with lateral branchlets.
For a certain distance from the base of the seta, its surface is
usually smooth, even when the rest of its extent is ornamented with
scales or branches. Moreover, the basal part of the seta is marked
off from its apical moiety by a sort of joint which is indicated by
a slight constriction, or by a peculiarity in the structure of the
cuticula at this point. A seta almost always takes its origin from
the bottom of a depression or pit of the layer of cuticle, from which
it is developed, and at its junction with the latter it is generally
thin and flexible, so that the seta moves easily in its socket. Each
seta contains a cavity, the boundaries of which generally follow the
outer contours of the seta. In a good many of the setæ, however, the
parietes, near the base of the seta, are thickened in such a manner
as almost, or completely, to obliterate the central cavity. However
thick the cuticle may be at the point from which the setæ take their
origin, it is always traversed by a funnel-shaped canal (fig. 56, B,
_d_), which usually expands beneath the base of the seta. Through
this canal the subjacent ectoderm extends up to the base of the seta,
and can even be traced for some distance into its interior.

It has already been mentioned that the apodemata and the tendons of
the muscles are infoldings of the cuticle, embraced and secreted by
corresponding involutions of the ectoderm. {199}

Thus the body of the crayfish is resolvable, in the first place, into
a repetition of similar segments, the _metameres_, each of which
consists of a somite and two appendages; the metameres are built
up out of a few simple _tissues_; and, finally, the tissues are
either aggregates of more or less modified nucleated _cells_, or are
products of such cells. Hence, in ultimate morphological analysis,
the crayfish is a multiple of the histological unit, the nucleated
cell.

What is true of the crayfish, is certainly true of all animals, above
the very lowest. And it cannot yet be considered certain that the
generalization fails to hold good even of the simplest manifestations
of animal life; since recent investigations have demonstrated the
presence of a nucleus in organisms in which it had hitherto appeared
to be absent.

However this may be, there is no doubt that in the case of man and
of all vertebrated animals, in that of all arthropods, mollusks,
echinoderms, worms, and inferior organisms down to the very lowest
sponges, the process of morphological analysis yields the same result
as in the case of the crayfish. The body is built up of tissues, and
the tissues are either obviously composed of nucleated cells; or,
from the presence of nuclei, they may be assumed to be the results of
the metamorphosis of such cells; or they are cuticular structures.

The essential character of the nucleated cell is that it consists
of a protoplasmic substance, one part of which differs somewhat
in its physical and chemical characters {200} from the rest, and
constitutes the nucleus. What part the nucleus plays in relation to
the functions, or vital activities, of the cell is as yet unknown;
but that it is the seat of operations of a different character from
those which go on in the body of the cell is clear enough. For, as we
have seen, however different the several tissues may be, the nuclei
which they contain are very much alike; whence it follows, that if
all these tissues were primitively composed of simple nucleated
cells, it must be the bodies of the cells which have undergone
metamorphosis, while the nuclei have remained relatively unchanged.

On the other hand, when cells multiply, as they do in all growing
parts, by the division of one cell into two, the signs of the process
of internal change which ends in fission are apparent in the nucleus
before they are manifest in the body of the cell; and, commonly, the
division of the former precedes that of the latter. Thus a single
cell body may possess two nuclei, and may become divided into two
cells by the subsequent aggregation of the two moieties of its
protoplasmic substance round each of them, as a centre.

In some cases, very singular structural changes take place in the
nuclei in the course of the process of cell-division. The granular
or fibrillar contents of the nucleus, the wall of which becomes less
distinct, arrange themselves in the form of a spindle or double
cone, formed of extremely delicate filaments; and in the plane
{201} of the base of the double cone the filaments present knots
or thickenings, just as if they were so many threads with a bead in
the middle of each. When the nuclear spindle is viewed sideways,
these beads or thickenings give rise to the appearance of a disk
traversing the centre of the spindle. Soon each bead separates into
two, and these move away from one another, but remain connected by
a fine filament. Thus the structure which had the form of a double
cone, with a disk in the middle, assumes that of a short cylinder,
with a disk and a cone at each end. But as the distance between the
two disks increases, the uniting filaments lose their parallelism,
converge in the middle, and finally separate, so that two separate
double cones are developed in place of the single one. Along with
these changes in the nucleus, others occur in the protoplasm of the
cell body, and its parts commonly display a tendency to arrange
themselves in radii from the extremities of the cones as a centre;
while, as the separation of the two secondary nuclear spindles
becomes complete, the cell body gradually splits from the periphery
inwards, in a direction at right angles to the common axis of the
spindles and between their apices. Thus two cells are formed, where,
previously, only one existed; and the nuclear spindles of each soon
revert to the globular form and confused arrangement of the contents,
characteristic of nuclei in their ordinary state. The formation of
these nuclear spindles is very beautifully seen in the epithelial
cells of the testis of the {202} crayfish (fig. 33, p. 132); but I
have not been able to find distinct evidence of it elsewhere in this
animal; and although the process has now been proved to take place in
all the divisions of the animal kingdom, it would seem that nuclei
may, and largely do, undergo division, without becoming converted
into spindles.

The most cursory examination of any of the higher plants shows that
the vegetable, like the animal body, is made up of various kinds of
tissues, such as pith, woody fibre, spiral vessels, ducts, and so on.
But even the most modified forms of vegetable tissue depart so little
from the type of the simple cell, that the reduction of them all to
that common type is suggested still more strongly than in the case
of the animal fabric. And thus the nucleated cell appears to be the
morphological unit of the plant no less than of the animal. Moreover,
recent inquiry has shown that in the course of the multiplication of
vegetable cells by division, the nuclear spindles may appear and run
through all their remarkable changes by stages precisely similar to
those which occur in animals.

The question of the universal presence of nuclei in cells may be left
open in the case of Plants, as in that of Animals; but, speaking
generally, it may justly be affirmed that the nucleated cell is the
morphological foundation of both divisions of the living world; and
the great generalisation of Schleiden and Schwann, that there is a
fundamental agreement in structure and {203} development between
plants and animals, has, in substance, been merely confirmed and
illustrated by the labours of the half century which has elapsed
since its promulgation.

Not only is it true that the minute structure of the crayfish is, in
principle, the same as that of any other animal, or of any plant,
however different it may be in detail; but, in all animals (save some
exceptional forms) above the lowest, the body is similarly composed
of three layers, ectoderm, mesoderm, and endoderm, disposed around
a central alimentary cavity. The ectoderm and the endoderm always
retain their epithelial character; while the mesoderm, which is
insignificant in the lower organisms, becomes, in the higher, far
more complicated even than it is in the crayfish.

Moreover, in the whole of the _Arthropoda_, and the whole of the
_Vertebrata_, to say nothing of other groups of animals, the body,
as in the crayfish, is susceptible of distinction into a series of
more or less numerous segments, composed of homologous parts. In
each segment these parts are modified according to physiological
requirements; and by the coalescence, segregation, and change of
relative size and position of the segments, well characterized
regions of the body are marked out. And it is remarkable that
precisely the same principles are illustrated by the morphology of
plants. A flower with its whorls of sepals, petals, stamens and
carpels has the same relation to a stem {204} with its whorls of
leaves, as a crayfish’s head has to its abdomen, or a dog’s skull to
its thorax.

       *       *       *       *       *

It may be objected, however, that the morphological generalisations
which have now been reached, are to a considerable extent of a
speculative character; and that, in the case of our crayfish, the
facts warrant no more than the assertion that the structure of that
animal may be consistently interpreted, on the supposition that the
body is made up of homologous somites and appendages, and that the
tissues are the result of the modification of homologous histological
elements or cells; and the objection is perfectly valid.

There can be no doubt that blood corpuscles, liver cells, and ova
are all nucleated cells; nor any that the third, fourth, and fifth
somites of the abdomen are constructed upon the same plan; for
these propositions are mere statements of the anatomical facts. But
when, from the presence of nuclei in connective tissue and muscles,
we conclude that these tissues are composed of modified cells; or
when we say that the ambulatory limbs of the thorax are of the
same type as the abdominal limbs, the exopodite being suppressed,
the statement, as the evidence stands at present, is no more than
a convenient way of interpreting the facts. The question remains,
has the muscle actually been formed out of nucleated cells? Has the
ambulatory limb ever possessed an exopodite, and lost it? {205}

The answer to these questions is to be sought in the facts of
individual and ancestral development.

An animal not only is, but becomes; the crayfish is the product of
an egg, in which not a single structure visible in the adult animal
exists: in that egg the different tissues and organs make their
appearance by a gradual process of evolution; and the study of this
process can alone tell us whether the unity of composition suggested
by the comparison of adult structures, is borne out by the facts of
their development in the individual or not. The hypothesis that the
body of the crayfish is made up of a series of homologous somites and
appendages, and that all the tissues are composed of nucleated cells,
might be only a permissible, because a useful, mode of colligating
the facts of anatomy. The investigation of the actual manner in which
the evolution of the body of the crayfish has been effected, is the
only means of ascertaining whether it is anything more. And, in this
sense, development is the criterion of all morphological speculations.

       *       *       *       *       *

The first obvious change which takes place in an impregnated ovum is
the breaking up of the yelk into smaller portions, each of which is
provided with a nucleus, and is termed a _blastomere_. In a general
morphological sense, a blastomere is a nucleated cell, and differs
from an ordinary cell only in size, and in the usual, though by no
means invariable, abundance of granular contents; and blastomeres
insensibly pass into ordinary cells, as {206} the process of
division of the yelk into smaller and smaller portions goes on.

In a great many animals, the splitting-up into blastomeres is
effected in such a manner that the yelk is, at first, divided into
equal, or nearly equal, masses; that each of these again divides into
two; and that the number of blastomeres thus increases in geometrical
progression until the entire yelk is converted into a mulberry-like
body, termed a _morula_, made up of a great number of small
blastomeres or nucleated cells. The whole organism is subsequently
built up by the multiplication, the change of position, and the
metamorphosis of these products of yelk division.

In such a case as this, yelk division is said to be _complete_. An
unessential modification of complete yelk division is seen when, at
an early period, the blastomeres produced by division are of unequal
sizes; or when they become unequal in consequence of division taking
place much more rapidly in one set than in another.

In many animals, especially those which have large ova, the
inequality of division is pushed so far that only a portion of
the yelk is affected by the process of fission, while the rest
serves merely as _food-yelk_, for nutriment to the blastomeres
thus produced. Over a greater or less extent of the surface of the
egg, the protoplasmic substance of the yelk segregates itself from
the rest, and, constituting a _germinal layer_, breaks up into the
blastomeres, which multiply at the expense of the {207} food-yelk,
and fabricate the body of the embryo. This process is termed
_partial_ or _incomplete_ yelk division.

The crayfish is one of those animals in the egg of which the yelk
undergoes partial division. The first steps of the process have not
yet been thoroughly worked out, but their result is seen in ova which
have been but a short time laid (fig. 57, A). In such eggs, the great
mass of the substance of the vitellus is destined to play the part
of food-yelk; and it is disposed in conical masses, which radiate
from a central spheroidal portion to the periphery of the yelk
(_v_). Corresponding with the base of each cone, there is a clear
protoplasmic plate, which contains a nucleus; and as these bodies are
all in contact by their edges, they form a complete, though thin,
investment to the food-yelk. This is termed the _blastoderm_ (_bl_).

Each nucleated protoplasmic plate adheres firmly to the corresponding
cone of granular food-yelk, and, in all probability, the two together
represent a blastomere; but, as the cones only indirectly subserve
the growth of the embryo, while the nucleated peripheral plates form
an independent spherical sac, out of which the body of the young
crayfish is gradually fashioned, it will be convenient to deal with
the latter separately.

[Illustration: FIG. 57.—_Astacus fluviatilis._—Diagrammatic sections
of embryos; partly after Reichenbach, partly original (× 20). A. An
ovum in which the blastoderm is just formed. B. An ovum in which
the invagination of the blastoderm to constitute the hypoblast or
rudiment of the mid-gut has taken place. (This nearly answers to the
stage represented in fig. 58, _A._) C. A longitudinal section of an
ovum, in which the rudiments of the abdomen, of the hind-gut, and
of the fore-gut have appeared. (This nearly answers to the stage
represented in fig. 58, E.) D. A similar section of an embryo in
nearly the same stage of development as that represented in C, fig.
59. E. An embryo just hatched, in longitudinal section; _a_, anus;
_bl_, blastoderm; _bp_, blastopore; _e_, eye; _ep. b._, epiblast;
_fg_, fore-gut; _fg_^1, its œsophageal, and _fg_^2, its gastric
portion; _h_, heart; _hg_, hind-gut; _m_, mouth; _mg_, hypoblast,
archenteron, or mid-gut; _v_, yelk. The dotted portions in D and E
represent the nervous system.]

Thus, at this period, the body of the developing crayfish is nothing
but a spherical bag, the thin walls of which are composed of a
single layer of nucleated cells, while its cavity is filled with
food-yelk. The first modification {209} which is effected in the
vesicular blastoderm manifests itself on that face of it which is
turned towards the pedicle of the egg. Here the layer of cells
becomes thickened throughout an oval area about 1‐25th of an inch in
diameter. Hence, when the egg is viewed by reflected light, a whitish
patch of corresponding form and size appears in this region. This may
be termed the _germinal disk_. Its long axis corresponds with that of
the future crayfish.

Next, a depression (fig. 58, A, _bp_) appears in the hinder third
of the germinal disk, in consequence of this part of the blastoderm
growing inwards, and thus giving rise to a small wide-mouthed
pouch, which projects into the food-yelk with which the cavity of
the blastoderm is filled (fig. 57, B, _mg_). As this infolding, or
invagination of the blastoderm, goes on, the pouch thus produced
increases, while its external opening, termed the _blastopore_ (fig.
57, B, and 58, A–E, _bp_), diminishes in size. Thus the body of
the embryo crayfish, from being a simple bag becomes a double bag,
such as might be produced by pushing in the wall of an incompletely
distended india-rubber ball with the finger. And, in this case, if
the interior of the bag contained porridge, the latter would very
fairly represent the food-yelk.

[Illustration: FIG. 58.—_Astacus fluviatilis._—Surface views of the
earlier stages in the development of the embryo, from the appearance
of the blastopore (A) to the assumption of the nauplius form (F)
(after Reichenbach, × about 23). _bp_, blastopore; _c_, carapace;
_fg_, fore-gut involution; _h_, heart; _hg_, hind-gut involution;
_lb_, labrum; _mg_, medullary groove; _o_, optic pit; _p_, endodermal
plug partly filling up the blastopore; _pc_, procephalic processes;
_ta_, abdominal elevation; _2_, antennules; _3_, antennæ; _4_,
mandibles.]

By this invagination a most important step has been taken in the
development of the crayfish. For, though the pouch is nothing but an
ingrowth of part of the blastoderm, the cells of which its wall is
composed {211} henceforward exhibit different tendencies from those
which are possessed by the rest of the blastoderm. In fact, it is
the primitive alimentary apparatus or _archenteron_, and its wall is
termed the _hypoblast_. The rest of the blastoderm, on the contrary,
is the primitive epidermis, and receives the name of _epiblast_.
If the food-yelk were away, and the archenteron enlarged until the
hypoblast came in contact with the epiblast, the entire body would be
a double-walled sac, containing an alimentary cavity, with a single
external aperture. This is the _gastrula_ condition of the embryo;
and some animals, such as the common fresh-water polype, are little
more than permanent _gastrulæ_.

Although the gastrula has not the slightest resemblance to a
crayfish, yet, as soon as the hypoblast and the epiblast are thus
differentiated, the foundations of some of the most important systems
of organs of the future crustacean are laid. The hypoblast will
give rise to the epithelial lining of the mid-gut; the epiblast
(which answers to the ectoderm in the adult) to the epithelia of the
fore-gut and hind-gut, to the epidermis, and to the central nervous
system.

The mesodermal structures, that is to say the connective tissue,
the muscles, the heart and vessels, and the reproductive organs,
which lie between the ectoderm and the endoderm, are not derived
directly from either the epiblast or the hypoblast, but have a
_quasi_-independent origin, from a mass of cells which first makes
its {212} appearance in the neighbourhood of the blastopore, between
the hypoblast and the epiblast, though they are probably derived
from the former. From this region they gradually spread, first over
the sternal, and then on to the tergal aspect of the embryo, and
constitute the _mesoblast_.

Epiblast, hypoblast, and mesoblast are at first alike constituted
of nothing but nucleated cells, and they increase in dimensions by
the continual fission and growth of these cells. The several layers
become gradually modelled into the organs which they constitute,
before the cells undergo any notable modification into tissues.
A limb, for example, is, at first, a mere cellular outgrowth, or
bud, composed of an outer coat of epiblast with an inner core of
mesoblast; and it is only subsequently that its component cells are
metamorphosed into well-defined epidermic and connective tissues,
vessels and muscles.

The embryo crayfish remains only a short while in the gastrula stage,
as the blastopore soon closes up, and the archenteron takes the form
of a sac, flattened out between the epiblast and the food-yelk, with
which its cells are in close contact (fig. 57, C and D).[12] Indeed,
as development proceeds, the cells of the hypoblast actually feed
upon the substance of the food-yelk, and turn it to account for the
general nutrition of the body. {213}

     [12] Whether, as some observers state, the hypoblastic cells
     grow over and inclose the food-yelk or not, is a question that
     may be left open. I have not been able to satisfy myself of this
     fact.

The sternal area of the embryo gradually enlarges until it occupies
one hemisphere of the yelk; in other words, the thickening of the
epiblast gradually extends outwards. Just in front of the blastopore,
as it closes, the middle of the epiblast grows out into a rounded
elevation (fig. 58, _t a_; fig. 59, _ab_), which rapidly increases
in length, and at the same time turns forwards. This is the rudiment
of the whole abdomen of the crayfish. Further forwards, two broad
and elongated, but flatter thickenings appear; one on each side of
the middle line (fig. 58, _p c_). As the free end of the abdominal
papilla now marks the hinder extremity of the embryo, so do these
two elevations, which are termed the _procephalic lobes_, define
its anterior termination. The whole sternal region of the body will
be produced by the elongation of that part of the embryo which lies
between these two limits.

A narrow longitudinal groove-like depression appears on the surface
of the epiblast, in the middle line, between the procephalic lobes
and the base of the abdominal papilla (fig. 58, C–F, _m g_). About
its centre, this groove becomes further depressed by the ingrowth
of the epiblast, which constitutes its floor, and gives rise to a
short tubular sac, which is the rudiment of the whole fore-gut (fig.
57, C, and fig. 58, E, _f g_). At first, this epiblastic ingrowth
does not communicate with the archenteron, but, after a while, its
blind end combines with the front and lower part of the hypoblast,
and an opening is formed by {214} which the cavity of the fore-gut
communicates with that of the mid-gut (fig. 57, E). Thus a gullet
and stomach, or rather the parts which will eventually give rise to
all these, are constituted. And it is important to remark that, in
comparison with the mid-gut, they are, at first, very small.

In the same way, the epiblast covering the sternal face of the
abdominal papilla undergoes invagination and is converted into a
narrow tube which is the origin of the whole hind-gut (fig. 57, C,
and fig. 58, E, _hg_). This, like the fore-gut, is at first blind;
but the shut front end soon applying itself to the hinder wall of the
archenteric sac, the two coalesce and open into one another (fig.
57, E). Thus the complete alimentary canal, consisting of a very
narrow, tubular, fore- and hind-gut, derived from the epiblast, and
a wider and more sac-like mid-gut, formed of the whole hypoblast, is
constituted.

The procephalic lobes become more convex; while, behind them, the
surface of the epiblast rises into six elevations disposed in pairs,
one on each side of the median groove. The hindermost of these, which
lie at the sides of the mouth, are the rudiments of the mandibles
(fig. 58, E and F, _4_); the other two become the antennæ (_3_) and
the antennules (_2_), while, at a later period, processes of the
procephalic lobes give rise to the eyestalks.

A short distance behind the abdomen, the epiblast rises into a
transverse ridge, which is concave forwards, {215} while its ends
are prolonged on each side nearly as far as the mouth. This is the
commencement of the free edge of the carapace (fig. 58, E and F,
and fig. 59, A, _c_)—the lateral parts of which, greatly enlarging,
become the branchiostegites (fig. 59, D, _c_).

In many animals allied to crayfish, the young, when it has reached
a stage in its development, which answers to this, undergoes rapid
changes of outward form and of internal structure, without making any
essential addition to the number of the appendages. The appendages
which represent the antennules, the antennæ, and the mandibles
elongate and become oar-like locomotive organs; a single median
eye is developed, and the young leaves the egg as an active larva,
which is known as a _Nauplius_. The crayfish, on the other hand,
is wholly incapable of an independent existence at this stage,
and continues its embryonic life within the egg case; but it is a
remarkable circumstance that the cells of the epiblast secrete a
delicate cuticula, which is subsequently shed. It is as if the animal
symbolized a nauplius condition by the development of this cuticle,
as the fœtal whalebone whale symbolizes a toothed condition by
developing teeth which are subsequently lost and never perform any
function.

[Illustration: FIG. 59.—_Astacus fluviatilis._—Ventral (A, B, C,
F) and lateral (D, E) views of the embryo in successive stages of
development (after Rathke, × 15). A is a little more advanced than
the embryo represented in fig. 58, F: D, E, and F are views of the
young crayfish when nearly ready to be hatched: in E, the carapace
is removed, and the limbs and abdomen are spread out. 1–14, the
cephalic and thoracic appendages; _ab_, abdomen; _br_, branchiæ;
_c_, carapace; _ep_, epipodite of the first maxillipede; _gg_,
green gland; _h_, heart; _lb_, labrum; _lr_, liver; _m_, mandibular
muscles.]

In fact, in the crayfish, the nauplius condition is soon left behind.
The sternal disk spreads more and more over the yelk; as the region
between the mouth and the root of the abdomen elongates, slight
transverse {217} depressions indicate the boundaries of the posterior
cephalic and the thoracic somites; and pairs of elevations, similar
to the rudiments of the antennules and antennæ, appear upon them in
regular order from before backwards (fig. 59, C).

In the meanwhile, the extremity of the abdomen flattens out and takes
on the form of an oval plate, the middle of the posterior margin of
which is slightly truncated or notched; while, finally, transverse
constrictions mark off six segments, the somites of the abdomen, in
front of this. Along with these changes, four pairs of tubercles grow
out from the sternal faces of the four middle abdominal somites,
and constitute the rudiments of the four middle pairs of abdominal
appendages. The first abdominal somite exhibits only two hardly
perceptible elevations in place of the appendages of the others,
while the sixth seems, at first, to have none. The appendages of the
sixth somite, however, are already formed, though, singularly enough,
they lie beneath the cuticle of the telson and are set free only
after the first ecdysis.

The rostrum grows out between the procephalic lobes; it remains
relatively very short up to the time that the young crayfish quits
the egg, and is directed more downwards than forwards. The lateral
portions of the carapacial ridge, becoming deeper, are converted into
the branchiostegites, and the cavities which they overarch are the
branchial chambers. The transverse portion of {218} the ridge, on
the other hand, remains relatively short, and constitutes the free
posterior margin of the carapace.

As these changes take place, the abdomen and the sternal region of
the thorax are constantly enlarging in proportion to the rest of
the ovum; and the food-yelk which lies in the cephalothorax is,
_pari passu_, being diminished. Hence the cephalothorax constantly
becomes relatively smaller and the tergal aspect of the carapace less
spherical; although, even when the young crayfish is ready to be
hatched, the difference between it and the adult in the form of the
cephalothoracic region, and in the size of the latter relatively to
the abdomen, is very marked.

The simple bud-like outgrowths of the somites, in which all the
appendages take their origin, are rapidly metamorphosed. The
eyestalks (fig. 59, _1_) soon attain a considerable relative size.
The extremities of the antennules (_2_) and of the antennæ (_3_)
become bifurcated; and the two divisions of the antennule remain
broad, thick, and of nearly the same size up to birth. On the other
hand, the inner or endopoditic division of the antenna becomes
immensely lengthened, and at the same time annulated, while the outer
or exopoditic division remains relatively short, and acquires its
characteristic scale-like form.

The labrum (_lb_) arises as a prolongation of the middle sternal
region in front of the mouth, while the bilobed metastoma is an
outgrowth of the sternal region behind it. {219}

The posterior cephalic and the thoracic appendages (_5–14_) elongate
and gradually approach the form which they possess in the adult. I
have not been able to discover, at any period of development, an
outer division or exopodite in any of the five posterior thoracic
limbs. And this is a very remarkable circumstance, inasmuch as such
an exopodite exists in the closely allied lobster in the larval
state; and, in many of the shrimp and prawn-like allies of the
crayfish, a complete or rudimentary exopodite is found in these
limbs, even in the adult condition.

When the crayfish is hatched (fig. 60) it differs from the adult in
many ways—not only is the cephalothorax more convex and larger in
proportion to the abdomen; but the rostrum is short and bent down
between the eyes. The sterna of the thorax are wider relatively,
and hence there is a greater interval between the bases of the legs
than in the adult. The proportion of the limbs to one another and
to the body are nearly the same as in the adult, but the chelæ of
the forceps are more slender. The tips of the chelæ are all strongly
incurved (fig. 8, B, p. 41), and the dactylopodites of the two
posterior thoracic limbs are hook-like. The appendages of the first
abdominal somite are undeveloped, and those of the last are inclosed
within the telson, which is, as has already been said, of a broad
oval form, usually notched in the middle of its hinder margin, and
devoid of any indication of transverse division. Its margins are
produced into a single series of short conical {220} processes, and
the disposition of the vascular canals in its interior gives it the
appearance of being radially striated.

The setæ, so abundant in the adult, are very scanty in the newly
hatched young; and the great majority of those which exist are simple
conical prolongations of the uncalcified cuticle, the bases of which
are not sunk in pits and which are devoid of lateral scales or
processes.

[Illustration: FIG. 60.—_Astacus fluviatilis._—Newly-hatched young
(× 6).]

The young animals are firmly attached to the abdominal appendages of
the parent in the manner already described. They are very sluggish,
though they move when touched; and at this period they do not feed,
but {221} are nourished by the food-yelk, of which a considerable
store still remains in the cephalothorax.

I imagine that they are set free during the first ecdysis, and
that the appendages of the sixth abdominal somite are at that time
expanded, but nothing is definitely known at present of these changes.

       *       *       *       *       *

The foregoing sketch of the general nature of the changes which
take place in the egg of the crayfish suffice to show that its
development is, in the strictest sense of the word, a process of
evolution. The egg is a relatively homogeneous mass of living
protoplasmic matter, containing much nutritive material; and the
development of the crayfish means the gradual conversion of this
comparatively simple body into an organism of great complexity. The
yelk becomes differentiated into formative and nutritive portions.
The formative portion is subdivided into histological units: these
arrange themselves into a blastodermic vesicle; the blastoderm
becomes differentiated into epiblast, hypoblast, and mesoblast;
and the simple vesicle assumes the gastrula condition. The layers
of the gastrula shape themselves into the body of the crayfish and
its appendages, while along with this, the cells of which all the
parts are built, become metamorphosed into tissues, each with its
characteristic properties. And all these wonderful changes are the
necessary consequences of the interaction of the molecular forces
resident in the substance of the {222} impregnated ovum, with the
conditions to which it is exposed; just as the forms evolved from a
crystallising fluid are dependent upon the chemical composition of
the dissolved matter and the influence of surrounding conditions.

Without entering into details which lie beyond the scope of the
present work, something must be said respecting the manner in which
the complicated internal organisation of the crayfish is evolved from
the cellular double sac of the gastrula stage.

It has been seen that the fore-gut is at first an insignificant
tubular involution of the epiblast in the region of the mouth. It
is, in fact, a part of the epiblast turned inwards, and the cells
of which it is composed secrete a thin cuticular layer, as do those
of the rest of the epiblast, which gives rise to the ectodermal or
epidermic part of the integument. As the embryo grows, the fore-gut
enlarges much faster than the mid-gut, increasing in height and from
before backwards, while its side-walls remain parallel, and are
separated by only a narrow cavity. At length, it takes on the shape
of a triangular bag (fig. 57, D, _fg_), attached by its narrow end
around the mouth and immersed in the food-yelk, which it gradually
divides into two lobes, one on the right and one on the left side.
At the same time a vertical plate of mesoblastic tissue, from which
the great anterior and posterior muscles are eventually developed,
connects it with the roof and with the front wall of the carapace.
{223} Becoming constricted in the middle, the fore-gut next appears
to consist of two dilatations of about equal size, connected by a
narrower passage (fig. 57, E, _fg_^1, _fg_^2). The front dilatation
becomes the œsophagus and the cardiac division of the stomach; the
hinder one, the pyloric division. At the sides of the front end of
the cardiac division two small pouches are formed shortly after
birth; in each of these a thick laminated deposit of chitin takes
place, and constitutes a minute crab’s-eye or gastrolith, which
has the same structure as in the adult, and is largely calcified.
This fact is the more remarkable as, at this time, the exoskeleton
contains very little calcareous deposit. In the position of the
gastric teeth, folds of the cellular wall of corresponding shape are
formed, and the chitinous cuticle of which the teeth are composed is,
as it were, modelled upon them.

The hind-gut occupies the whole length of the abdomen, and its cells
early arrange themselves into six ridges, and secrete a cuticular
layer.

The mid-gut, or hypoblastic sac, very soon gives off numerous small
prolongations on each side of its hinder extremity, and these are
converted into the cæca of the liver (fig. 57, E, _mg_). The cells
of its tergal wall are in close contact with the adjacent masses of
food-yelk; and it is probable that the gradual absorption of the
food-yelk is chiefly effected by these cells. At birth, however, the
lateral lobes of the food-yelk are still large, and occupy the space
left between the stomach and liver {224} on the one hand, and the
cephalic integument on the other.

The mesoblastic cells give rise to the layer of connective tissue
which forms the deeper portion of the integument, and to that which
invests the alimentary canal; to all the muscles; and to the heart,
the vessels, and the corpuscles of the blood. The heart appears very
early as a solid mass of mesoblastic cells in the tergal region of
the thorax, just in front of the origin of the abdomen (figs. 57, 58,
59, _h_). It soon becomes hollow, and its walls exhibit rhythmical
contractions.

The branchiæ are, at first, simple papillæ of the integument of the
region from which they take their rise. These papillæ elongate into
stems, which give off lateral filaments. The podobranchiæ are at
first similar to the arthrobranchiæ, but an outgrowth soon takes
place near the free end of the stem, and becomes the lamina, while
the attached end enlarges into the base.

The renal organ is stated to arise by a tubular involution of the
epiblast, which soon becomes convoluted, and gives rise to the green
gland.

The central nervous system is wholly a product of the epiblast. The
cells which lie at the sides of the longitudinal groove already
mentioned (fig. 58, _mg_), grow inwards, and give rise to two cords
which are at first separate from one another and continuous with
the rest of the epiblast. At the front end of the groove a {225}
depression arises, and its cells form a mass which connects these
two cords in front of the mouth, and gives rise to the cerebral
ganglia. The epiblastic linings of two small pits (fig. 58, _o_)
which appear very early on the surface of the procephalic lobes,
are also carried inwards in the same way, and, uniting with the
foregoing, produce the optic ganglia.

The cells of the longitudinal cords become differentiated into nerve
fibres and nerve cells, and the latter, gathering towards certain
points, give rise to the ganglia which eventually unite in the middle
line. By degrees, the ingrowth of epiblastic cells, from which all
these structures are developed, becomes completely separated from
the rest of the epiblast, and is invested by mesoblastic cells. The
central nervous system, therefore, in a crayfish, as in a vertebrated
animal, is at first, as a part of the ectoderm, morphologically one
with the epidermis; and the deep and protected position which it
occupies in the adult is only a consequence of the mode in which the
nervous portion of the ectoderm grows inwards and becomes detached
from the epidermic portion.

The visual rods of the eye are merely modified cells of the ectoderm.
The auditory sac is formed by an involution of the ectoderm of the
basal joint of the antennule. At birth it is a shallow wide-mouthed
depression, and contains no otoliths.

Lastly, the reproductive organs result from the segregation and
special modification of cells of the mesoblast {226} behind the
liver. Rathke states that the sexual apertures are not visible until
the young crayfish has attained the length of an inch; and that the
first pair of abdominal appendages of the male appear still later in
the form of two papillæ, which gradually elongate and take on their
characteristic forms.

{227}



CHAPTER V.

THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH.—THE STRUCTURE AND THE
DEVELOPMENT OF THE CRAYFISH COMPARED WITH THOSE OF OTHER LIVING
BEINGS.


Up to this point, our attention has been directed almost exclusively
to the common English crayfish. Except in so far as the crayfish is
dependent for its maintenance upon other animals, or upon plants,
we might have ignored the existence of all living things except
crayfishes. But, it is hardly necessary to observe, that innumerable
hosts of other forms of life not only tenant the waters and the
dry land, but throng the air; and that all the crayfishes in the
world constitute a hardly appreciable fraction of its total living
population.

Common observation leads us to see that these multitudinous living
beings differ from not-living things in many ways; and when the
analysis of these differences is pushed as far as we are at present
able to carry it, it shews us that all living beings agree with the
crayfish and differ from not-living things in the same particulars.
Like the crayfish, they are constantly wasting away by {228}
oxidation, and repairing themselves by taking into their substance
the matters which serve them for food; like the crayfish, they shape
themselves according to a definite pattern of external form and
internal structure; like the crayfish, they give off germs which grow
and develop into the shapes characteristic of the adult. No mineral
matter is maintained in this fashion; nor grows in the same way; nor
undergoes this kind of development; nor multiplies its kind by any
such process of reproduction.

Again, common observation early leads to the discrimination of
living things into two great divisions. Nobody confounds ordinary
animals with ordinary plants, nor doubts that the crayfish belongs
to the former category and the waterweed to the latter. If a living
thing moves and possesses a digestive receptacle, it is held to be
an animal; if it is motionless and draws its nourishment directly
from the substances which are in contact with its outer surface,
it is held to be a plant. We need not inquire, at present, how far
this rough definition of the differences which separate animals
from plants holds good. Accepting it for the moment, it is obvious
that the crayfish is unquestionably an animal,—as much an animal
as the vole, the perch, and the pond-snail, which inhabit the same
waters. Moreover, the crayfish has, in common with these animals, not
merely the motor and digestive powers characteristic of animality,
but they all, like it, possess a complete alimentary canal; special
{229} apparatus for the circulation and the aëration of the blood;
a nervous system with sense-organs; muscles and motor mechanisms;
reproductive organs. Regarded as pieces of physiological apparatus,
there is a striking similarity between all three. But, as has already
been hinted in the preceding chapter, if we look at them from a
purely morphological point of view, the differences between the
crayfish, the perch, and the pond-snail, appear at first sight so
great, that it may be difficult to imagine that the plan of structure
of the first can have any relation to that of either of the last two.
On the other hand, if the crayfish is compared with the water-beetle,
notwithstanding wide differences, many points of similarity between
the two will manifest themselves; while, if a small lobster is set
side by side with a crayfish, an unpractised observer, though he will
readily see that the two animals are somewhat different, may be a
long time in making out the exact nature of the differences.

Thus there are degrees of likeness and unlikeness among animals, in
respect of their outward form and internal structure, or, in other
words, in their morphology. The lobster is very like a crayfish,
the beetle is remotely like one; the pond-snail and the perch
are extremely unlike crayfishes. Facts of this kind are commonly
expressed in the language of zoologists, by saying that the lobster
and the crayfish are closely allied forms; that the beetle and the
crayfish present a remote affinity; and that there is no affinity
between the {230} crayfish and the pond-snail, or the crayfish and
the perch.

The exact determination of the resemblances and differences of animal
forms by the comparison of the structure and the development of one
with those of another, is the business of comparative morphology.
Morphological comparison, fully and thoroughly worked out, furnishes
us with the means of estimating the position of any one animal in
relation to all the rest; while it shews us with what forms that
animal is nearly, and with what it is remotely, allied: applied to
all animals, it furnishes us with a kind of map, upon which animals
are arranged in the order of their respective affinities; or a
classification, in which they are grouped in that order. For the
purpose of developing the results of comparative morphology in the
case of the crayfish, it will be convenient to bring together, in a
summary form, those points of form and structure, many of which have
already been referred to and which characterise it as a separate kind
of animal.

       *       *       *       *       *

Full-grown English crayfishes usually measure about three inches and
a half from the extremity of the rostrum in front to that of the
telson behind. The largest specimen I have met with measured four
inches.[13] The {231} males are commonly somewhat larger, and they
almost always have longer and stronger forceps than the females. The
general colour of the integument varies from a light reddish-brown
to a dark olive-green; and the hue of the tergal surface of the body
and limbs is always deeper than that of the sternal surface, which is
often light yellowish-green, with more or less red at the extremities
of the forceps. The greenish hue of the sternal surface occasionally
passes into yellow in the thorax and into blue in the abdomen.

     [13] The dimensions of crayfishes at successive ages given
     at p. 31, beginning at the words “By the end of the year,”
     refer to the “écrevisse à pieds rouges” of France; not to the
     English crayfish, which is considerably smaller. Doubtless, the
     proportional rate of increment is much the same, in the two
     kinds; but in the English crayfish it has not been actually
     ascertained.

The distance from the orbit to the posterior margin of the carapace
is nearly equal to that from the posterior margin of the carapace
to the base of the telson, when the abdomen is fully extended, but
this measurement of the carapace is commonly greater than that of the
abdomen in the males and less in the females.

The general contour of the carapace (fig. 61), without the rostrum,
is that of an oval, truncated at the ends: the anterior end being
narrower than the posterior. Its surface is evenly arched from side
to side. The greatest breadth of the carapace lies midway between the
cervical groove and its posterior edge. Its greatest vertical depth
is on a level with the transverse portion of the cervical groove.

The length of the rostrum, measured from the orbit {232} to its
extremity, is greater than half the distance from the orbit to the
cervical groove. It is trihedral in section, and its free end is
slightly curved upwards (fig. 41). It gradually becomes narrower for
about three-fourths of its whole length. At this point it has rather
less than half the width which it has at its base (fig. 61, A); and
its raised, granular and sometimes distinctly serrated margins are
produced into two obliquely directed spines, one on each side. Beyond
these, the rostrum rapidly narrows to a fine point; and this part of
the rostrum is equal in length to the width between the two spines.

The tergal surface of the rostrum is flattened and slightly excavated
from side to side, except in its anterior half, where it presents a
granular or finely serrated median ridge, which gradually passes into
a low elevation in the posterior half, and, as such, may generally be
traced on to the cephalic region of the carapace. The inclined sides
of the rostrum meet ventrally in a sharp edge, convex from before
backwards; the posterior half of this edge gives rise to a small,
usually bifurcated, spine, which descends between the eye-stalks
(fig. 41). The raised and granulated lateral margins of the rostrum
are continued back on to the carapace for a short distance, as two
linear ridges (fig. 61, A). Parallel with each of these ridges, and
close to it, there is another longitudinal elevation (_a, b_), the
anterior end of which is raised into a prominent spine (_a_), which
is situated immediately behind the orbit, and may, therefore, be
termed the _post-orbital_ {233} _spine_. The elevation itself may
be distinguished as the _post-orbital ridge_. The flattened surface
of this ridge is marked by a longitudinal depression or groove. The
{234} posterior end of the ridge passes into a somewhat broader and
less marked elevation, the hinder end of which turns inwards, and
then comes to an end at a point midway between the orbit and the
cervical groove. Generally this hinder elevation appears like a mere
continuation of the post-orbital ridge; but, sometimes, the two are
separated by a distinct depression. I have never seen any prominent
spine upon the posterior elevation, though it is sometimes minutely
spinulose. The post-orbital ridges of each side, viewed together,
give rise to a characteristic lyrate mark upon the cephalic region of
the carapace.

[Illustration: FIG. 61.—A, D, & G, _Astacus torrentium_; B, E, & H,
_A. nobilis_; C, F, & I, _A. nigrescens_ (nat. size). A–C, Dorsal
views of carapace; D–F, side views of third abdominal somites; G–I,
Dorsal views of telson. _a, b_, post-orbital ridge and spines; _c_,
branchio-cardiac grooves inclosing the areola.]

A faintly marked, curved, linear depression runs from the hinder end
of the post-orbital ridge, at first directly downwards, and then
curves backwards to the cervical groove. It corresponds with the
anterior and inferior boundary of the attachment of the adductor
muscle of the mandible.

Below the level of this, and immediately behind the cervical groove,
there are usually three spines, arranged in a series, which follow
the cervical groove. The points of all are directed obliquely
forwards, and the lowest is the largest. Sometimes there is only one
prominent spine, with one or two very small ones; sometimes there are
as many as five of these _cervical spines_.

The cardiac region is marked out by two grooves which run backwards
from the cervical groove (fig. 61, A, _c_), and terminate at a
considerable distance from the posterior {235} edge of the carapace.
Each groove runs, at first, obliquely inwards, and then takes a
straight course parallel with its fellow. The area thus defined is
termed the _areola_; its breadth is equal to about one-third of the
total transverse diameter of the carapace in this region.

No such distinct lines indicate the lateral boundary of the region
in front of the cervical groove which answers to the stomach. But
the middle part of the carapace, or that which is comprised in
the gastric and cardiac regions, has its surface sculptured in a
different way from the branchiostegites and the lateral regions of
the head. In the former, the surface is excavated by shallow pits,
separated by relatively broad flat-topped ridges; but, in the latter,
the ridges become more prominent, and take the form of tubercles, the
apices of which are directed forwards. Minute setæ spring from the
depressions between these tubercles.

The branchiostegite has a thickened rim, which is strongest below and
behind (fig. 1). The free edge of this rim is fringed with close-set
setæ.

The pleura of the second to the sixth abdominal somites are broadly
lanceolate and obtusely pointed at their free ends (fig. 61, D); the
anterior edge is longer and more convex than the posterior edge. In
the females, the pleura are larger, and are directed more outwards
and less downwards than in the males. The pleura of the second somite
are much larger than the rest, and overlap the very small pleura of
the first somite (fig. 1). The {236} pleura of the sixth somite are
narrow, and their posterior edges are concave.

The pits and setæ of the cuticle which clothes the tergal surfaces
of the abdominal somites are so few and scattered, that the latter
appear almost smooth. In the telson, however, especially in its
posterior division, the markings are coarser and the setæ more
apparent.

The telson (fig. 61, G) presents an anterior quadrate division and
a posterior half-oval part, the free curved edge of which is beset
with long setæ, and is sometimes slightly notched in the middle.
The posterior division is freely movable upon the anterior, in
consequence of the thinness and pliability of the cuticle along
a transverse line which joins the postero-external angles of the
anterior division, each of which is produced into two strong spines,
of which the outer is the longer. The length of the posterior
division of the telson, measured from the middle of the suture, is
equal to, or but very little less than, that of the anterior division.

On the under side of the head, the basal joints of the antennules are
visible, internal to those of the antennæ, but the attachment of the
latter is behind and below that of the former (fig. 3, A). Behind
these, and in front of the mouth, the epistoma (fig. 39, A, II, III)
presents a broad area of a pentagonal form. The posterior boundary of
this area is formed by two thickened transverse ridges, which meet
on the middle line at a very open angle, the apex of which is turned
forwards. {237} The posterior edges of these ridges are continuous
with the labrum. The anterior margin is produced in the middle into a
_fleur de lys_ shaped process, the summit of which terminates between
the antennules. At the sides of this process, the anterior margin
of the epistoma is deeply excavated to receive the basal joints of
the antennæ. Following the contours of these excavated margins, the
surface of the epistoma presents two lateral convexities. The widest
and most prominent part of each of these lies towards the outer edge
of the epistoma, and is produced into a conical spine. Sometimes
there is a second smaller spine beside the principal one. Between the
two convexities lies a triangular median depressed area.

The distance from the apex of the anterior median process to the
posterior ridge is equal to a little more than half the width of the
epistoma.

The corneal surface of the eye is transversely elongated and
reniform, and its pigment is black. The eye-stalks are much broader
at their bases than at their corneal ends (fig. 48, A). The
antennules are about twice as long as the rostrum. The tergal surface
of the trihedral basal joint of the antennule, on which the eye-stalk
rests, is concave; the outer surface is convex, the inner flat
(figs. 26, A, and 48, B). Near the anterior end of the sternal edge
which separates the two latter faces, there is a strong curved spine
directed forwards (fig. 48, B, _a_). When the setæ, which proceed
from the outer edge of {238} the auditory aperture and hide it, are
removed, it is seen to be a wide, somewhat triangular cleft, which
occupies the greater part of the hinder half of the tergal surface of
the basal joint (fig. 26, A).

The exopodites, or squames, of the antennæ extend as far as the apex
of the rostrum, or even project beyond it, when they are turned
forwards, while they reach to the commencement of the filament of
the endopodite (_Frontispiece_). The squame is fully twice as long
as it is broad, with a general convexity of its tergal and concavity
of its sternal surface. The outer edge is straight and thick, the
inner, which is fringed with long setæ, is convex and thin (fig. 48,
C). Where these two edges join in front, the squame is produced into
a strong spine. A thick outer portion of the squame is marked off
from the thinner inner portion by a longitudinal groove on the tergal
side, and by a strong ridge on the sternal side. One or two small
spines generally project from the posterior and external angle of the
squame; but they may be very small or absent in individual specimens.
Close beneath these, the outer angle of the next joint is produced
into a strong spine. When the abdomen is straightened out, if the
antennæ are turned back as far as they will go without damage, the
ends of their filaments usually reach the tergum of the third somite
of the abdomen (_Frontispiece_). I have not observed any difference
between the sexes in this respect.

The inner edge of the ischiopodite of the third {239} maxillipede
is strongly serrated and wider in front than behind (fig. 44); the
meropodite possesses four or five spines in the same region; and
there are one or two spines at the distal end of the carpopodite.
When straightened out, the maxillipedes extend as far as, or even
beyond, the end of the rostrum.

The inner or sternal edge of the ischiopodite of the forceps is
serrated; that of the meropodite presents two rows of spines, the
inner small and numerous, the outer large and few. There are several
strong spines at the anterior end of the outer or tergal face of this
joint. The carpopodite has two strong spines on its under or sternal
surface, while its sharp inner edge presents many strong spines. Its
upper surface is marked by a longitudinal depression, and is beset
with sharp tubercles. The length of the propodite, from its base to
the extremity of the fixed claw of the chela, measures rather more
than twice as much as the extreme breadth of its base, the thickness
of which is less than a third of this length (fig. 20, p. 93). The
external angular process, or fixed claw, is of the same length as the
base, or a little shorter. Its inner edge is sharp and spinose, and
the outer more rounded and simply tuberculated. The apex of the fixed
claw is produced into a slightly incurved spine. Its inner edge has a
sinuous curvature, convex posteriorly, concave anteriorly, and bears
a series of rounded tubercles, of which one near the summit of the
convexity, and one near the apex of the claw are the most prominent.
{240}

The apex of the dactylopodite, like that of the propodite, is
formed by a slightly incurved spine (fig. 20), while its outer,
sharper, edge presents a curvature, the inverse of that of the edge
of the fixed claw against which it is applied. This edge is beset
with rounded tubercles, the most prominent of which are one at the
beginning, and one at the end of the concave posterior moiety of the
edge. When the dactylopodite is brought up to the fixed claw, these
tubercles lie, one in front of and one behind the chief tubercle of
the convexity of the latter. The whole surface of the propodite and
dactylopodite is covered with minute elevations, those of the upper
surface being much more prominent than those of the lower surface.

The length of the fully extended forceps generally equals the
distance between the posterior margin of the orbit and the base of
the telson, in well characterized males; and, in individual examples,
they are even longer; while it may not be greater than the distance
between the orbit and the hinder edge of the fourth abdominal somite,
in females; and, in massiveness and strength, the difference of the
forceps in the two sexes is still more remarkable (fig. 2). Moreover
there is a good deal of variation in the form and size of the chelæ
in individual males. The right and left chelæ present no important
differences.

The ischiopodites of the four succeeding thoracic limbs are devoid
of any recurved spines in either sex (_Front._, fig. 46). The first
pair are the stoutest, the second the {241} longest: and when the
latter are spread out at right angles to the body, the distance from
tip to tip of the dactylopodites is equal to, or rather greater
than, the extreme length of the body from the apex of the rostrum to
the posterior edge of the telson, in both sexes. In both sexes, the
length of the swimmerets hardly exceeds half the transverse diameter
of the somites to which they are attached.

The exopodites of the appendages of the sixth abdominal somite (the
extreme length of which is rather greater than that of the telson)
are divided into a larger proximal, and a smaller distal portion
(fig. 37, F, p. 144). The latter is about half as long as the former,
and has a rounded free edge, setose like that of the telson. There
is a complete flexible hinge between the two portions, and the
overlapping free edge of the proximal portion, which is slightly
concave, is beset with conical spines, the outermost of which are
the longest. The endopodite has a spine at the junction of its outer
straight edge with the terminal setose convex edge. A faintly marked
longitudinal median ridge, or keel, ends close to the margin in a
minute spine. The tergal distal edge of the protopodite is deeply
bilobed, and the inner lobe ends in two spines, while the outer,
shorter and broader lobe, is minutely serrated.

In addition to the characters distinctive of sex, which have
already been fully detailed (pp. 7, 20, and 145), there is a marked
difference in the form of the sterna of the three posterior thoracic
somites between the males and females. {242} Comparing a male and
a female of the same size, the triangular area between the bases of
the penultimate and ante-penultimate thoracic limbs is considerably
broader at the base in the female. In both sexes, the hinder part of
the penultimate sternum is a rounded transverse ridge separated by a
groove from the anterior part; but this ridge is much larger and more
prominent in the female than in the male, and it is often obscurely
divided into two lobes by a median depression. Moreover, there are
but few setæ on this region in the female; while, in the male, the
setæ are long and numerous.

The sternum of the last thoracic somite of the female is divided by a
transverse groove into two parts, of which the posterior, viewed from
the sternal aspect, has the form of a transverse elongated ridge,
which narrows to each end, is moderately convex in the middle, and
is almost free from setæ. In the male, the corresponding posterior
division of the last thoracic sternum is produced downwards and
forwards into a rounded eminence which gives attachment to a sort of
brush of long setæ (fig. 35, p. 136).

The importance of this long enumeration of minute details[14] will
appear by and by. It is simply a statement of the more obvious
external characters in which all the full-grown English crayfishes
which have come under my {243} notice agree. No one of these
individual crayfishes was exactly like the other; and to give an
account of any single crayfish as it existed in nature, its special
peculiarities must be added to the list of characters given above;
which, considered together with the facts of structure discussed
in previous chapters, constitutes a definition, or diagnosis, of
the English kind, or _species_, of crayfish. It follows that the
species, regarded as the sum of the morphological characters in
question and nothing else, does not exist in nature; but that it is
an abstraction, obtained by separating the structural characters in
which the actual existences—the individual crayfishes—agree, from
those in which they differ, and neglecting the latter.

     [14] The student of systematic zoology will find the comparison
     of a lobster with a crayfish in all the points mentioned to be
     an excellent training of the faculty of observation.

A diagram, embodying the totality of the structural characters
thus determined by observation to be common to all our crayfishes,
might be constructed; and it would be a picture of nothing which
ever existed in nature; though it would serve as a very complete
plan of the structure of all the crayfishes which are to be found
in this country. The morphological definition of a species is, in
fact, nothing but a description of the plan of structure which
characterises all the individuals of that species.

       *       *       *       *       *

California is separated from these islands by a third of the
circumference of the globe, one-half of the interval being occupied
by the broad North Atlantic ocean. The fresh waters of California,
however, contain crayfishes which are {244} so like our own, that
it is necessary to compare the two in every point mentioned in
the foregoing description in order to estimate the value of the
differences which they present. Thus, to take one of the kinds of
crayfishes found in California, which has been called _Astacus
nigrescens_; the general structure of the animal may be described
in precisely the same terms as those used for the English crayfish.
Even the branchiæ present no important difference, except that the
rudimentary pleurobranchiæ are rather more conspicuous; and that
there is a third small one, in front of the two which correspond with
those possessed by the English crayfish.

The Californian crayfish is larger and somewhat differently coloured,
the undersides of the forceps particularly presenting a red hue.
The limbs, and especially the forceps of the males, are relatively
longer; the chelæ of the forceps have more slender proportions; the
areola is narrower relatively to the transverse diameter of the
carapace (fig. 61, C). More definite distinctions are to be found in
the rostrum, which is almost parallel-sided for two-thirds of its
length, then gives off two strong lateral spines and suddenly narrows
to its apex. Behind these spines, the raised lateral edges of the
rostrum present five or six other spines which diminish in size from
before backwards. The postorbital spine is very prominent, but the
ridge is represented, in front, by the base of this spine, which is
slightly grooved; and behind, by a distinct spine which is not so
strong as the postorbital spine. {245} There are no cervical spines,
and the middle part of the cervical groove is angulated backwards
instead of being transverse.

[Illustration: FIG. 62. A & D, _Astacus torrentium_; B & E, _A.
nobilis_; C & F, _A. nigrescens_. A–C, 1st abdominal appendage of the
male; D–F, endopodite of second appendage (× 3). _a_, anterior, and
_b_, posterior rolled edge; _c_, _d_, _e_, corresponding parts of the
appendages in each species; _f_, rolled plate of endopodite; _g_,
terminal division of endopodite.]

The abdominal pleura are narrow, equal-sided, and acutely pointed in
the males (fig. 61, F)—slightly broader, more obtuse, and with the
anterior edges {246} rather more convex than the posterior, in the
females. The tergal surface of the telson is not divided into two
parts by a suture (fig. 61, I). The anterior process of the epistoma
is of a broad rhomboidal shape, and there are no distinct lateral
spines.

The squame of the antenna is not so broad relatively to its length;
its inner edge is less convex, and its outer edge is slightly
concave; the outer basal angle is sharp but not produced into a
spine. The opposed edges of the fixed and movable claws of the
chelæ of the forceps are almost straight and present no conspicuous
tubercles. In the males, the forceps are vastly larger than in the
females, and the two claws of the chelæ are bowed out, so that a
wide interval is left when their apices are applied together; in the
females, the claws are straight and the edges fit together, leaving
no interval. Both the upper and the under surfaces of the claws
are almost smooth. The median ridge of the endopodite of the sixth
abdominal appendage is more marked, and ends close to the margin in a
small prominent spine.

In the females, the posterior division of the sternum of the
penultimate thoracic somite is prominent and deeply bilobed; and
there are some small differences in form in the abdominal appendages
of the males. More especially, the rolled inner process of the
endopodite of the second appendage (fig. 62 F, _f_) is disposed very
obliquely, and its open mouth is on a level with the base of the
jointed part of the endopodite (_g_) instead of reaching almost to
{247} the free end of the latter and being nearly parallel with
it. In the first appendage (C), the anterior rolled edge (_a_)
more closely embraces the posterior (_b_), and the groove is more
completely converted into a tube.

       *       *       *       *       *

It will be observed that the differences between the English and
the Californian crayfishes amount to exceedingly little; but, on
the assumption that these differences are constant, and that no
transitional forms between the English and the Californian crayfishes
are to be met with, the individuals which present the characteristic
peculiarities of the latter are said to form a distinct species,
_Astacus nigrescens_; and the definition of that species is, like
that of the English species, a morphological abstraction, embodying
an account of the plan of that species, so far as it is distinct from
that of other crayfishes.

We shall see by and by that there are sundry other kinds of
crayfishes, which differ no more from the English or the Californian
kinds, than these do from one another; and, therefore, they are all
grouped as species of the one genus, _Astacus_.

[Illustration: FIG. 63. _Cambarus Clarkii_, male (1/2 nat. size),
after Hagen.]

If, leaving California, we cross the Rocky Mountains and enter the
eastern States of the North American Union, many sorts of crayfishes,
which would at once be recognised as such by any English visitor,
will be found to be abundant. But on careful examination it will be
discovered that all of these differ, both from the English crayfish,
and from _Astacus nigrescens_, to a much greater {248} extent
than those do from one another. The gills are, in fact, reduced
to seventeen on each side, in consequence of the absence of the
pleuro-branchia of the last thoracic somite; and there are some other
differences to which it is not needful to refer at present. It is
convenient to {249} distinguish these seventeen-gilled crayfishes,
as a whole, from the eighteen-gilled species; and this is effected by
changing the generic name. They are no longer called _Astacus_, but
_Cambarus_ (fig. 63).

All the individual crayfish referred to thus far, therefore, have
been sorted out, first into the groups termed _species_; and then
these species have been further sorted into two divisions, termed
_genera_. Each genus is an abstraction, formed by summing up the
common characters of the species which it includes, just as each
species is an abstraction, composed of the common characters of the
individuals which belong to it; and the one has no more existence
in nature than the other. The definition of the genus is simply a
statement of the plan of structure which is common to all the species
included under that genus; just as the definition of the species is a
statement of the common plan of structure which runs throughout the
individuals which compose the species.

[Illustration: FIG. 64.—_Parastacus brasiliensis_ (1/2 nat. size).
From southern Brazil.]

Again, crayfishes are found in the fresh waters of the Southern
hemisphere; and almost the whole of what has been said respecting the
structure of the English crayfish applies to these; in other words,
their general plan is the same. But, in these southern crayfishes,
the podobranchiæ have no distinct lamina, and the first somite of
the abdomen is devoid of appendages in both sexes. The southern
crayfishes, like those of the Northern hemisphere, are divisible
into many species; and these species {250} are susceptible of
being grouped into six genera—_Astacoides_ (fig. 65), _Astacopsis_,
_Chæraps_, _Parastacus_ (fig. 64), _Engæus_, and _Paranephrops_—on
the same principle as that which has led to the grouping of the
Northern forms into two genera. But the same convenience which has
{252} led to the association of groups of similar species into
genera, has given rise to the combination of allied genera into
higher groups, which are termed _Families_. It is obvious that the
definition of a family, as a statement of the characters in which a
certain number of genera agree, is another morphological abstraction,
which stands in the same relation to generic, as generic do to
specific abstractions. Moreover, the definition of the family is a
statement of the plan of all the genera comprised in that family.

[Illustration: FIG. 65.—_Astacoides madagascarensis_ (2/3 nat. size).
From Madagascar.]

The family of the Northern crayfishes is termed _Potamobiidæ_; that
of the Southern crayfishes, _Parastacidæ_. But these two families
have in common all those structural characters which are special
to neither; and, carrying out the metaphorical nomenclature of the
zoologist a stage further, we may say that the two form a _Tribe_—the
definition of which describes the plan which is common to both
families.

[Illustration: FIG. 66.—Diagram of the morphological relations of the
Astacina.]

It may conduce to intelligibility if these results are put into a
graphic form. In fig. 66, A. is a diagram representing the plan
of an animal in which all the externally visible parts which are
found, more or less modified, in the natural objects which we call
individual crayfishes are roughly sketched. It represents the plan
of the tribe. B. is a diagram exhibiting such a modification of
A. as converts it into the plan common to the whole family of the
_Parastacidæ_. C. stands in the same relation to the _Potamobiidæ_.
If the scheme were thoroughly worked out, diagrams representing the
peculiarities of {254} form which characterize each of the genera
and species, would appear in the place of the names of the former, or
of the circles which represent the latter. All these figures would
represent abstractions—mental images which have no existence outside
the mind. Actual facts would begin with drawings of individual
animals, which we may suppose to occupy the place of the dots above
the upper line in the diagram.

That all crayfishes may be regarded as modifications of the common
plan A, is not an hypothesis, but a generalization obtained by
comparing together the observations made upon the structure of
individual crayfishes. It is simply a graphic method of representing
the facts which are commonly stated in the form of a definition of
the tribe of crayfishes, or _Astacina_.

This definition runs as follows:—

Multicellular animals provided with an alimentary canal and with a
chitinous cuticular exoskeleton; with a ganglionated central nervous
system traversed by the œsophagus; possessing a heart and branchial
respiratory organs.

The body is bilaterally symmetrical, and consists of twenty metameres
(or somites and their appendages), of which six are associated into
a head, eight into a thorax, and six into an abdomen. A telson is
attached to the last abdominal somite.

The somites of the abdominal region are all free, those of the head
and thorax, except the hindermost, which is {255} partially free,
are united into a cephalothorax, the tergal wall of which has the
form of a continuous carapace. The carapace is produced in front into
a rostrum, at the sides into branchiostegites.

The eyes are placed at the ends of movable stalks. The antennules
are terminated by two filaments. The exopodite of the antenna has
the form of a mobile scale. The mandible has a palp. The first and
second maxillæ are foliaceous; the second being provided with a
large scaphognathite. There are three pairs of maxillipedes, and the
endopodites of the third pair are narrow and elongated. The next pair
of thoracic appendages is much larger than the rest, and is chelate,
as are the two following pairs, which are slender ambulatory limbs.
The hindmost two pairs of thoracic appendages are ambulatory limbs,
like the foregoing, but not chelate. The abdominal appendages are
small swimmerets, except the sixth pair, which are very large, and
have the exopodite divided by a transverse joint.

All the crayfishes have a complex gastric armature. The seven
anterior thoracic limbs are provided with podobranchiæ, but the first
of these is always more or less completely reduced to an epipodite.
More or fewer arthrobranchiæ always exist. Pleurobranchiæ may be
present or absent.

In this tribe of _Astacina_ there are two families, the _Potamobiidæ_
and the _Parastacidæ_; and the definition of each of these families
is formed by superadding to the {256} definition of the tribe the
statement of the special peculiarities of the family.

Thus, the _Potamobiidæ_ are those _Astacina_ in which the
podobranchiæ of the second, fourth, fifth, and sixth thoracic
appendages are always provided with a plaited lamina, and that of
the first is an epipodite devoid of branchial filaments. The first
abdominal somite invariably bears appendages in the males, and
usually in both sexes. In the males these appendages are styliform,
and those of the second somite are always peculiarly modified. The
appendages of the four following somites are relatively small. The
telson is very generally divided by a transverse incomplete hinge.
None of the branchial filaments are terminated by hooks; nor are
any of the coxopoditic setæ, or the longer setæ of the podobranchiæ
hooked, though hooked tubercles occur on the stem and on the laminæ
of the latter. The coxopoditic setæ are always long and tortuous.

In the _Parastacidæ_, on the other hand, the podobranchiæ are
devoid of more than a rudiment of a lamina, though the stem may be
alate. The podobranchia of the first maxillipede has the form of
an epipodite; but, in almost all cases, it bears a certain number
of well developed branchial filaments. The first abdominal somite
possesses no appendages in either sex: and the appendages of the
four following somites are large. The telson is never divided by a
transverse hinge. More or fewer of the branchial filaments of the
{257} podobranchiæ are terminated by short hooked spines; and the
coxopoditic setæ, as well as those which beset the stems of the
podobranchiæ, have hooked apices.

The definitions of the genera would in like manner be given by adding
the distinctive characters of each genus to the definitions of the
family; and those of the species by adding its character to those
of the genus. But at present it is unnecessary to pursue this topic
further.

       *       *       *       *       *

There are no other inhabitants of the fresh waters, or of the land,
which could be mistaken for crayfishes; but certain marine animals,
familiar to every one, are so strikingly similar to them, that one
of these was formerly included in the same genus, _Astacus_; while
another is very often known as the “Sea-crayfish.” These are the
“Common Lobster,” the “Norway Lobster,” and the “Rock Lobster” or
“Spiny Lobster.”

The common lobster (_Homarus vulgaris_, fig. 67) presents the
following distinctive characters. The last thoracic somite is firmly
adherent to the rest; the exopodite of the antenna is so small as to
appear like a mere movable scale; all the abdominal appendages are
well developed in both sexes; and, in the males, the two anterior
pairs are somewhat like those of the male _Astacus_, but less
modified.

[Illustration: FIG. 67. _Homarus vulgaris_ (1/3 nat. size).]

The principal difference from the _Astacina_ is exhibited by
the gills, of which there are twenty on each side; namely, six
podobranchiæ, ten arthrobranchiæ, and four {259} fully developed
pleurobranchiæ. Moreover, the branchial filaments of these gills
are much stiffer and more closely set than in most crayfishes. But
the most important distinction is presented by the podobranchiæ,
in which the stem is, as it were, completely split into two parts
longitudinally (as in fig. 68, B); one half (_ep_) corresponding with
the lamina of the crayfish gill, and the other (_pl_) with its plume.
Hence the base (_b_) of the podobranchia bears the gill in front;
while, behind, it is continued into a broad epipoditic plate (_ep_)
slightly folded upon itself longitudinally but not plaited, as in the
crayfish.

[Illustration: FIG. 68. Podobranchiæ of A, _Parastacus_; B,
_Nephrops_; C, _Palæmon_. A′, C′, transverse sections of A and C
respectively. _a_, point of attachment; _al_, wing-like expansion of
the stem; _b_, base; _br_, branchial filaments; _ep_, epipodite; _l_,
branchial laminæ; _pl_, plume; _st_, stem.]

[Illustration: FIG. 69. _Nephrops norvegicus_ (1/2 nat. size).]

The Norway Lobster (_Nephrops norvegicus_, fig. 69) {261} resembles
the lobster in those respects in which the latter differs from the
crayfishes: but the antennary squame is large; and, in addition, the
branchial plume of the podobranchia of the second maxillipede is very
small or absent, so that the total number of functional branchiæ is
reduced to nineteen on each side.

These two genera, _Homarus_ and _Nephrops_, therefore, represent a
family, _Homarina_, constructed upon the same common plan as the
crayfishes, but differing so far from the _Astacina_ in the structure
of the branchiæ and in some other points, that the distinction must
be expressed by putting them into a different tribe. It is obvious
that the special characteristics of the plan of the _Homarina_ give
it much more likeness to that of the _Potamobiidæ_ than to that of
the _Parastacidæ_.

The Rock Lobster (_Palinurus_, fig. 70) differs much more from the
crayfishes than either the common lobster or the Norway lobster does.
Thus, to refer only to the more important distinctions, the antennæ
are enormous; none of the five posterior pairs of thoracic limbs
are chelate, and the first pair are not so large in proportion to
the rest as in the crayfishes and lobsters. The posterior thoracic
sterna are very broad, not comparatively narrow, as in the foregoing
genera. There are no appendages to the first somite of the abdomen
in either sex. In this respect, it is curious to observe that, in
contradistinction from the _Homarina_, the Rock Lobsters are more
closely allied to the _Parastacidæ_ than to the _Potamobiidæ_. {263}
The gills are similar to those of the lobsters, but reach the
number of twenty-one on each side.

[Illustration: FIG. 70. _Palinurus vulgaris_ (about 1/4 nat. size).]

In their fundamental structure the rock lobsters agree with
the crayfishes; hence the plans of the two may be regarded as
modifications of a plan common to both. To this end, the only
considerable changes needful in the tribal plan of the crayfishes,
are the substitution of simple for chelate terminations to the middle
thoracic limbs and the suppression of the appendages of the first
somite of the abdomen.

Thus not only all the crayfishes, but all the lobsters and rock
lobsters, different as they are in appearance, size, and habits of
life, reveal to the morphologist unmistakable signs of a fundamental
unity of organization; each is a comparatively simple variation of
the general theme—the common plan.

Even the branchiæ, which vary so much in number in different members
of these groups, are constructed upon a uniform principle, and the
differences which they present are readily intelligible as the result
of various modifications of one and the same primitive arrangement.

In all, the gills are _trichobranchiæ_; that is, each gill is
somewhat like a bottle-brush, and presents a stem beset, more or
less closely, with many series of branchial filaments. The largest
number of complete branchiæ possessed by any of the _Potamobiidæ_,
_Parastacidæ_, _Homaridæ_, or _Palinuridæ_, is twenty-one on each
side; {264} and when this number is present, the total is made up of
the same numbers of podobranchiæ, arthrobranchiæ, and pleurobranchiæ
attached to corresponding somites. In _Palinurus_ and in the genus
_Astacopsis_ (which is one of the _Parastacidæ_), for example, there
are six podobranchiæ attached to the thoracic limbs from the second
to the seventh inclusively; five pairs of arthrobranchiæ are attached
to the interarticular membranes of the thoracic limbs from the third
to the seventh inclusively, and one to that of the second, making
eleven in all; while four pleurobranchiæ are fixed to the epimera of
the four hindmost thoracic somites. Moreover, in _Astacopsis_, the
epipodite of the first thoracic appendage (the first maxillipede)
bears branchial filaments, and is a sort of reduced gill.

These facts may be stated in a tabular form as follows:—

 _The branchial formula of Astacopsis._

 Somites and                   Arthrobranchiæ.
 their        Podobranchiæ.   /───────^───────\     Pleuro-
 Appendages.                 Anterior. Posterior.  branchiæ.

  VII.         0 (ep. r.)      0         0         0       =   0 (ep. r.)

 VIII.         1               1         0         0       =   2

   IX.         1               1         1         0       =   3

    X.         1               1         1         0       =   3

   XI.         1               1         1         1       =   4

  XII.         1               1         1         1       =   4

 XIII.         1               1         1         1       =   4

  XIV.         0               0         0         1       =   1
              ──              ──         ──       ──          ──
               6 + ep. r.  +   6    +     5   +    4       =  21 + ep. r.

{265}

This tabular “branchial formula” exhibits at a glance not only the
total number of branchiæ, but that of each kind of branchia; and that
of all kinds connected with each somite; and it further indicates
that the podobranchia of the first thoracic somite has become so far
modified, that it is represented only by an epipodite, with branchial
filaments scattered upon its surface.

In _Palinurus_, these branchial filaments are absent and the
branchial formula therefore becomes—

 Somites and                     Arthrobranchiæ.
 their          Podobranchiæ.   /───────^───────\    Pleuro-
 Appendages.                   Anterior. Posterior.  branchiæ.

  VII.           0 (ep.)        0         0           0       =   0 (ep.)

 VIII.           1              1         0           0       =   2

   IX.           1              1         1           0       =   3

    X.           1              1         1           0       =   3

   XI.           1              1         1           1       =   4

  XII.           1              1         1           1       =   4

 XIII.           1              1         1           1       =   4

  XIV.           0              0         0           1       =   1
                ──             ──        ──          ──          ──
                 6 + ep.   +    6    +    5     +     4       =  21 + ep.

In the lobster, the solitary arthrobranchia of the eighth somite
disappears, and the branchiæ are reduced to twenty on each side.

In _Astacus_, this branchia remains; but, in the English crayfish,
the most anterior of the pleurobranchiæ has vanished, and mere
rudiments of the two next remain. It has been mentioned that other
_Astaci_ present a rudiment of the first pleurobranchia. {266}

 _The branchial formula of Astacus._

 Somites and                 Arthrobranchiæ
 their      Podobranchiæ.  /───────^───────\     Pleuro-
 Appendages.              Anterior. Posterior.  branchiæ.

  VII.        0 (ep.)        0          0         0              = 0 (ep.)

 VIII.        1              1          0         0              = 2

   IX.        1              1          1         0              = 3

    X.        1              1          1         0              = 3

   XI.        1              1          1         0 or _r_       = 3 or 3 + _r_

  XII.        1              1          1         _r_            = 3 + _r_

 XIII.        1              1          1         _r_            = 3 + _r_

  XIV.        0              0          0         1              = 1
             ──             ──         ──        ──               ──
              6 + ep.   +    6    +     5    +    1 + 2 or 3_r_ = 18 + ep. + 2 or 3_r_.

In _Cambarus_, the number of the branchiæ is reduced to seventeen by
the disappearance of the last pleurobranchia; while, in _Astacoides_,
the process of reduction is carried so far, that only twelve complete
branchiæ are left, the rest being either represented by mere
rudiments, or disappearing altogether.

 _The branchial formula of Astacoides._

 Somites and                      Arthrobranchiæ
 their           Podobranchiæ.   /───────^───────\       Pleuro-
 Appendages.                    Anterior. Posterior.    branchiæ.


  VII.             0 (ep. r.)     0          0           0       = 0 (cp. r.)

 VIII.             1              _r_        0           0       = 1 + _r_

   IX.             1              1          0           0       = 2

    X.             1              1          _r_         0       = 2 + _r_

   XI.             1              1          _r_         0       = 2 + _r_

  XII.             1              1          _r_         0       = 2 + _r_

 XIII.             1              1          _r_         0       = 2 + _r_

  XIV.             0              0          0           1       = 1

                   6 + _ep. r_    5 + _r_ +  0 + 4 _r_ + 1       = 12 + _ep. r._ + 5 _r_.

{267}

As these formulæ show, those trichobranchiate crustacea, which
possess fewer than twenty-one complete branchiæ on each side,
commonly present traces of the missing ones, either in the shape
of epipodites, as in the case of the podobranchiæ, or of minute
rudiments, in the case of the arthrobranchiæ and the pleurobranchiæ.

In the marine, prawn-like, genus Penæus (fig. 73, Chap. VI.), the
gills are curiously modified trichobranchiæ. The number of functional
branchiæ is, as in the lobster, twenty; but the study of their
disposition shows that the total is made up in a very different way.

 _The branchial formula of Penæus._

 Somites and                 Arthrobranchiæ.
   their                    /───────^───────\     Pleuro-
 Appendages. Podobranchiæ. Anterior. Posterior.   branchiæ.

  VII.         0 (ep.)         1         0          0    =    1 + ep.

 VIII.         0 (ep.)         1         1          1    =    3 + ep.

   IX.         0 (ep.)         1         1          1    =    3 + ep.

    X.         0 (ep.)         1         1          1    =    3 + ep.

   XI.         0 (ep.)         1         1          1    =    3 + ep.

  XII.         0 (ep.)         1         1          1    =    3 + ep.

 XIII.         0               1         1          1    =    3

  XIV.         0               0         0          1    =    1
              ──              ──        ──         ──        ──
               0 + 6 ep.  +    7    +    6      +   7    =   20 + 6 ep.

This case is very interesting; for it shows that the whole of the
podobranchiæ may lose their branchial character, and be reduced
to epipodites, as is the case with the first in the crayfish and
lobster, and indeed in most of the forms under consideration. And
since all but one of the somites bear both arthrobranchiæ and
pleurobranchiæ, {268} the suggestion arises that each hypothetically
complete thoracic somite should possess four gills on each side,
giving the following


 _Hypothetically complete branchial formula._

 Somites and                      Arthrobranchiæ.
   their       Podobranchiæ.    /───────^───────\       Pleuro-
 Appendages.                   Anterior.   Posterior.   branchiæ.

    VII.           1               1           1          1      = 4
   VIII.           1               1           1          1      = 4
     IX.           1               1           1          1      = 4
      X.           1               1           1          1      = 4
     XI.           1               1           1          1      = 4
    XII.           1               1           1          1      = 4
   XIII.           1               1           1          1      = 4
    XIV.           1               1           1          1      = 4
                  ──              ──          ──         ──       ──
                   8       +       8     +     8   +      8     = 32

Starting from this hypothetically complete branchial formula, we
may regard all the actual formulæ as produced from it by the more
or less complete suppression of the most anterior, or of the most
posterior branchiæ, or of both, in each series. In the case of the
podobranchiæ, the branchiæ are converted into epipodites; in that of
the other branchiæ, they become rudimentary, or disappear.

       *       *       *       *       *

In general appearance a common prawn (_Palæmon_, fig. 71) is very
similar to a miniature lobster or crayfish. Nor does a closer
examination fail to reveal a complete fundamental likeness. The
number of the somites, and of the appendages, and their general
character and {269} disposition, are in fact the same. But, in the
prawn, the abdomen is much larger in proportion to the cephalothorax;
the basal scale, or expodite of the antenna, is much larger; the
external maxillipedes are longer, and differ less from the succeeding
thoracic appendages. The first pair of these, which answers to the
forceps of the crayfish, is chelate, but it is very slender; the
second pair, also chelate, is always larger than the first, and
is sometimes exceedingly {270} long and strong (fig. 71, B); the
remaining thoracic limbs are terminated by simple claws. The five
anterior abdominal somites are all provided with large swimmerets,
which are used like paddles, when the animal swims quietly; and, in
the males, the first pair is only slightly different from the rest.
The rostrum is very large, and strongly serrated.

[Illustration: FIG. 71. _Palæmon jamaicensis_ (about 5/7 nat. size).
A, female; B, fifth thoracic appendage of male.]

None of these differences from the crayfish, however, is so great, as
to prepare us for the remarkable change observable in the respiratory
organs. The total number of the gills is only eight. Of these, five
are large pleurobranchiæ, attached to the epimera of the five hinder
thoracic somites; two are arthrobranchiæ, fixed to the interarticular
membrane of the external maxillipede; and one, which is the only
complete podobranchia, belongs to the second maxillipede. The
podobranchiæ of the first and third maxillipedes are represented only
by small epipodites. The branchial formula therefore is:—

 Somites and                      Arthrobranchiæ.
   their       Podobranchiæ.     /───────^───────\      Pleuro-
 Appendages.                   Anterior.   Posterior.   branchiæ.

    VII.           0 (ep.)       0           0             0    = 0 (ep.)
   VIII.           1             0           0             0    = 1
     IX.           0 (ep.)       1           1             0    = 2 (ep.)
      X.           0             0           0             1    = 1
     XI.           0             0           0             1    = 1
    XII.           0             0           0             1    = 1
   XIII.           0             0           0             1    = 1
    XIV.           0             0           0             1    = 1
                  ──            ──          ──            ──     ──
                   1 + 2 ep.  +  1      +    1      +      5    = 8 + 2 ep.

{271}

The prawn, in fact, presents us with an extreme case of that kind of
modification of the branchial system, of which _Penæus_ has furnished
a less complete example. The series of the podobranchiæ is reduced
almost to nothing, while the large pleurobranchiæ are the chief
organs of respiration.

But this is not the only difference. The prawn’s gills are not
brush-like, but are foliaceous. They are not _trichobranchiæ_, but
_phyllobranchiæ_; that is to say, the central stem of the branchia,
instead of being beset with numerous series of slender filaments,
bears only two rows of broad flat lamellæ (fig. 68, C, C′, _l_),
which are attached to opposite sides of the stem (C′, _s_), and
gradually diminish in size from the region of the stem by which it is
fixed, upwards and downwards. These lamellæ are superimposed closely
upon one another, like the leaves of a book; and the blood traversing
the numerous passages by which their substance is excavated, comes
into close relation with the currents of aerated water, which are
driven between the branchial leaflets by a respiratory mechanism of
the same nature as that of the crayfish.

Different as these phyllobranchiæ of the prawns are in appearance
from the trichobranchiæ of the preceding _Crustacea_, they are easily
reduced to the same type. For in the genus _Axius_, which is closely
allied to the lobsters, each branchial stem bears a single series
of filaments on its opposite sides; and if these biserial filaments
are supposed to widen out into broad leaflets, the transition from
{272} the trichobranchia to the phyllobranchia will be very easily
effected.

The shrimp (_Crangon_) also possesses phyllobranchiæ, and differs
from the prawn chiefly in the character of its locomotive and
prehensile thoracic limbs.

       *       *       *       *       *

There are yet other very well-known marine animals, which, in common
appreciation, are always associated with the lobsters and crayfishes,
although the difference of general appearance is vastly greater than
in any of the cases which have yet been considered. These are the
Crabs.

In all the forms we have hitherto been considering, the abdomen is as
long as, or longer than, the cephalothorax, while its width is the
same, or but little less. The sixth somite has very large appendages,
which, together with the telson, make up a powerful tail-fin; and
the large abdomen is thus fitted for playing an important part in
locomotion.

Again, the length of the cephalothorax is much greater than its
width, and it is produced in front into a long rostrum. The bases
of the antennæ are freely movable, and they are provided with a
movable exopodite. Moreover, the eye-stalks are not inclosed in a
cavity or orbit, and the eyes themselves appear above and in front
of the antennules. The external maxillipedes are narrow, and their
endopodites are more or less leg-like.

[Illustration: FIG. 72. _Cancer pagerus_, male (1/3 nat. size). A,
dorsal view, with the abdomen extended; B, front view of “face.”
_as_, antennary sternum; _or_, orbit; _r_, rostrum; 1. eyestalk; 2.
antennule; 3. base of antenna; 3′, free portion of antenna.]

None of these statements apply to the crabs. In these {273} animals
the abdomen is short, flattened, and apt to escape immediate notice,
as it is habitually kept closely applied against the under surface
of the cephalothorax. It is {274} not used as a swimming organ; and
the sixth somite possesses no appendages whatever. The breadth of
the cephalothorax is often greater than its length, and there is no
prominent rostrum. In its place there is a truncated process (fig.
72, B, _r_), which sends down a vertical partition, and divides
from one another two cavities, in which the swollen basal joints
of the small antennules (_2_) are lodged. The outer boundary of
each of these cavities is formed by the basal part of the antenna
(_3_), which is firmly fixed to the edge of the carapace. There is
no exopoditic scale; and the free part of the antenna (_3′_) is very
small. The convex corneal surface of the eye appears outside the base
of the antenna, lodged in a sort of orbit (_or_), the inner margin
of which is formed by the base of the antenna, while the upper and
outer boundaries are constituted by the carapace. Thus, while in all
the preceding forms, the eye is situated nearest the middle line,
and is most forward, while the antennule lies outside and behind it,
and the antenna comes next; in the crab, the antennule occupies the
innermost place, the antenna comes next, and the eye appears to be
external to and behind the other two. But there is no real change in
the attachments of the eye-stalks. For if the antennule and the basal
joint of the antenna are removed, it will be seen that the base of
the eye-stalk is attached, as in the crayfish, close to the middle
line, on the inner side, and in front of the antennule. But it is
very long and extends outwards, behind the antennule and the an--tenna;
{275} its corneal surface alone being visible, as it projects into
the orbit.

Again, the ischiopodites of the external maxillipedes are expanded
into broad quadrate plates, which meet in the middle line, and close
over the other manducatory organs, like two folding-doors set in a
square doorway. Behind these there are great chelate forceps, as in
the crayfish; but the succeeding four pairs of ambulatory limbs are
terminated by simple claws.

When the abdomen is forcibly turned back, its sternal surface is
seen to be soft and membranous. There are no swimmerets; but, in the
female, the four anterior pairs of abdominal limbs are represented by
singular appendages, which give attachment to the eggs; while in the
males there are two pairs of styliform organs attached to the first
and second somites of the abdomen, which correspond with those of the
male crayfishes.

The ventral portions of the branchiostegites are sharply bent
inwards, and their edges are so closely applied throughout the
greater part of their length to the bases of the ambulatory limbs,
that no branchial cleft is left. In front of the bases of the
forceps, however, there is an elongated aperture, which can be
shut or opened by a sort of valve, connected with the external
maxillipede, which serves for the entrance of water into the
branchial cavity. The water employed in respiration, and kept in
constant motion by the action of the scaphognathite, is baled out
through two apertures, which {276} are separated from the foregoing
by the external maxillipedes, and lie at the sides of the quadrate
space in which these organs are set.

There are only nine gills on each side, and these, as in the
prawn and shrimp, are phyllobranchiæ. Seven of the branchiæ are
pyramidal in shape, and for the most part of large size. When the
branchiostegite is removed, they are seen lying close against its
inner walls, their apices converging towards its summit. The two
hindermost of these gills are pleurobranchiæ, the other five are
arthrobranchiæ. The two remaining gills are podobranchiæ, and
belong to the second and the third maxillipedes respectively. Each
is divided into a branchial and an epipoditic portion, the latter
having the form of a long curved blade. The branchial portion of the
podobranchia of the second maxillipede is long, and lies horizontally
under the bases of the four anterior arthrobranchiæ; while the
gill of the podobranchia of the third maxillipede is short and
triangular, and fits in between the bases of the second and the third
arthrobranchiæ. The epipodite of the third maxillipede is very long,
and its base furnishes the valve of the afferent aperture of the
branchial cavity, which has been mentioned above. The podobranchia of
the first maxillipede is represented only by a long curved epipoditic
blade, which can sweep over the outer surface of the gills, and
doubtless serves to keep them clear of foreign bodies. {277}

 _The branchial formula of Cancer pagurus._

 Somites and                      Arthrobranchiæ.
   their       Podobranchiæ.    /───────^───────\       Pleurobranchiæ.
 Appendages.                   Anterior.   Posterior.


    VII.           0 (ep.)          0           0           0         = 0
   VIII.           1                1           0           0         = 2
     IX.           1                1           1           0         = 3
      X.           0                1           1           0         = 2
     XI.           0                0           0           1         = 1
    XII.           0                0           0           1         = 1
   XIII.           0                0           0           0         = 0
    XIV.           0                0           0           0         = 0
                  ──               ──          ──          ──          ──
                   2 + ep.  +       3     +     2    +      2         = 9 + ep.


It will be observed that the suppression of branchiæ has here taken
place in all the series, and at both the anterior and the posterior
ends of each. But the defect in total number is made up by the
increase of size, not of the pleurobranchiæ alone, as in the case
of the prawns, but of the arthrobranchiæ as well. At the same time
the whole apparatus has become more specialized and perfected as a
breathing organ. The close fitting of the edges of the carapace,
and the possibility of closing the inhalent and exhalent apertures,
render the crabs much more independent of actual immersion in water
than most of their congeners; and some of them habitually live on dry
land and breathe by means of the atmospheric air which they take into
and expel from their branchial cavities.

Notwithstanding all these wide departures from the structure and
habits of the crayfishes, however, attentive examination shows that
the plan of construction of the {278} crab is, in all fundamental
respects, the same as that of the crayfish. The body is made up of
the same number of somites. The appendages of the head and of the
thorax are identical in number, in function, and even in the general
pattern of their structure. But two pairs of abdominal appendages
in the female, and four pairs in the male, have disappeared. The
exopodites of the antennæ have vanished, and not even epipodites
remain to represent the podobranchiæ of the posterior five pairs
of thoracic limbs. The exceedingly elongated eye-stalks are turned
backwards and outwards, above the bases of the antennules and
the antennæ, and the bases of the latter have become united with
the edges of the carapace in front of them. In this manner the
extraordinary face, or _metope_ (fig. 72, B) of the crab results from
a simple modification of the arrangement of parts, every one of which
exists in the crayfish. The same common plan serves for both.

       *       *       *       *       *

The foregoing illustrations are taken from a few of our commonest
and most easily obtainable _Crustacea_; but they amply suffice
to exemplify the manner in which the conception of a plan of
organization, common to a multitude of animals of extremely diverse
outward forms and habits, is forced upon us by mere comparative
anatomy.

Nothing would be easier, were the occasion fitting, than to extend
this method of comparison to the whole of the several thousand
species of crab-like, crayfish-like, or {279} prawn-like animals,
which, from the fact that they all have their eyes set upon
movable stalks, are termed the _Podophthalmia_, or stalk-eyed
_Crustacea_; and by arguments of similar force to prove that they
are all modifications of the same common plan. Not only so, but the
sand-hoppers of the sea-shore, the wood-lice of the land, and the
water-fleas or the monoculi of the ponds, nay, even such remote forms
as the barnacles which adhere to floating wood, and the acorn shells
which crowd every inch of rock on many of our coasts, reveal the same
fundamental organization. Further than this, the spiders and the
scorpions, the millipedes and the centipedes, and the multitudinous
legions of the insect world, show us, amid infinite diversity of
detail, nothing which is new in principle to any one who has mastered
the morphology of the crayfish.

Given a body divided into somites, each with a pair of appendages;
and given the power to modify those somites and their appendages in
strict accordance with the principles by which the common plan of
the _Podophthalmia_ is modified in the actually existing members of
that order; and the whole of the _Arthropoda_, which probably make
up two-thirds of the animal world, might readily be educed from one
primitive form.

And this conclusion is not merely speculative. As a matter of
observation, though the _Arthropoda_ are not all evolved from one
primitive form, in one sense of the words, yet they are in another.
For each can be traced {280} back in the course of its development
to an ovum, and that ovum gives rise to a blastoderm, from which the
parts of the embryo arise in a manner essentially similar to that in
which the young crayfish is developed.

Moreover, in a large proportion of the _Crustacea_, the embryo leaves
the egg under the form of a small oval body, termed a _Nauplius_
(fig. 73, D), provided with (usually) three pairs of appendages,
which play the part of swimming limbs, and with a median eye. Changes
of form accompanied by sheddings of the cuticle take place, in
virtue of which the larva passes into a new stage, when it is termed
a _Zoæa_ (C). In this, the three pairs of locomotive appendages
of the _Nauplius_ are metamorphosed into rudimentary antennules,
antennæ, and mandibles, while two or more pairs of anterior thoracic
appendages provided with exopodites and hence appearing bifurcated,
subserve locomotion. The abdomen has grown out and become a notable
feature of the Zoæa, but it has no appendages.

In some _Podophthalmia_, as in _Penæus_ (fig, 73), the young leaves
the egg as a Nauplius, and the Nauplius becomes a Zoæa. The hinder
thoracic appendages, each provided with an epipodite, appear; the
stalked eyes and the abdominal members are developed, and the larva
passes into what is sometimes called the _Mysis_ or _Schizopod_
stage. The adult state differs from this chiefly in the presence of
branchiæ and the rudimentary character of the exopodites of the five
posterior thoracic limbs. {281}

[Illustration: FIG. 73. _Penæus semisulcatus._ A, adult (after de
Haan. 1/2 nat. size); B, Zoæa, and C, less advanced Zoæa of a species
of _Penæus_. D, Nauplius. (B, C, and D, after Fritz Müller.)]

In the Opossum-shrimps (_Mysis_) the young does not leave the pouch
of the mother until it is fully {282} developed; and, in this case,
the _Nauplius_ state is passed through so rapidly and in so early and
imperfect a condition of the embryo, that it would not be recognized
except for the cuticle which is developed and is subsequently shed.

[Illustration: FIG. 74. _Cancer pagurus._ A, newly hatched Zoæa; B,
more advanced Zoæa; C, dorsal, and D, side view of Megalopa (after
Spence Bate). (The figures A and B are more magnified than C and D.)]

{283}

In the great majority of the _Podophthalmia_, the Nauplius stage
seems to be passed over without any such clear evidence of its
occurrence, and the young is set free as a Zoæa. In the lobsters,
which have, throughout life, a large abdomen provided with
swimmerets, the Zoæa, after going through a Mysis or Schizopod stage,
passes into the adult form.

In the crab, the young leaves the egg as a Zoæa (fig. 74, A and B).
But this is not followed by a Schizopod stage, inasmuch as the five
hinder pair of thoracic limbs are apparently, from the first, devoid
of exopodites. But the Zoæa, after it has acquired stalked eyes and
a complete set of thoracic and abdominal members, and has passed
into what is called the _Megalopa_ stage (fig. 74, C and D), suffers
a more complete metamorphosis. The carapace widens, the fore part
of the head is modified so as to bring about the formation of the
characteristic metope: and the abdomen, losing more or fewer of its
posterior appendages, takes up its final position under the thorax.

In the Zoæa state, those thoracic limbs which give rise to the
maxillipedes are provided with well-developed exopodites, and
in the free Mysis state all these limbs have exopodites. In the
Opossum-shrimps these persist throughout life; in _Penæus_, the
rudiments of them only remain; in the lobster, they disappear
altogether.

Thus, in these animals, there is no difficulty in demonstrating
that embryological uniformity of type of all the {284} limbs,
complete evidence of which was not furnished by the development of
the crayfish. In this crustacean, in fact, it would appear that the
process of development has undergone its maximum of abbreviation.
The embryo presents no distinct and independent Nauplius or Zoæa
stages, and, as in the crab, there is no Schizopod or Mysis stage.
The abdominal appendages are developed very early, and the new born
young, which resembles the Megalopa stage of the crab, differs only
in a few points from the adult animal.

       *       *       *       *       *

Guided by comparative morphology, we are thus led to admit that the
whole of the _Arthropoda_ are connected by closer or more remote
degrees of affinity with the crayfish. If we were to study the perch
and the pond-snail with similar care, we should be led to analogous
conclusions. For the perch is related by similar gradations, in the
first place, with other fishes; then more remotely, with frogs and
newts, reptiles, birds, and mammals; or, in other words, with the
whole of the great division of the _Vertebrata_. The pond-snail, by
like reasoning upon analogous data, is connected with the _Mollusca_,
in all their innumerable kinds of slugs, shellfish, squids, and
cuttlefish. And, in each case, the study of development takes us back
to an egg as the primary condition of the animal, and to the process
of yelk division, the formation of a blastoderm, and the conversion
of that blastoderm into a more or less modified {285} gastrula, as
the early stages of development. The like is true of all the worms,
sea-urchins, starfishes, jellyfishes, polypes, and sponges; and it is
only in the minutest and simplest forms of animal life that the germ,
or representative of the ovum becomes metamorphosed into the adult
form without the preliminary process of division.

In the majority even of these _Protozoa_, the typical structure
of the nucleated cell is retained, and the whole animal is the
equivalent of a histological unit of one of the higher organisms.
An _Amœba_ is strictly comparable, morphologically, to one of the
corpuscles of the blood of the crayfish.

Thus, to exactly the same extent as it is legitimate to represent
all the crayfishes as modifications of the common astacine plan,
it is legitimate to represent all the multicellular animals as
modifications of the gastrula, and the gastrula itself as a
peculiarly disposed aggregate of cells; while the _Protozoa_ are such
cells either isolated, or otherwise aggregated.

It is easy to demonstrate that all plants are either cell aggregates,
or simple cells; and as it is impossible to draw any precise line
of demarcation, either physiological or morphological, between the
simplest plants, and the simplest of the _Protozoa_, it follows that
all forms of life are morphologically related to one another; and
that in whatever sense we say that the English and the Californian
crayfish are allied, in the same sense, though not to the same
degree, must we admit that all living things {286} are allied.
Given one of those protoplasmic bodies, of which we are unable to
say certainly whether it is animal or plant, and endow it with such
inherent capacities of self-modification as are manifested daily
under our eyes by developing ova, and we have a sufficient reason for
the existence of any plant, or of any animal.

This is the great result of comparative morphology; and it is
carefully to be noted that this result is not a speculation, but
a generalisation. The truths of anatomy and of embryology are
generalised statements of facts of experience; the question whether
an animal is more or less like another in its structure and in its
development, or not, is capable of being tested by observation; the
doctrine of the unity of organisation of plants and animals is simply
a mode of stating the conclusions drawn from experience. But, if it
is a just mode of stating these conclusions, then it is undoubtedly
conceivable that all plants and all animals may have been evolved
from a common physical basis of life, by processes similar to those
which we every day see at work in the evolution of individual animals
and plants from that foundation.

That which is conceivable, however, is by no means necessarily true;
and no amount of purely morphological evidence can suffice to prove
that the forms of life have come into existence in one way rather
than another.

There is a common plan among churches, no less than {287} among
crayfishes; nevertheless the churches have certainly not been
developed from a common ancestor, but have been built separately.
Whether the different kinds of crayfishes have been built separately,
is a problem we shall not be in a position to grapple with, until we
have considered a series of facts connected with them, which have not
yet been touched upon.

{288}



CHAPTER VI.

THE DISTRIBUTION AND THE ÆTIOLOGY OF THE CRAYFISHES.


So far as I have been able to discover, all the crayfishes which
inhabit the British islands agree in every point with the full
description given above, at p. 230. They are abundant in some of our
rivers, such as the Isis, and other affluents of the Thames; and
they have been observed in those of Devon;[15] but they appear to
be absent from many others. I cannot hear of any, for example, in
the Cam or the Ouse, on the east, or in the rivers of Lancashire and
Cheshire, on the west. It is still more remarkable that, according
to the best information I can obtain, they are absent in the
Severn, though they are plentiful in the Thames and Severn canal.
Dr. M^cIntosh, who has paid particular attention to the fauna of
Scotland, assures me that crayfish are unknown north of the Tweed. In
Ireland, on the other hand, they occur in many localities;[16] but
the question whether their diffusion, and even their introduction
into this {289} island, has or has not been effected by artificial
means, is involved in some obscurity.

     [15] Moore. Magazine of Natural History. New Series, III., 1839.

     [16] Thompson. Annals and Magazine of Natural History, XI., 1843.

English zoologists have always termed our crayfish _Astacus
fluviatilis_; and, up to a recent period, the majority of Continental
naturalists have included a corresponding form of _Astacus_ under
that specific name.

Thus M. Milne Edwards, in his classical work on the _Crustacea_,[17]
published in 1837, observes under the head of “Écrevisse commune.
_Astacus fluviatilis_:” “There are two varieties of this crayfish;
in the one, the rostrum gradually becomes narrower from its base
onwards, and the lateral spines are situated close to its extremity;
in the other, the lateral edges of the rostrum are parallel in their
posterior half and the lateral spines are stronger and more remote
from the end.”

The “first variety,” here mentioned, is known under the name of
“Écrevisse à pieds blancs”[18] in France, by way of distinction from
the “second variety,” which is termed “Écrevisse à pieds rouges,” on
account of the more or less extensive red coloration of the forceps
and ambulatory limbs. This second variety is the larger, commonly
attaining five inches in length, and sometimes reaching much larger
dimensions; and it is more highly esteemed for the market, on account
of its better flavour.

     [17]  “Histoire Naturelle des Crustacés.”

     [18]  Carbonnier. “L’Écrevisse,” p. 8.

In Germany, the two forms have long been popularly distinguished, the
former by the name of “Steinkrebs,” {290} or “stone crayfish,” and
the latter by that of “Edelkrebs,” or “noble crayfish.”

Milne Edwards, it will be observed, speaks of these two forms of
crayfish as “varieties” of the species _Astacus fluviatilis_; but,
even as far back as the year 1803 some zoologists began to regard the
“stone crayfish” as a distinct species, to which Schrank applied the
name of _Astacus torrentium_, while the “noble crayfish” remained
in possession of the old denomination, _Astacus fluviatilis_;
and, subsequently, various forms of “stone-crayfishes” have been
further distinguished as the species _Astacus saxatilis_, _A.
tristis_, _A. pallipes_, _A. fontinalis_, &c. On the other hand,
Dr. Gerstfeldt,[19] who has devoted especial attention to the
question, denies that these are anything more than varieties of one
species; but he holds this and Milne Edwards’s “second variety” to be
specifically distinct from one another.

We thus find ourselves in the presence of three views respecting the
English and French crayfishes.

1. They are all varieties of one species—_A. fluviatilis_.

2. There are two species—_A. fluviatilis_, and _A. torrentium_, of
which last there are several varieties.

3. There are, at fewest, five or six distinct species.

Before adopting the one or the other of these views, it is necessary
to form a definite conception of the meaning of the terms “species”
and “variety.” {291}

     [19] “Ueber die Flusskrebse Europas.” Mém. de l’Acad. de St.
     Petersburg, 1859.

The word “species” in Biology has two significations; the one based
upon morphological, the other upon physiological considerations.

A species, in the strictly morphological sense, is simply an
assemblage of individuals which agree with one another, and differ
from the rest of the living world in the sum of their morphological
characters; that is to say, in the structure and in the development
of both sexes. If the sum of these characters in one group is
represented by A, and that in another by A + _n_; the two are
morphological species, whether _n_ represents an important or an
unimportant difference.

The great majority of species described in works on Systematic
Zoology are merely morphological species. That is to say, one or more
specimens of a kind of animal having been obtained, these specimens
have been found to differ from any previously known by the character
or characters _n_; and this difference constitutes the definition of
the new species, and is all we really know about its distinctness.

But, in practice, the formation of specific groups is more or less
qualified by considerations based upon what is known respecting
variation. It is a matter of observation that progeny are never
exactly like their parents, but present small and inconstant
differences from them. Hence, when specific identity is predicated
of a group of individuals, the meaning conveyed is not that they
are all exactly alike, but only that their differences are so {292}
small, and so inconstant, that they lie within the probable limits
of individual variation.

Observation further acquaints us with the fact, that, sometimes, an
individual member of a species may exhibit a more or less marked
variation, which is propagated through all the offspring of that
individual, and may even become intensified in them. And, in this
manner, a _variety_, or _race_, is generated within the species;
which variety, or race, if nothing were known respecting its origin,
might have every claim to be regarded as a separate morphological
species. The distinctive characters, of a race, however, are rarely
equally well marked in all the members of the race. Thus suppose the
species A to develop the race A + _x_; then the difference _x_ is apt
to be much less in some individuals than in others; so that, in a
large suite of specimens, the interval between A + _x_ and A will be
filled up by a series of forms in which _x_ gradually diminishes.

Finally, it is a matter of observation that modification of the
physical conditions under which a species lives favours the
development of varieties and races.

Hence, in the case of two specimens having respectively the
characters A and A + _n_, although, _primâ facie_, they are of
distinct species; yet if a large collection shows us that the
interval between A and A + _n_ is filled up by forms of A having
traces of _n_, and forms of A + _n_ in which _n_ becomes less and
less, then it will be {293} concluded that A and A + _n_ are races
of one species and not separate species. And this conclusion will
be fortified if A and A + _n_ occupy different stations in the same
geographical area.

Even when no transitional forms between A and A + _n_ are
discoverable, if _n_ is a small and unimportant difference, such
as of average size, colour, or ornamentation, it may be fairly
held that A and A + _n_ are mere varieties; inasmuch as experience
proves that such variations may take place comparatively suddenly;
or the intermediate forms may have died out and thus the evidence of
variation may have been effaced.

From what has been said it follows that the groups termed
morphological species are provisional arrangements, expressive simply
of the present state of our knowledge.

We call two groups species, if we know of no transitional forms
between them, and if there is no reason to believe that the
differences which they present are such as may arise in the ordinary
course of variation. But it is impossible to say whether the progress
of inquiry into the characters of any group of individuals may prove
that what have hitherto been taken for mere varieties are distinct
morphological species; or whether, on the contrary, it may prove that
what have hitherto been regarded as distinct morphological species
are mere varieties.

What has happened in the case of the crayfish is this: {294} the
older observers lumped all the Western European forms which came
under their notice under one species, _Astacus fluviatilis_; noting,
more or less distinctly, the stone crayfish and the noble crayfish
as races or varieties of that species. Later zoologists, comparing
crayfishes together more critically, and finding that the stone
crayfish is ordinarily markedly different from the noble crayfish,
concluded that there were no transitional forms, and made the former
into a distinct species, tacitly assuming that the differential
characters are not such as could be produced by variation.

It is at present an open question whether further investigation will
or will not bear out either of these assumptions. If large series
of specimens of both stone crayfishes and noble crayfishes from
different localities are carefully examined, they will be found to
present great variations in size and colour, in the tuberculation of
the carapace and limbs, and in the absolute and relative sizes of the
forceps.

The most constant characters of the stone crayfish are:—

1. The tapering form of the rostrum and the approximation of the
lateral spines to its point; the distance between these spines being
about equal to their distance from the apex of the rostrum (fig. 61,
A).

2. The development of one or two spines from the ventral margin of
the rostrum.

3. The gradual subsidence of the posterior part of {295} the
post-orbital ridge, and the absence of spines on its surface.

4. The large relative size of the posterior division of the telson
(G).

On the contrary, in the noble crayfish:—

1. The sides of the posterior two-thirds of the rostrum are nearly
parallel, and the lateral spines are fully a third of the length of
the rostrum from its point; the distance between them being much less
than their distance from the apex of the rostrum (B).

2. No spine is developed from the ventral margin of the rostrum.

3. The posterior part of the post-orbital ridge is a more or less
distinct, sometimes spinous elevation.

4. The posterior division of the telson is smaller relatively to the
anterior division (H).

I may add that I have found three rudimentary pleurobranchiæ in the
noble crayfish, and never more than two in the stone crayfish.

In order to ascertain whether no crayfish exist in which the
characters of the parts here referred to are intermediate between
those defined, it would be necessary to examine numerous examples
of each kind of crayfish from all parts of the areas which they
respectively inhabit. This has been done to some extent, but by no
means thoroughly; and I think that all that can be safely said, at
present, is that the existence of intermediate forms is not proven.
But, whatever the constancy of the {296} differences between the
two kinds of crayfishes, there can surely be no doubt as to their
insignificance; and no question that they are no more than such as,
judging by analogy, might be produced by variation.

From a morphological point of view, then, it is really impossible to
decide the question whether the stone crayfish and the noble crayfish
should be regarded as species or as varieties. But, since it will,
hereafter, be convenient to have distinct names for the two kinds, I
shall speak of them as _Astacus torrentium_ and _Astacus nobilis_.[20]

     [20] According to strict zoological usage the names should
     be written _A. fluviatilis_ (var. _torrentium_) and _A.
     fluviatilis_ (var. _nobilis_) on the hypothesis that the
     stone crayfish and the noble crayfish are varieties; and _A.
     torrentium_ and _A. fluviatilis_ on the hypothesis that they are
     species; but as I neither wish to prejudge the species question,
     nor to employ cumbrously long names, I take a third course.

In the physiological sense, a species means, firstly, a group of
animals the members of which are capable of completely fertile union
with one another, but not with the members of any other group; and,
secondly, it means all the descendants of a primitive ancestor or
ancestors, supposed to have originated otherwise than by ordinary
generation.

It is clear that, even if crayfishes had an unbegotten ancestor,
there is no means of knowing whether the stone crayfish and the noble
crayfish are descendants of the same, or of different ancestors, so
that the second sense of species hardly concerns us. As to the first
sense, there is no evidence to show whether the two {297} kinds of
crayfish under consideration are capable of fertile union or whether
they are sterile. It is said, however, that hybrids or mongrels are
not met with in the waters which are inhabited by both kinds, and
that the breeding season of the stone crayfish begins earlier than
that of the noble crayfish.

M. Carbonnier, who practises crayfish culture on a large scale, gives
some interesting facts bearing on this question in the work already
cited. He says that, in the streams of France, there are two very
distinct kinds of crayfishes—the red-clawed crayfish (L’Écrevisse à
pieds rouges), and the white-clawed crayfish (L’Écrevisse à pieds
blancs), and that the latter inhabit the swifter streams. In a piece
of land converted into a crayfish farm, in which the white-clawed
crayfish existed naturally in great abundance, 300,000 red-clawed
crayfish were introduced in the course of five years; nevertheless,
at the end of this time, no intermediate forms were to be seen, and
the “pieds rouges” exhibited a marked superiority in size over the
“pieds blancs.” M. Carbonnier, in fact, says that they were nearly
twice as big.

On the whole, the facts as at present known, seem to incline rather
in favour of the conclusion that _A. torrentium_ and _A. nobilis_ are
distinct species; in the sense that transitional forms have not been
clearly made out, and that, possibly, they do not interbreed.

       *       *       *       *       *

As I have already remarked, the very numerous {298} specimens of
English and Irish crayfishes which have passed through my hands,
have all presented the character of _Astacus torrentium_, with
which also the description given in works of recognised authority
coincides as far as it goes.[21] The same form is found in many parts
of France, as far south as the Pyrenees, and it is met with as far
east as Alsace and Switzerland. I have recently[22] been enabled,
by the kindness of Dr. Bolivar, of Madrid, who sent me a number of
crayfishes from the neighbourhood of that city, to satisfy myself
that the Spanish peninsula contains crayfishes altogether similar
to those of Britain, except that the subrostral spine is less
developed. Further, I have no doubt that Dr. Heller[23] is right
in his identification of the English crayfish with a form which
he describes under the name of _A. saxatilis_. He says that it is
especially abundant in Southern Europe, and that it occurs in Greece,
in Dalmatia, in the islands of Cherso and Veglia, at Trieste, in the
Lago di Garda, and at Genoa. Further, _Astacus torrentium_ appears
to be widely distributed in North Germany. The eastern limit of this
crayfish is uncertain; but, according to Kessler,[24] it does not
occur within the limits of the Russian empire. {299}

     [21] See Bell. “British Stalk-eyed Crustacea,” p. 237.

     [22] Since the statement respecting the occurrence of crayfishes
     in Spain on p. 44 was printed.

     [23] “Die Crustaceen des Südlichen Europas,” 1863.

     [24] “Die Russischen Flusskrebse.” Bulletin de la Société
     Impériale des Naturalistes de Moscow, 1874.

_Astacus torrentium_ appears to be particularly addicted to rapid
highland streams and the turbid pools which they feed.

_Astacus nobilis_ is indigenous to France, Germany, and the Italian
peninsula. It is said to be found at Nice and at Barcelona, though I
cannot hear of it elsewhere in Spain. Its south-eastern limit appears
to be the Lake of Zirknitz, in Carniola, not far from the famous
caves of Adelsberg. It is not known in Dalmatia, in Turkey, nor in
Greece. In the Russian empire, according to Kessler, this crayfish
chiefly inhabits the watershed of the Baltic. The northern limit of
its distribution lies between Christianstad, in the Gulf of Bothnia
(62° 16′ N), and Serdobol, at the northern end of Lake Ladoga.
“Eastward of Lake Ladoga it is found in the Uslanka, a tributary
of the Swir. It appears to be the only crayfish which exists in
the waters which flow from the south into the Gulf of Finland and
into the Baltic; except in those streams and lakes which have been
artificially connected with the Volga, and in which it is partially
replaced by _A. leptodactylus_.” It still inhabits the Lakes of
Beresai and Bologoe, as well as the affluents of the Msta and the
Wolchow; and it is met with in affluents of the Dnieper, as far as
Mohilew. _Astacus nobilis_ is also found in Denmark and Southern
Sweden; but, in the latter country, its introduction appears to have
been artificial. This crayfish is said occasionally to be met with on
the Livonian coast in the waters of the Baltic, which, however, it
must {300} be remembered, are much less salt than ordinary sea water.

It will be observed that while the two forms, _A. torrentium_ and
_A. nobilis_, are intermixed over a large part of Central Europe,
_A. torrentium_ has a wider north-westward, south-westward, and
south-eastward extension, being the sole occupant of Britain, and
apparently of the greater part of Spain and of Greece. On the other
hand, in the northern and eastern parts of Central Europe, _A.
nobilis_ appears to exist alone.

Further to the east, a new form, _Astacus leptodactylus_ (fig. 75),
makes its appearance. Whether _A. leptodactylus_ exists in the upper
waters of the Danube, does not appear, but in the lower Danube and in
the Theiss it is the dominant, if not the exclusive, crayfish. From
hence it extends through all the rivers which flow into the Black,
Azov, and Caspian Seas, from Bessarabia and Podolia on the west,
to the Ural mountains on the east. In fact, the natural habitat of
this crayfish appears to be the watershed of the Pontocaspian area,
excluding that part of the Black Sea which lies southward of the
Caucasus on the one hand, and of the mouths of the Danube on the
other.[25]

     [25] These statements rest on the authority of Kessler and
     Gerstfeldt, in their memoirs already cited.

It is a remarkable circumstance that this crayfish not only thrives
in the brackish waters of the estuaries of the rivers which debouche
into the Black Sea and the Sea of Azov, but that it is found even in
the salter {302} southern parts of the Caspian, in which it lives
at considerable depths.

[Illustration: FIG. 75.—Astacus leptodactylus (after Rathke, 1/3 nat.
size).]

In the north, _Astacus leptodactylus_ is met with in the rivers
which flow into the White Sea, as well as in many streams and lakes
about the Gulf of Finland. But it has probably been introduced
into these streams by the canals which have been constructed to
connect the basin of the Volga with the rivers which flow into the
Baltic and into the White Sea. In the latter, the invading _A.
leptodactylus_ is everywhere overcoming and driving out _A. nobilis_
in the struggle for existence, apparently in virtue of its more rapid
multiplication.[26]

     [26] Kessler (Die Russischen Flusskrebse, l. c. p. 369–70), has
     an interesting discussion of this question.

In the Caspian and in the brackish waters of the estuaries of the
Dniester and the Bug, a somewhat different crayfish, which has been
called _Astacus pachypus_, occurs; another closely allied form (_A.
angulosus_) is met with in the mountain streams of the Crimea and
of the northern face of the Caucasus; and a third, _A. colchicus_,
has recently been discovered in the Rion, or Phasis of the ancients,
which flows into the eastern extremity of the Black Sea.

With respect to the question whether these Pontocaspian crayfishes
are specifically distinct from one another, and whether the most
widely distributed kind, _A. leptodactylus_, is distinct from _A.
nobilis_, exactly the same difficulties arise as in the case of
the west European {303} crayfishes. Gerstfeldt, who has had the
opportunity of examining large series of specimens, concludes that
the Pontocaspian crayfishes and _A. nobilis_ are all varieties
of one species. Kessler, on the contrary, while he admits that
_A. angulosus_ is, and _A. pachypus_ may be, a variety of _A.
leptodactylus_, affirms that the latter is specifically distinct from
_A. nobilis_.

Undoubtedly, well marked examples of _A. leptodactylus_ are very
different from _A. nobilis_.

1. The edges of the rostrum are produced into five or six sharp
spines, instead of being smooth or slightly serrated as in _A.
nobilis_.

2. The fore part of the rostrum has no serrated spinous median keel,
such as commonly, though not universally, exists in _A. nobilis_.

3. The posterior end of the post-orbital ridge is still more distinct
and spiniform than in _A. nobilis_.

4. The abdominal pleura of _A. leptodactylus_ are narrower, more
equal sided, and triangular in shape.

5. The chelæ of the forceps, especially in the males, are more
elongated; and the moveable and fixed claws are slenderer and have
their opposed edges straighter and less tuberculated.

But, in all these respects, individual specimens of _A. nobilis_ vary
in the direction of _A. leptodactylus_ and _vice versâ_; and if _A.
angulosus_ and _A. pachypus_ are varieties of _A. leptodactylus_, I
cannot see why Gerstfeldt’s conclusion that _A. nobilis_ is another
variety of {304} the same form need be questioned on morphological
grounds. However, Kessler asserts that, in those localities in which
_A. leptodactylus_ and _A. nobilis_ live together, no intermediate
forms occur, which is presumptive evidence that they do not intermix
by breeding.

       *       *       *       *       *

No crayfishes are known to inhabit the rivers of the northern Asiatic
watershed, such as the Obi, Yenisei, and Lena. None are known[27]
in the sea of Aral, or the great rivers Oxus and Jaxartes, which
feed that vast lake; nor any in the lakes of Balkash and Baikal.
If further exploration verifies this negative fact, it will be not
a little remarkable; inasmuch as two[28], if not more, kinds of
crayfishes are found in the basin of the great river Amur, which
drains a large area of north-eastern Asia, and debouches into the
Gulf of Tartary, in about the latitude of York.

Japan has one species (_A. japonicus_), perhaps more; but no crayfish
has as yet been made known in any part of eastern Asia, south of
Amurland. There are certainly none in Hindostan; none are known in
Persia, Arabia, or Syria. In Asia Minor the only recorded locality is
the Rion. No crayfish has yet been discovered in the whole continent
of Africa.[29] {305}

     [27] It would be hazardous, however, to assume that none exist,
     especially in the Oxus, which formerly flowed into the Caspian.

     [28] _A. dauricus_ and _A. Schrenckii_.

     [29] Whatever the so-called _Astacus capensis_ of the Cape
     Colony may be, it is certainly not a crayfish.

Thus, on the continent of the old world, the crayfishes are
restricted to a zone, the southern limit of which coincides with
certain great geographical features; on the west, the Mediterranean,
with its continuation, the Black Sea; then the range of the Caucasus,
followed by the great Asiatic highlands, as far as the Corea on
the east. On the north, though there is no such physical boundary,
the crayfishes appear to be entirely excluded from the Siberian
river basins; while east and west, though a sea-barrier exists, the
crayfishes extend beyond it, to reach the British islands and those
of Japan.

Crossing the Pacific, we meet with some half-a-dozen kinds of
crayfishes,[30] different from those of the old world, but still
belonging to the genus _Astacus_, in British Columbia, Oregon, and
California. Beyond the Rocky Mountains, from the Great Lakes to
Guatemala, crayfishes abound, as many as thirty-two different species
having been described, but they all belong to the genus _Cambarus_
(fig. 63, p. 248). Species of this genus also occur in Cuba,[31]
but, so far as is at present known, not in any of the other West
Indian islands. The occurrence of a curious dimorphism among the male
_Cambari_ has been described by Dr. Hagen; and a blind _Cambarus_
{306} is found, along with other blind animals, in the subterranean
caves of Kentucky.

     [30] Dr. Hagen in his “Monograph of the North American
     Astacidæ,” enumerates six species; _A. Gambelii_, _A.
     klamathensis_, _A. leenisculus_, _A. nigrescens_, _A. oreganus_,
     _and A. Trowbridgii_.

     [31] Von Martens. _Cambarus cubensis._ Archiv. für
     Naturgeschichte, xxxviii.

All the crayfishes of the northern hemisphere belong to the
_Potamobiidæ_, and no members of this family are known to exist
south of the equator. The crayfishes of the southern hemisphere,
in fact, all belong to the division of the _Parastacidæ_, and in
respect of the number and variety of forms and the size which they
reach, the head-quarters of the _Parastacidæ_ is the continent of
Australia. Some of the Australian crayfishes (fig. 76) attain a foot
or more in length, and are as large as full-sized lobsters. The genus
_Engæus_ of Tasmania comprises small crayfish which, like some of the
_Cambari_, live habitually on land, in burrows which they excavate in
the soil.

New Zealand has a peculiar genus of crayfishes, _Paranephrops_, a
species of which is found in the Fiji Islands, but none are known to
occur elsewhere in Polynesia.

Two kinds of crayfish have been obtained in southern Brazil, and
have been described by Dr. v. Martens,[32] as _A. pilimanus_ and _A.
brasiliensis_. I have shown that they belong to a peculiar genus,
_Parastacus_. The former was procured at Porto Alegre, which is
situated in 30° S. Latitude, close to the mouth of the Jacuhy, at the
north end of the great Laguna do Patos, which {308} communicates
by a narrow passage with the sea; and also at Sta. Cruz in the upper
basin of the Rio Pardo, an affluent of the Jacuhy, “by digging it out
of holes in the ground.” The latter (_P. brasiliensis_, fig. 64) was
obtained at Porto Alegre, and further inland, in the region of the
primitive forest at Rodersburg, in shallow streams.

     [32] Südbrasilische Süss- und Brackwasser Crustaceen, nach den
     Sammlungen des Dr. Reinh. Hensel. Archiv. für Naturgeschichte,
     XXXV. 1869.

[Illustration: FIG. 76.—Australian Crayfish (1/3 nat. size).[33]]

     [33] The nomenclature of the Australian crayfishes requires
     thorough revision. I therefore, for the present, assign no name
     to this crayfish. It is probably identical with the _A. nobilis_
     of Dana and the _A. armatus_ of Von Martens.

In addition to these, no crayfish have as yet been found in any of
the great rivers, such as the Orinoko; the Amazon, in which they were
specially sought for by Agassiz; or in the La Plata, on the eastern
side of the Andes. But, on the west, an “_Astacus_” _chilensis_ is
described in the “Histoire Naturelle des Crustacées,” (vol. ii. p.
333). It is here stated that this crayfish “habite les côtes du
Chili,” but the freshwaters of the Chilian coast are doubtless to be
understood.

Finally, Madagascar has a genus and species of crayfish (_Astacoides
madagascariensis_, fig. 65) peculiar to itself.

       *       *       *       *       *

On comparing the results obtained by the study of the geographical
distribution of the crayfishes with those brought to light by
the examination of their morphological characters, the important
fact that there is a broad and general correspondence between the
two becomes apparent. The wide equatorial belt of the earth’s
surface which separates the crayfishes of the northern from those
of the southern hemisphere, is a sort of geographical {310}
representation of the broad morphological differences which mark
off the _Potamobiidæ_ from the _Parastacidæ_. Each group occupies a
definite area of the earth’s surface, and the two are separated by an
extensive border-land untenanted by crayfishes.

[Illustration: FIG. 77.—MAP OF THE WORLD, showing the geographical
distribution of the Crayfishes. I. Eur-asiatic Crayfishes; II.
Amurland Crayfishes; III. Japanese Crayfishes; IV. Western North
American Crayfishes; V. Eastern North American Crayfishes; VI.
Brazilian Crayfishes; VII. Chilian Crayfishes; VIII. Novozelanian
Crayfishes; IX. Fijian Crayfishes; X. Tasmanian Crayfishes; XI.
Australian Crayfishes; XII. Mascarene Crayfishes.]

A similar correspondence is exhibited, though less distinctly, when
we consider the distribution of the genera and species of each
group. Thus, among the _Potamobiidæ_, _Astacus torrentium_ and
_nobilis_ belong essentially to the northern, western, and southern
watersheds of the central European highlands, the streams of which
flow respectively into the Baltic and the North Seas, the Atlantic
and the Mediterranean (fig. 77, I.); _A. leptodactylus_, _pachypus_,
_angulosus_, and _colchicus_, appertain to the Pontocaspian
watershed, the rivers of which drain into the Black Sea and the
Caspian (I.); while _Astacus dauricus_ and _A. Schrenckii_ are
restricted to the widely separated basin of the Amur, which sheds its
waters into the Pacific (II.) The _Astaci_ of the rivers of western
North America, which flow into the Pacific (IV.), and the _Cambari_
of the Eastern or Atlantic water-shed (V.) are separated by the great
physical barrier of the Rocky Mountain ranges. Finally, with regard
to the _Parastacidæ_, the widely separated geographical regions of
New Zealand (VIII.), Australia (IX.), Madagascar (XII.), and South
America (VI. and VII.), are inhabited by generically distinct groups.

But when we look more closely into the matter, it will {311}
be found that the parallel between the geographical and the
morphological facts cannot be quite strictly carried out.

_Astacus torrentium_, as we have seen, inhabits both the British
Islands and the continent of Europe; nevertheless, there is every
reason to believe that twenty miles of sea water is an insuperable
barrier to the passage of crayfishes from one land to the other. For
though some crayfishes live in brackish water, there is no evidence
that any existing species can maintain themselves in the sea. A fact
of the same character meets us at the other side of the Eurasiatic
continent, the Japanese and the Amurland crayfishes being closely
allied; although it is not clear that there are any identical species
on the two sides of the Sea of Japan.

Another circumstance is still more remarkable. The West American
crayfishes are but little more different from the Pontocaspian
crayfishes, than these are from _Astacus torrentium_. On the face
of the matter, one might therefore expect the Amurland and Japanese
crayfishes, which are intermediate in geographical position, to be
also intermediate, morphologically, between the Pontocaspian and the
West American forms. But this is not the case. The branchial system
of the Amurland _Astaci_ appears to be the same as that of the rest
of the genus; but, in the males, the third joint (ischiopodite) of
the second and third pair of ambulatory limbs is provided with a
conical, recurved, hook-like process; while, in the females, the
hinder edge of the penultimate thoracic {312} sternum is elevated
into a transverse prominence, on the posterior face of which there is
a pit or depression.[34]

In both these characters, but more especially in the former, the
Amurland and Japanese _Astaci_ depart from both the Pontocaspian and
the West American _Astaci_, and approach the _Cambari_ of Eastern
North America.

[Illustration: FIG. 78.—_Cambarus_ (Guatemala) penultimate leg.
_cxp_, coxopodite; _cxs_, coxopoditic setæ; _pdb_, podobranchia;
_bp_, basipodite; _ip_, ischiopodite; _mp_, meropodite; _cp_,
carpopodite; _pp_, propodite; _dp_, dactylopodite.]

In these crayfishes, in fact, one or both of the same pairs of legs
in the male are provided with similar hook-like processes; while, in
the females, the modification of the penultimate thoracic sternum
is carried still further and gives rise to the curious structure
described by Dr. Hagen as the “annulus ventralis.”

     [34]  Kessler, l. c.

In all the _Cambari_, the pleurobranchiæ appear to be entirely
suppressed, and the hindermost podobranchia has no lamina; while
the areola is usually extremely narrow. The proportional size of
the areola in the Amurland {313} crayfishes is not recorded; in
the Japanese crayfish, judging by the figure given by De Haan, it
is about the same as in the western _Astaci_. On the other hand,
in the West American crayfishes it is distinctly smaller; so that,
in this respect, they perhaps more nearly approach the _Cambari_.
Unfortunately, nothing is known as to the branchiæ of the Amurland
crayfishes. According to De Haan, those of the Japanese species
resemble those of the western _Astaci_: as those of the West American
_Astaci_ certainly do.

With respect to the _Parastacidcæ_; in the remarkable length and
flatness of the epistoma, the crayfishes of Australia, Madagascar,
and South America, resemble one another. But in its peculiar
truncated rostrum (see fig. 65) and in the extreme modification
of its branchial system, which I have described elsewhere, the
Madagascar genus stands alone.

The _Paranephrops_ of New Zealand and the Fijis, with its wide and
short epistoma, long rostrum, and large antennary squames, is much
more unlike the Australian forms than might be expected from its
geographical position. On the other hand, considering their wide
separation by sea, the amount of resemblance between the New Zealand
and the Fiji species is very remarkable.

       *       *       *       *       *

If the distribution of the crayfishes is compared with that of
terrestrial animals in general, the points of {314} difference are
at least as remarkable as the resemblances.

With respect to the latter, the area occupied by the _Potamobiidæ_,
corresponds roughly with the Palæarctic and Nearctic divisions of the
great Arctogæal provinces of distribution indicated by mammals and
birds; while distinct groups of crayfishes occupy a larger or smaller
part of the other, namely, the Austro-Columbian, Australian, and
Novozelanian primary distributional provinces of mammals and birds.
Again, the peculiar crayfishes of Madagascar answer to the special
features of the rest of the fauna of that island.

But the North American crayfishes extend much further South than
the limits of the Nearctic fauna in general; while the absence of
any group of crayfishes in Africa, or in the rest of the old world,
south of the great Asiatic table-land, forms a strong contrast to the
general resemblance of the North African and Indian fauna to that of
the rest of Arctogæa. Again, there is no such vast difference between
the crayfishes of New Zealand, Australia, and South America, as there
is between the mammals and the birds of those regions.

It may be concluded, therefore, that the conditions which have
determined the distribution of crayfishes have been very different
from those which have governed the distribution of mammals and birds.
But if we compare with the distribution of the crayfishes, not that
of terrestrial animals in general, but only that of freshwater {315}
fishes, some very curious points of approximation become manifest.
The _Salmonidæ_, or fishes of the salmon and trout kind, a few of
which are exclusively marine, many both marine and freshwater, while
others are confined to fresh water, are distributed over the northern
hemisphere, in a manner which recalls the distribution of the
Potamobine crayfishes,[35] though they do not extend so far to the
South in the new world, while they go a little further, namely, as
far as Algeria, Northern Asia Minor, and Armenia, in the old world.
With the exception of the single genus _Retropinna_, which inhabits
New Zealand, no true salmonoid fish occurs south of the equator; but,
as Dr. Günther has pointed out, two groups of freshwater fishes,
the _Haplochitonidæ_ and the _Galaxidæ_, which stand in somewhat
the same relation to the _Salmonidæ_ as the _Parastacidæ_ do to the
_Potamobiidæ_, take the place of the _Salmonidæ_ in the fresh waters
of New Zealand, Australia, and South America. There are two species
of _Haplochiton_ in Tierra del Fuego; and of the closely allied
genus _Prototroctes_, one species is found in South Australia, and
one in New Zealand; of the _Galaxidæ_, the same species, _Galaxias
attennuatus_, occurs in the streams of New Zealand, Tasmania, the
Falkland Islands, and Peru.

     [35] According to Dr. Günther their southern range is similarly
     limited by the Asiatic Highlands. But they abound in the rivers
     both of the old and new worlds which flow into the Arctic sea;
     and though those on the western side of the Rocky Mountains are
     different from the Eastern American forms, yet there are species
     common to both the Asiatic and the American coasts of the North
     Pacific.

Thus, these fish avoid South Africa, as the crayfishes {316} do; but
I am not aware that any member of the group is found in Madagascar,
and thus completes the analogy.

       *       *       *       *       *

The preservation of the soft parts of animals in the fossil state
depends upon favourable conditions of rare occurrence; and, in the
case of the _Crustacea_, it is not often that one can hope to meet
with such small hard parts as the abdominal members, in a good state
of preservation. But without recourse to the branchial apparatus,
and to the abdominal appendages, it might be very difficult to
say whether a given crustacean belonged to the Astacine, or to
the closely allied Homarine group. Of course, if the accompanying
fossils indicated that the deposit in which the remains occur, was of
freshwater origin, the presumption in favour of their Astacine nature
would be very strong; but if they were inhabitants of the sea, the
problem whether the crustacean in question was a marine Astacine, or
a true Homarine, might be very hard to solve.

Undoubted remains of crayfishes have hitherto been discovered only
in freshwater strata of late tertiary age. In Idaho, North America,
Professor Cope[36] found, in association with _Mastodon mirificus_,
and _Equus excelsus_, several species, which he considers to be
distinct from {317} the existing American crayfishes; whether they
are _Cambari_ or _Astaci_ does not appear. But, in the lower chalk of
Ochtrup, in Westphalia, and therefore in a marine deposit, Von der
Marck and Schlüter[37] have obtained a single, somewhat imperfect,
specimen of a crustacean, which they term _Astacus politus_, and
which, singularly enough, has the divided telson found only in the
genus _Astacus_. It would be very desirable to know more about this
interesting fossil. For the present it affords a strong presumption
that a marine Potamobine existed as far back as the earlier part of
the cretaceous epoch.

     [36] On three extinct _Astaci_ from the freshwater Tertiary
     of Idaho. Proceedings of the American Philosophical Society,
     1869–70.

     [37] Neue Fische und Krebse aus der Kreide von Westphalen.
     Palæontographica, Bd. XV., p. 302; tab. XLIV., figs. 4 and 5.

       *       *       *       *       *

Such are the more important facts of Morphology, Physiology, and
Distribution, which make up the sum of our present knowledge of the
Biology of Crayfishes. The imperfection of that knowledge, especially
as regards the relations between Morphology and Distribution, becomes
a serious drawback when we attack the final problem of Biology, which
is to find out why animals of such structure and active powers, and
so localized, exist?

It would appear difficult to frame more than two fundamental
hypotheses in attempting to solve this problem. Either we must seek
the origin of crayfishes in conditions extraneous to the ordinary
course of natural {318} operations, by what is commonly termed
Creation; or we must seek for it in conditions afforded by the
usual course of nature, when the hypothesis assumes some shape of
the doctrine of Evolution. And there are two forms of the latter
hypothesis; for, it may be assumed, on the one hand, that crayfishes
have come into existence, independently of any other form of
living matter, which is the hypothesis of spontaneous or equivocal
generation, or abiogenesis; or, on the other hand, we may suppose
that crayfishes have resulted from the modification of some other
form of living matter; and this is what, to borrow a useful word from
the French language, is known as _transformism_.

I do not think that any hypothesis respecting the origin of
crayfishes can be suggested, which is not referable to one or other
of these, or to a combination of them.

As regards the hypothesis of creation, little need be said. From a
scientific point of view, the adoption of this speculation is the
same thing as an admission that the problem is not susceptible of
solution. Moreover, the proposition that a given thing has been
created, whether true or false, is not capable of proof. By the
nature of the case direct evidence of the fact is not obtainable.
The only indirect evidence is such as amounts to proof that natural
agencies are incompetent to cause the existence of the thing in
question. But such evidence is out of our reach. The most that {319}
can be proved, in any case, is that no known natural cause is
competent to produce a given effect; and it is an obvious blunder to
confound the demonstration of our own ignorance with a proof of the
impotence of natural causes. However, apart from the philosophical
worthlessness of the hypothesis of creation, it would be a waste
of time to discuss a view which no one upholds. And, unless I am
greatly mistaken, at the present day, no one possessed of knowledge
sufficient to give his opinion importance is prepared to maintain
that the ancestors of the various species of crayfish were fabricated
out of inorganic matter, or brought from nothingness into being, by a
creative fiat.

Our only refuge, therefore, appears to be the hypothesis of
evolution. And, with respect to the doctrine of abiogenesis, we may
also, in view of a proper economy of labour, postpone its discussion
until such time as the smallest fragment of evidence that a crayfish
can be evolved by natural agencies from not living matter, is brought
forward.

In the meanwhile, the hypothesis of transformism remains in
possession of the field; and the only profitable inquiry is, how
far are the facts susceptible of interpretation, on the hypothesis
that all the existing kinds of crayfish are the product of the
metamorphosis of other forms of living beings; and that the
biological phenomena which they exhibit are the results of the
interaction, through past time, of two series of {320} factors:
the one, a process of morphological and concomitant physiological
modification; the other, a process of change in the condition of the
earth’s surface.

If we set aside, as not worth serious consideration, the assumption
that the _Astacus torrentium_ of Britain was originally created
apart from the _Astacus torrentium_ of the Continent; it follows,
either that this crayfish has passed across the sea by voluntary
or involuntary migration; or that the _Astacus torrentium_ existed
before the English Channel, and spread into England while these
islands were still continuous with the European mainland; and that
the present isolation of the English crayfishes from the members of
the same species on the Continent is to be accounted for by those
changes in the physical geography of western Europe which, as there
is abundant evidence to prove, have separated the British Islands
from the mainland.

There is no evidence that our crayfish has been purposely introduced
by human agency into Great Britain; and from the mode of life of
crayfish and the manner in which the eggs are carried about by the
parent during their development, transport by birds or floating
timber would seem to be out of the question. Again, although _Astacus
nobilis_ is said to venture into the brackish waters of the Gulf of
Finland, and _A. leptodactylus_, as we have seen, makes itself at
home in the more or less salt Caspian, there is no reason to believe
that _Astacus torrentium_ is capable of existing in {321} sea-water,
still less of crossing the many miles of sea which separate England
from even the nearest point of the Continent. In fact, the existence
of the same kind of crayfish on both sides of the Channel appears to
be only a case of the general truth, that the Fauna of the British
Islands is identical with a part of that of the Continent; and as
our foxes, badgers, and moles certainly have neither swum across,
nor been transported by man, but existed in Britain while it was
still continuous with western Europe, and have been isolated by the
subsequent intervention of the sea, so we may confidently explain the
presence of _Astacus torrentium_ by reference to the same operation.

If we take into account the occurrence of _Astacus nobilis_ over
so large a part of the area occupied by _Astacus torrentium_; its
absence in the British Islands, and in Greece; and the closer
affinity which exists between _A. nobilis_ and _A. leptodactylus_,
than between _A. nobilis_ and _A. torrentium_; it seems not
improbable that Astacus torrentium was the original tenant of the
whole western European area outside the Ponto-Caspian watershed; and
that _A. nobilis_ is an invading offshoot of the Ponto-Caspian or
_leptodactylus_ form which has made its way into the western rivers
in the course of the many changes of level which central Europe has
undergone; in the same way as _A. leptodactylus_ is now passing into
the rivers of the Baltic provinces of Russia.

The study of the glacial phenomena of central Europe {322} has led
Sartorius von Waltershausen[38] to the conclusion that at the time
when the glaciers of the Alps had a much greater extension than at
present, a vast mass of freshwater extended from the valley of the
Danube to that of the Rhone, around the northern escarpment of the
Alpine chain, and connected the head-waters of the Danube with those
of the Rhine, the Rhone, and the northern Italian rivers. As the
Danube debouches into the Black Sea, and this was formerly connected
with the Aralo-Caspian Sea, an easy passage would thus be opened up
by which crayfishes might pass from the Aralo-Caspian area to western
Europe. If they spread by this road, the _Astacus torrentium_ may
represent the first wave of migration westward, while _A. nobilis_
answers to a second, and _A. leptodactylus_, with its varieties,
remains as the representative of the old Aralo-Caspian crayfishes.
And thus the crayfishes would present a curious parallel with the
Iberian, Aryan, and Mongoloid streams of westward movement among
mankind.

If we thus suppose the western Eurasiatic crayfishes to be simply
varieties of a primitive Aralo-Caspian stock, their limitation to the
south by the Mediterranean and by the great Asiatic highlands becomes
easily intelligible.

     [38] “Untersuchungen ueber die Klimate der Gegenwart und der
     Vorwelt.” Natuurkundige Verhandelingen van de Hollandsche
     Maatschappij der Wetenschappen te Haarlem, 1865.

The extremely severe climatal conditions which obtain in northern
Siberia may sufficiently account for the {323} absence of crayfishes
(if they are really absent) in the rivers Obi, Yenisei, and Lena,
and in the great lake Baikal, which lies more than 1,300 feet above
the sea, and is frozen over from November to May. Moreover, there
can be no doubt that, at a comparatively recent period, the whole of
this region, from the Baltic to the mouth of the Lena, was submerged
beneath a southward extension of the waters of the Arctic ocean to
the Aralo-Caspian Sea and Lake Baikal, and a westward extension to
the Gulf of Finland.

The great lakes and inland seas which stretch, at intervals, from
Baikal, on the east, to Wenner in Sweden, on the west, are simply
pools, isolated partly by the rising of the ancient sea-bottom and
partly by evaporation; and often completely converted into fresh
water by the inflow of the surrounding land-drainage. But the
population of these pools was originally the same as that of the
Northern Ocean, and a few species of marine crustaceans, mollusks,
and fish, besides seals, remain in them as living evidences of the
great change which has taken place. The same process which, as we
shall see, has isolated the _Mysis_ of the Arctic seas in the lakes
of Sweden and Finland, has shut up with it other arctic marine
crustacea, such as species of _Gammarus_ and _Idothea_. And the very
same species of _Gammarus_ is imprisoned, along with arctic seals, in
the waters of Lake Baikal.

The distribution of the American crayfishes agrees equally well with
the hypothesis of the northern origin of {324} the stock from which
they have been evolved. Even under existing geographical conditions,
an affluent of the Mississippi, the St. Peter’s river, communicates
directly, in rainy weather, with the Red river, which flows into Lake
Winnipeg, the southernmost of the long series of intercommunicating
lakes and streams, which occupy the low and flat water-parting
between the southern and the northern watersheds of the North
American Continent. But the northernmost of these, the Great Slave
Lake, empties itself by the Mackenzie river into the Arctic Ocean,
and thus provides a route by which crayfishes might spread from the
north over all parts of North America east of the Rocky Mountains.

The so-called Rocky Mountain range is, in reality, an immense
table-land, the edges of which are fringed by two principal lines of
mountainous elevations. The table-land itself occupies the place of
a great north and south depression which, in the cretaceous epoch,
was occupied by the sea and probably communicated with the ocean
at its northern, as well as at its southern end. During and since
this epoch it became gradually filled up, and it now contains an
immense thickness of deposits of all ages from the cretaceous to
the pliocene—the earlier marine, the later more and more completely
freshwater. During the tertiary epoch, various portions of this
area have been occupied by vast lakes, the more northern of which
doubtless had outlets into the Northern sea. That crayfish existed in
the vicinity of the Rocky Mountains {325} in the latter part of the
tertiary epoch is testified by the Idaho fossils. And there is thus
no difficulty in understanding their presence in the rivers which
have now cut their way to the Pacific coast.

The similarity of the crayfish of the Amurland and of Japan is a
fact of the same order as the identity of the English crayfish with
the _Astacus torrentium_ of the European Continent, and is to be
explained in an analogous fashion. For there can be no doubt that the
Asiatic continent formerly extended much further to the eastward than
it does at present, and included what are now the islands of Japan.
Even with this alteration of the geographical conditions, however, it
is not easy to see how crayfishes can have got into the Amur-Japanese
fresh waters. For a north-eastern prolongation of the Asiatic
highlands, which ends to the north in the Stanovoi range, shuts in
the Amur basin on the west; while the Amur debouches into the sea
of Okhotsk, and the Pacific ocean washes the shores of the Japanese
islands.

But there are many grounds for the conclusion that, in the latter
half of the tertiary epoch, eastern Asia and North America were
connected, and that the chain of the Kurile and Aleutian islands may
indicate the position of a great extent of submerged land. In that
case, the sea of Okhotsk and Behring’s sea may occupy the site of
inland waters which formerly placed the mouth of the Amur in direct
communication with the Northern Ocean, just as the Black Sea, at
present, brings the basin of the {326} Danube into connection, first
with the Mediterranean and then with the western Atlantic; and, as
in former times, it gave access from the south to the vast area
now drained by the Volga. When the Black Sea communicated with the
Aralo-Caspian sea, and this opened to the north into the Arctic sea,
a chain of great inland waters must have skirted the eastern frontier
of Europe, just such as would now lie on the eastern frontier of Asia
if the present coast underwent elevation.

Supposing, however, that the ancestral forms of the _Potamobiidæ_
obtained access to the river basins in which they are now found, from
the north, the hypothesis that a mass of fresh water once occupied a
great part of the region which is now Siberia and the Arctic Ocean,
would be hardly tenable, and it is, in fact, wholly unnecessary for
our present purpose.

The vast majority of the stalk-eyed crustaceans are, and always have
been, exclusively marine animals; the crayfishes, the _Atyidæ_, and
the fluviatile crabs (_Thelphusidæ_), being the only considerable
groups among them which habitually confine themselves to fresh
waters. But even in such a genus as _Penæus_, most of the species of
which are exclusively marine, some, such as _Penæus brasiliensis_,
ascend rivers for long distances. Moreover, there are cases in which
it cannot be doubted that the descendants of marine _Crustacea_ have
gradually accustomed themselves to fresh water conditions, and have,
at the same time, become more or less modified, {327} so that they
are no longer absolutely identical with those descendants of their
ancestors which have continued to live in the sea.[39]

In several of the lakes of Norway, Sweden and Finland, and in Lake
Ladoga, in Northern Europe; in Lake Superior and Lake Michigan,
in North America; a small crustacean, _Mysis relicta_, occurs in
such abundance as to furnish a great part of the supply of food to
the fresh water fishes which inhabit these lakes. Now, this _Mysis
relicta_ is hardly distinguishable from the _Mysis oculata_ which
inhabits the Arctic seas, and is certainly nothing but a slight
variety of that species.

In the case of the lakes of Norway and Sweden, there is independent
evidence that they formerly communicated with the Baltic, and were,
in fact, fiords or arms of the sea. The communication of these fiords
with the sea having been gradually cut off, the marine animals they
contained have been imprisoned; and as the water has been slowly
changed from salt to fresh by the drainage of the surrounding land,
only those which were able to withstand the altered conditions have
survived. Among these is the _Mysis oculata_, which has in the
meanwhile undergone the slight variation which has converted it into
_Mysis relicta_. Whether the same explanation {328} applies to Lakes
Superior and Michigan, or whether the _Mysis oculata_ has not passed
into these masses of fresh water by channels of communication with
the Arctic Ocean which no longer exist, is a secondary question. The
fact remains that _Mysis relicta_ is a primitively marine animal
which has become completely adapted to fresh-water life.

     [39] See on this interesting subject: Martens, “On the
     occurrence of marine animal forms in fresh water.” Annals of
     Natural History, 1858: Lovèn. “Ueber einige im Wetter und Wener
     See gefundene Crustaceen.” Halle Zeitschrift für die Gesammten
     Wissenschaften, xix., 1862: G. O. Sars, “Histoire Naturelle des
     Crustacés d’eau douce de Norvège,” 1867.

Several species of prawns (_Palæmon_) abound in our own seas. Other
marine prawns are found on the coasts of North America, in the
Mediterranean, in the South Atlantic and Indian Oceans, and in the
Pacific as far south as New Zealand. But species of the same genus
(_Palæmon_) are met with, living altogether in fresh water, in
Lake Erie, in the rivers of Florida, in the Ohio, in the rivers of
the Gulf of Mexico, of the West India Islands and of eastern South
America, as far as southern Brazil, if not further; in those of Chili
and those of Costa Rica in western South America; in the Upper Nile,
in West Africa, in Natal, in the Islands of Johanna, Mauritius, and
Bourbon, in the Ganges, in the Molucca and Philippine Islands, and
probably elsewhere.

Many of these fluviatile prawns differ from the marine species not
only in their great size (some attaining a foot or more in length),
but still more remarkably in the vast development of the fifth pair
of thoracic appendages. These are always larger than the slender
fourth pair (which answer to the forceps of the crayfishes); and,
in the males especially, they are very long and strong, and {329}
are terminated by great chelæ, not unlike those of the crayfishes.
Hence these fluviatile prawns (known in many places by the name of
“Cammarons”) are not unfrequently confounded with true crayfishes;
though the fact that there are only three pair of ordinary legs
behind the largest, forceps-like pair, is sufficient at once to
distinguish them from any of the _Astacidæ_.

[Illustration: FIG. 79. _Palæmon jamaicensis_ (about 5/7 nat. size).
A, female; B, fifth thoracic appendage of male.]

Species of these large-clawed prawns live in the {330} brackish
water lagoons of the Gulf of Mexico, but I am not aware that any
of them have yet been met with in the sea itself. The _Palæmon
lacustris_ (_Anchistia migratoria_, Heller) abounds in fresh-water
ditches and canals between Padua and Venice, and in the Lago di
Garda, as well as in the brooks of Dalmatia; but its occurrence in
the Adriatic or the Mediterranean, which has been asserted, appears
to be doubtful. So the Nile prawn, though very similar to some
Mediterranean prawns, does not seem to be identical with any at
present known.[40]

In all these cases, it appears reasonable to apply the analogy of the
_Mysis relicta_, and to suppose that the fluviatile prawns are simply
the result of the adaptive modification of species which, like their
congeners, were primitively marine.

     [40] Heller, “Die Crustaceen des südlichen Europas,” p. 259.
     Klunzinger, “Ueber eine Süsswasser-crustacee im Nil,” with
     the notes by von Martens and von Siebold: Zeitschrift für
     Wissenschaftliche Zoologie, 1866.

But if the existing sea prawns were to die out, or to be beaten in
the struggle for existence, we should have, scattered over the world
in isolated river basins, more or less distinct species of freshwater
prawns,[41] the areas inhabited by which might hereafter be
indefinitely enlarged or diminished, by alteration in the elevation
of the {331} land and by other changes in physical geography. And,
indeed, under these circumstances, the freshwater prawns themselves
might become so much modified, that, even if the descendants of their
ancestors remained unchanged in structure and habits in the sea, the
relationship of the two might no longer be obvious.

     [41] This seems actually to have happened in the case of the
     widely-spread allies and companions of the fluviatile prawns,
     _Atya_ and _Caridina_. I am not aware that truly marine species
     of these genera are known.

These considerations appear to me to indicate the direction in which
we must look for a rational explanation of the origin of crayfishes
and their present distribution.

I have no doubt that they are derived from ancestors which lived
altogether in the sea, as the great majority of the _Mysidæ_ and
many of the prawns do now; and that, of these ancestral crayfishes,
there were some which, like _Mysis oculata_ or _Penæus brasiliensis_,
readily adapted themselves to fresh water conditions, ascended
rivers, and took possession of lakes. These, more or less modified,
have given rise to the existing crayfishes, while the primitive stock
would seem to have vanished. At any rate, at the present time, no
marine crustacean with the characters of the _Astacidæ_ is known.

As crayfishes have been found in the later tertiaries of North
America, we shall hardly err in dating the existence of these
marine crayfishes at least as far back as the miocene epoch;
and I am disposed to think that, during the earlier tertiary
and later mesozoic periods, these _Crustacea_ not only had as
wide a distribution as the Prawns and _Penæi_ have now, but were
differentiated into two groups, one with the general characters of
the {332} _Potamobiidæ_ in the northern hemisphere, and another,
with those of the _Parastacidæ_, in the southern hemisphere.

The ancestral Potamobine form probably presented the peculiarities
of the _Potamobiidæ_ in a less marked degree than any existing
species does. Probably the four pleurobranchiæ were all equally
well developed; the laminæ of the podobranchiæ smaller and less
distinct from the stem; the first and second abdominal appendages
less specialised; and the telson less distinctly divided. So far as
the type was less specially Potamobine, it must have approached the
common form in which _Homarus_ and _Nephrops_ originated. And it
is to be remarked that these also are exclusively confined to the
northern hemisphere.

The wide range and close affinity of the genera _Astacus_ and
_Cambarus_ appear to me to necessitate the supposition that they are
derived from some one already specialised Potamobine form; and I have
already mentioned the grounds upon which I am disposed to believe
that this ancestral Potamobine existed in the sea which lay north of
the miocene continent in the northern hemisphere.

In the marine primitive crayfishes south of the equator, the
branchial apparatus appears to have suffered less modification, while
the suppression of the first abdominal appendages, in both sexes, has
its analogue among the _Palinuridæ_, the headquarters of which are in
the southern hemisphere. That they should have ascended {333} the
rivers of New Zealand, Australia, Madagascar, and South America, and
become fresh water _Parastacidæ_, is an assumption which is justified
by the analogy of the fresh-water prawns. It remains to be seen
whether marine _Parastacidæ_ still remain in the South Pacific and
Atlantic Oceans, or whether they have become extinct.

       *       *       *       *       *

In speculating upon the causes of an effect which is the product of
several co-operating factors, the nature of each of which has to be
divined by reasoning backwards from its effects, the probability of
falling into error is very great. And this probability is enhanced
when, as in the present case, the effect in question consists of a
multitude of phenomena of structure and distribution about which much
is yet imperfectly known. Hence the preceding discussion must rather
be regarded as an illustration of the sort of argumentation by which
a completely satisfactory theory of the ætiology of the crayfish
will some day be established, than as sufficing to construct such a
theory. It must be admitted that it does not account for the whole of
the positive facts which have been ascertained; and that it requires
supplementing, in order to furnish even a plausible explanation of
various negative facts.

The positive fact which presents a difficulty is the closer
resemblance between the Amur-Japanese crayfish and the East American
_Cambari_, than between the {334} latter and the West American
_Astaci_; and the closer resemblance between the latter and the
Pontocaspian crayfish, than either bear to the Amur-Japanese form.
If the facts had been the other way, and the West American and
Amur-Japanese crayfish had changed places, the case would have been
intelligible enough. The primitive Potamobine stock might then have
been supposed to have differentiated itself into a western astacoid,
and an eastern cambaroid form;[42] the latter would have ascended
the American, and the former the Asiatic rivers. As the matter
stands, I do not see that any plausible explanation can be offered
without recourse to suppositions respecting a former more direct
communication between the mouth of the Amur, and that of the North
American rivers, in favour of which no definite evidence can be
offered at present.

The most important negative fact which remains to be accounted for
is the absence of crayfishes in the rivers of a large moiety of the
continental lands, and in numerous islands. Differences of climatal
conditions are obviously inadequate to account for the absence of
crayfishes in Jamaica, when they are present in Cuba; for their
absence in Mozambique, and the islands of Johanna and Mauritius, when
they are present in Madagascar; and for their absence in the Nile,
when they exist in Guatemala. {335}

     [42] Just as there is an American form of _Idothea_ and an
     Asiatic form in the Arctic ocean at the present day.

At present, I confess that I do not see my way to a perfectly
satisfactory explanation of the absence of crayfishes in so many
parts of the world in which they mighty _à priori_, be expected to
exist; and I can only suggest the directions in which an explanation
may be sought.

The first of these is the existence of physical obstacles to the
spread of crayfishes, at the time at which the Potamobine and the
Parastacine stocks respectively began to take possession of the
rivers, some of which have now ceased to exist; and the second is
the probability that, in many rivers which have been accessible to
crayfishes, the ground was already held by more powerful competitors.

If the ancestors of the Potamobine crayfishes originated only among
those primitive crayfishes which inhabited the seas north of the
miocene continent, their present limitation to the south, in the old
world, is as easily intelligible as is their extension southward,
in the course of the river basins of Northern America as far as
Guatemala, but no further. For the elevation of the Eurasiatic
highlands had commenced in the miocene epoch, while the isthmus of
Panama was interrupted by the sea.

With respect to the Southern hemisphere, the absence of crayfishes in
Mauritius and in the islands of the Indian Ocean, though they occur
in Madagascar, may be due to the fact that the former islands are of
comparatively late volcanic origin; while Madagascar is the remnant
of {336} a very ancient continental area, the oldest indigenous
population of which, in all probability, is directly descended from
that which occupied it at the beginning of the tertiary epoch. If
Parastacine _Crustacea_ inhabited the southern hemisphere at this
period, and subsequently became extinct as marine animals, their
preservation in the freshwaters of Australia, New Zealand, and the
older portions of South America may be understood. The difficulty
of the absence of crayfishes in South Africa[43] remains; and all
that can be said is, that it is a difficulty of the same nature as
that which confronts us when we compare the fauna of South Africa in
general with that of Madagascar. The population of the latter region
has a more ancient aspect than that of the former; and it may be that
South Africa, in its present shape, is of very much later date than
Madagascar.

     [43] But it must be remembered that we have as yet everything to
     learn respecting the fauna of the great inland lakes and river
     systems of South Africa.

With respect to the second point for consideration, it is to be
remarked that, in the temperate regions of the world, the crayfishes
are by far the largest and strongest of any of the inhabitants of
freshwater, except the _Vertebrata_; and that while frogs and the
like fall an easy prey to them, they must be formidable enemies
and competitors even to fishes, aquatic reptiles, and the smaller
aquatic mammals. In warm climates, however, not only the large
prawns which have been mentioned, but _Atyæ_ {337} and fluviatile
crabs (_Thelphusa_) compete for the possession of the freshwaters;
and it is not improbable that under some circumstances, they may be
more than a match for crayfishes; so that the latter might either
be driven out of territory they already occupied, as _Astacus
leptodactylus_ is driving out _A. nobilis_ in the Russian rivers; or
might be prevented from entering rivers already tenanted by their
rivals.

In connection with this speculation, it is worthy of remark that the
area occupied by the fluviatile crabs is very nearly the same as that
zone of the earth’s surface from which crayfish are excluded, or in
which they are scanty. That is to say, they are found in the hotter
parts of the eastern side of the two Americas, the West Indies,
Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan,
Burmah, China, Japan, and the Sandwich Islands. The large-clawed
fluviatile prawns are found in the same regions of America, on both
east and west coasts, in Africa, Southern Asia, the Moluccas, and the
Philippine Islands; while the _Atyidæ_ not only cover the same area,
but reach Japan, extend over Polynesia, to the Sandwich Islands,
on the north, and New Zealand, on the south, and are found on both
shores of the Mediterranean; a blind form (_Troglocaris Schmidtii_),
in the Adelsberg caves, representing the blind _Cambarus_ of the
caves of Kentucky.

       *       *       *       *       *

The hypothesis respecting the origin of crayfishes {338} which
has been tentatively put forward in the preceding pages, involves
the assumption that marine Crustacea of the astacine type were in
existence during the deposition of the middle tertiary formations,
when the great continents began to assume their present shape.
That such was the case there can be no doubt, inasmuch as abundant
remains of Crustacea of that type occur still earlier in the mesozoic
rocks. They prove the existence of ancient crustaceans, from which
the crayfishes may have been derived, at that period of the earth’s
history when the conformation of the land and sea were such as to
admit of their entering the regions in which we now find them.

The materials which have, up to the present, time been collected are
too scanty to permit of the tracing out of all the details of the
genealogy of the crayfish. Nevertheless, the evidence which exists
is perfectly clear, as far as it goes, and is in complete accordance
with the requirements of the doctrine of evolution.

Mention has been made of the close affinity between the
crayfishes and the lobsters—the _Astacina_ and the _Homarina_;
and it fortunately happens that these two groups, which may be
included under the common name of the _Astacomorpha_, are readily
distinguishable from all the other _Podophthalmia_ by peculiarities
of their exoskeleton which are readily seen in all well-preserved
fossils. In all, as in the crayfish, there are large forceps,
followed by two pairs of chelate ambulatory limbs, while {339} the
succeeding two pairs of legs are terminated by simple claws. The
exopodite of the last abdominal appendage is divided into two parts
by a transverse suture. The pleura of the second abdominal somite
are larger than the others, and overlap those of the first somite,
which are very small. Any fossil crustacean which presents all these
characters, is certainly one of the _Astacomorpha_.

The _Astacina_, again, are distinguished from the _Homarina_ by the
mobility of the last thoracic somite, and the characters of the first
and second abdominal appendages, when they are present; or by their
entire absence. But it is so difficult to make out anything about
either of these characters in fossils, that, so far as I am aware,
we know nothing about them in any fossil Astacomorph. And hence, it
may be impossible to say to which division any given form belongs,
unless its resemblances to known types are so minute and so close as
to remove doubt.

For the present purpose, the series of the fossiliferous rocks may
be grouped as follows:—1. Recent and Quaternary. 2. Newer Tertiary
(Pliocene and Miocene). 3. Older Tertiary (Eocene). 4. Cretaceous
(Chalk, Greensand and Gault). 5. Wealden. 6. Jurassic (Purbeck
to Inferior Oolite). 7. Liassic. 8. Triassic. 9. Permian. 10.
Carboniferous. 11. Devonian. 12. Silurian. 13. Cambrian.

[Illustration: FIG. 80.—A, _Pseudastacus pustulosus_ (nat. size). B,
_Eryma modestiformis_ (× 2). Both figures after Oppel.]

Now the oldest known member of the group of the {341} decapod
_Podophthalmia_ to which the _Astacomorpha_ belong occurs in the
Carboniferous formation. It is the genus _Anthrapalæmon_—a small and
very curious crustacean, about which nothing more need be said at
present, as it does not appear to have special affinities with the
_Astacomorpha_. In the later formations, up to the top of the Trias,
podophthalmatous _Crustacea_ are very rare; and, unless the Triassic
genus _Pemphix_ is an exception, no Astacomorphs are known to occur
in them. The specimens of _Pemphix_ which I have examined are not
sufficiently complete to enable me to express any opinion about them.

The case is altered when we reach the Middle Lias. In fact this
yields several forms of a genus, _Eryma_ (fig. 80, B), which also
occurs in the overlying strata almost up to the top of the Jurassic
series, and presents so many variations that nearly forty different
species have been recognised. _Eryma_ is, in all respects, an
Astacomorph, and so far as can be seen, it differs from the existing
genera only in such respects as those in which they differ from one
another. Thus it is quite certain that Astacomorphous _Crustacea_
have existed since a period so remote as the older part of the
Mesozoic period; and any hesitation in admitting this singular
persistency of type on the part of the crayfishes, is at once removed
by the consideration of the fact that, along with _Eryma_, in the
Middle Lias, prawn-like _Crustacea_, generically identical with
the existing _Penæus_, flourished in the sea {342} and left their
remains in the mud of the ancient sea bottom.

[Illustration: FIG. 81.—_Hoploparia longimana_ (2/3 nat. size).—_cp_,
carapace; _r_, rostrum, T, telson; XV., XVI., first and second
abdominal somites; 10, forceps; 20, last abdominal appendage.]

_Eryma_ is the only crustacean, which can be certainly ascribed to
the _Astacomorpha_, that has hitherto been found in the strata from
the Middle Lias to the lithographic slates; which last lie in the
upper part of the Jurassic series. In the freshwater beds of the
Wealden, no _Astacomorpha_ are known, and although no very great
weight is to be attached to a negative fact of this kind, it is, so
far, evidence that the _Astacomorpha_ had not yet taken to freshwater
life. In the marine deposits of the Cretaceous epoch, however,
astacomorphous forms, which {343} are known by the generic names of
_Hoploparia_ and _Enoploclytia_, are abundant.

The differences between these two genera, and between both and
_Eryma_, are altogether insignificant from a broad morphological
point of view. They appear to me to be of less importance than those
which obtain between the different existing genera of crayfishes.

_Hoploparia_ is found in the London clay. It therefore extends beyond
the bounds of the Mesozoic epoch into the older Tertiary. But when
this genus is compared with the existing _Homarus_ and _Nephrops_,
it is found partly to resemble the one and partly the other. Thus,
on one line, the actual series of forms which have succeeded one
another from the Liassic epoch to the present day, is such as must
have existed if the common lobster and the Norway lobster are the
descendants of _Erymoid_ crustaceans which inhabited the seas of the
Liassic epoch.

Side by side with _Eryma_, in the lithographic slates, there is a
genus, _Pseudastacus_ (fig. 80, A), which, as its name implies, has
an extraordinarily close resemblance to the crayfishes of the present
day. Indeed there is no point of any importance in which (in the
absence of any knowledge of the abdominal appendages in the males)
it differs from them. On the other hand, in some features, as in
the structure of the carapace, it differs from _Eryma_, much as the
existing crayfishes differ from _Nephrops_. Thus, in the latter part
of the Jurassic epoch, the Astacine type {344} was already distinct
from the Homarine type, though both were marine; and, since _Eryma_
begins at least as early as the Middle Lias, it is possible that
_Pseudastacus_ goes back as far, and that the common protastacine
form is to be sought in the Trias. _Pseudastacus_ is found in the
marine cretaceous rocks of the Lebanon, but has not yet been traced
into the Tertiary formations.

I am disposed to think that _Pseudastacus_ is comparable to such a
form as _Astacus nigrescens_ rather than to any of the _Parastacidæ_,
as I doubt the existence of the latter group at any time in northern
latitudes.

In the chalk of Westphalia (also a marine deposit) a single specimen
of another Astacomorph has been discovered, which possesses an
especial interest as it is a true _Astacus_ (_A. politus_, Von der
Marck and Schlüter), provided with the characteristic transversely
divided telson which is found in the majority of the _Potamobiidæ_.

If we arrange the results of palæontological inquiry which have now
been stated in the form of a table such as that which is given on the
following page, the significance of the succession of astacomorphous
forms, in time, becomes apparent. {345}


 SUCCESSIVE FORMS OF THE ASTACOMORPHOUS TYPE.

 I. Recent.                     _Potamobiidæ._     _Homarina._       _Penæus._
 ────────────────────────────────────|─────────────────|────────────────────|──
 II. Later Tertiary.  _Astacus_      |                 |                    |
                     (Idaho).        |                 |                    |
 ────────────────────────|───────────|─────────────────|────────────────────|──
 III. Earlier Tertiary.  |           |             _Hoploparia._            |
 ────────────────────────|───────────|──────────────────────────────────────|──
 IV. Cretaceous.  _Astacus._ _Pseudastacus._ _Enoploclytia._ _Hoploparia._  |
 ────────────────────────────\─────────────────────────────/────────────────|──
 V. Wealden                   \                           /                 |
     (Fresh Water).            \                         /                  |
 ───────────────────────────────\───────────────────────/───────────────────|──
 VI. Jurassic.              _Pseudastacus._         _Eryma._          _Penæus._
 ─────────────────────────────────|────────────────────|────────────────────|──
 VII. Liassic.                    |                 _Eryma._          _Penæus._
 ──────────────────────────────────────────────────────────────────────────────
 VIII. Triassic.
 ──────────────────────────────────────────────────────────────────────────────
 IX. Permian.
 ──────────────────────────────────────────────────────────────────────────────
 X. Carboniferous.                 _Anthrapalæmon._
 ──────────────────────────────────────────────────────────────────────────────
 XI. Devonian.
 ──────────────────────────────────────────────────────────────────────────────
 XII. Silurian.
 ──────────────────────────────────────────────────────────────────────────────
 XIII. Cambrian.

If an Astacomorphous crustacean, having characters intermediate
between those of _Eryma_ and those of _Pseudastacus_, existed
in the Triassic epoch or earlier; if it gradually diverged into
Pseudastacine and Erymoid forms; if these again took on Astacine
and Homarine {346} characters, and finally ended in the existing
_Potamobiidæ_ and _Homarina_, the fossil forms left in the track of
this process of evolution would be very much what they actually are.
Up to the end of the Mesozoic epoch the only known _Potamobiidæ_
are marine animals. And we have already seen that the facts of
distribution suggest the hypothesis that they must have been so, at
least up to this time.

Thus, with respect to the Ætiology of the crayfishes, all the known
facts are in harmony with the requirements of the hypothesis that
they have been gradually evolved in the course of the Mesozoic
and subsequent epochs of the world’s history from a primitive
Astacomorphous form.

And it is well to reflect that the only alternative supposition is,
that these numerous successive and coexistent forms of insignificant
animals, the differences of which require careful study for their
discrimination, have been separately and independently fabricated,
and put into the localities in which we find them. By whatever verbal
fog the question at issue may be hidden, this is the real nature of
the dilemma presented to us not only by the crayfish, but by every
animal and by every plant; from man to the humblest animalcule; from
the spreading beech and towering pine to the _Micrococci_ which lie
at the limit of microscopic visibility.

{347}



NOTES.


NOTE I., CHAPTER I., p. 17.

THE CHEMICAL COMPOSITION OF THE EXOSKELETON.

The harder parts of the exoskeleton of the crayfish contain rather
more than half their weight of calcareous salts. Of these nearly
seven-eighths consist of carbonate of lime, the rest being phosphate
of lime.

The animal matter consists for the most part of a peculiar
substance termed _Chitin_, which enters into the composition of
the hard parts not only of the _Arthropoda_ in general but of many
other invertebrated animals. Chitin is not dissolved even by hot
caustic alkalies, whence the use of solutions of caustic potash
and soda in cleaning the skeletons of crayfishes. It is soluble
in cold concentrated hydrochloric acid without change, and may be
precipitated from its solution by the addition of water.

Chitin contains nitrogen, and according to the latest investigations
(Ledderhose, “Ueber Chitin und seine Spaltungs-produkte:” Zeitschrift
für Physiologische Chemie, II. 1879) its composition is represented
by the formula C_{15}H_{26}N_{2}O_{10} .


NOTE II., CHAPTER I., p. 29.

THE CRAB’S EYES, OR GASTROLITHS.

The “Gastroliths,” as the “crab’s eyes” may be termed, are found
fully developed only in the latter part of the summer season, just
before ecdysis sets in. They then give rise to rounded prominences,
one on {348} each side of the anterior part of the cardiac division
of the stomach. The proper wall of the stomach is continued over the
outer surface of the prominence; and, in fact, forms the outer wall
of the chamber in which the gastrolith is contained, the inner wall
being formed by the cuticular lining of the stomach. When the outer
wall is cut through, it is readily detached from the convex outer
surface of the gastrolith, with which it is in close contact. The
inner surface of the gastrolith is usually flat or slightly concave.
Sometimes it is strongly adherent to the chitonous cuticula; but when
fully formed it is readily detached from the latter. Thus the proper
wall of the stomach invests only the outer face of the gastrolith,
the inner face of which is adherent to, or at any rate in close
contact with, the cuticula. The gastrolith is by no means a mere
concretion, but is a cuticular growth, having a definite structure.
Its inner surface is smooth, but the outer surface is rough, from
the projection of irregular ridges which form a kind of meshwork.
A vertical section shows that it is composed of thin superimposed
layers, of which the inner are parallel with the flat inner surface,
while the outer becomes gradually concentric with the outer surface.
Moreover, the inner layers are less calcified than the outer, the
projections of the outer surface being particularly dense and hard.
In fact, the gastroliths are very similar to other hard parts of the
exoskeleton in structure, except that the densest layers are nearest
the epithelial substratum, instead of furthest away from it.

When ecdysis occurs, the gastroliths are cast off along with the
gastric armature in general, into the cavity of the stomach, and are
there dissolved, a new cuticle being formed external to them from
the proper wall of the stomach. The dissolved calcareous matter is
probably used up in the formation of the new exoskeleton.

According to the observations of M. Chantran (Comptes Rendus,
LXXVIII. 1874) the gastroliths begin to be formed about forty days
before ecdysis takes place in crayfish of four years’ old; but the
interval is less in younger crayfish, and is not more than ten days
during the first year after birth. When shed into the stomach during
ecdysis they are ground down, not merely dissolved. The process
of destruction and absorption takes twenty-four to thirty hours
in very young crayfish, seventy to eighty hours in adults. Unless
the gastroliths are normally developed and re-absorbed, ecdysis is
not healthily effected, and the crayfish dies in the course of the
process. {349}

According to Dulk (“Chemische Untersuchung der Krebsteine:” Müller’s
Archiv. 1835), the gastroliths have the following composition:—

 Animal matter soluble in water                     11·43
 Animal matter insoluble in water (probably chitin)  4·33
 Phosphate of lime                                  18·60
 Carbonate of lime                                  63·16
 Soda reckoned as carbonate                          1·41
                                                    ─────
                                                    98·93
                                                    ─────

The proportion of mineral to animal matter and of phosphate to
carbonate of lime is therefore greater in the gastroliths than in the
exoskeleton in general.


NOTE III., CHAPTER I., p. 31.

GROWTH OF CRAYFISH.

The statements in the text, after the words “By the end of the year,”
regarding the sizes of the crayfish at different ages, are given
on the authority of M. Carbonnier (L’Écrevisse. Paris, 1869); but
they obviously apply only to the large “Écrevisse à pieds rouges”
of France, and not to the English crayfish, which appears to be
identical with the “Écrevisse à pieds blancs,” and is of much smaller
size. According to M. Carbonnier (l. c. p. 51), the young crayfish
just born is “un centimètre et demi environ,” that is to say,
three-fifths of an inch long. The young of the English crayfish still
attached to the mother, which I have seen, rarely exceeds half this
length.

M. Soubeiran (“Sur l’histoire naturelle et l’education des
Écrevisses:” Comptes Rendus, LX. 1865) gives the result of his study
of the growth of the crayfishes reared at Clairefontaine, near
Rambouillet, in the following table:

                          Mean length.  Mean weight.
                            Metres.      Grammes.

 Crayfish of the year        0·025         0·50
 Crayfish 1 year old         0·050         1·50
 Crayfish 2 years old        0·070         3·50
 Crayfish 3 years old        0·090         6·50
 Crayfish 4 years old        0·110        17·50
 Crayfish 5 years old        0·125        18·50
 Crayfish indeterminate      0·160        30·00
 Crayfish very old           0·190       125·00

These observations must also apply to the “Écrevisse à pieds rouges.”

{350}


NOTE IV., CHAPTER I., p. 37.

THE ECDYSES OF CRAYFISHES.

There is a good deal of discrepancy between different observers as to
the frequency of the process of ecdysis in crayfishes. In the text
I have followed M. Carbonnier, but M. Chantran (“Observations sur
l’histoire naturelle des Écrevisses:” Comptes Rendus, LXXI. 1870,
and LXXIII. 1871), who appears to have studied the question (on the
“écrevisse à pieds rouges” apparently) very carefully, declares that
the young crayfish moults no fewer than eight times in the course of
the first twelve months. The first moult takes place ten days after
it is hatched; the second, third, fourth, and fifth, at intervals
of from twenty to twenty-five days, so that the young animal moults
five times in the course of the ninety to one hundred days of July,
August, and September. From the latter month to the end of April in
the following year, no ecdysis takes place. The sixth takes place in
May, the seventh in June, and the eighth in July. In the second year
of its age, the crayfish moults five times, that is to say, in August
and in September, and in May, June, and July following. In the third
year, the crayfish commonly moults only twice, namely in July and in
September. At a greater age than this, the females moult only once a
year, from August to September; while the males moult twice, first in
June and July; afterwards in August and September.

The details of the process of ecdysis are discussed by Braun,
“Ueber die histologischen Vorgänge bei der Häutung von _Astacus
fluviatilis_.” Würzburg Arbeiten, Bd. II.


NOTE V., CHAPTER I., p. 39.

REPRODUCTION IN CRAYFISHES.

The males are said to approach the females in November, December,
and January, in the case of the French crayfishes. In England
they certainly begin as early as the beginning of October, if not
earlier. According to M. Chantran (Comptes Rendus, 1870), and M.
Gerbe (Comptes Rendus, 1858), the male seizes the female with his
pincers, throws her on her back, and deposits the spermatic matter,
firstly, on the external plates of the caudal fin; secondly, on the
thoracic sterna around the external openings of the oviducts. During
this operation, the appendages of the two first abdominal somites are
carried backwards, {351} the extremities of the posterior pair are
inclosed in the groove of the anterior pair; and the end of the vas
deferens becoming everted and prominent, the seminal matter is poured
out, and runs slowly along the groove of the anterior appendage
to its destination, where it hardens and assumes a vermicular
aspect. The filaments of which it is composed are, in fact, tubular
spermatophores, and consist of a tough case or sheath filled with
seminal matter. The spoon-shaped extremity of the second abdominal
appendage, working backwards and forwards in the groove of the
anterior appendage, clears the seminal matter out of it, and prevents
it from becoming choked.

After an interval which varies from ten to forty-five days,
oviposition takes place. The female, resting on her back, bends the
end of the abdomen forward over the hinder thoracic sterna, so that a
chamber is formed into which the oviducts open. The eggs are passed
into the chamber by one operation, usually during the night, and
are plunged into a viscous greyish mucus with which it is filled.
The spermatozoa pass out of the vermicular spermatophores, and mix
with this fluid, in which the peculiarity of their form renders them
readily recognisable. The spermatozoa are thus brought into close
relation with the ova, but what actually becomes of them is unknown.

The origin of the viscous matter which fills the abdominal
chamber when the eggs are deposited in it, and the manner in
which these become fixed to the abdominal limbs is discussed by
Lereboullet (“Recherches sur le mode de fixation des œufs aux
faux pattes abdominaux dans les Écrevisses.” Annales des Sciences
Naturelles, 4e Ee. T. XIV. 1860), and by Braun (Arbeiten aus dem
Zoologisch-Zootomischen Institut in Würzburg, II.).


NOTE VI., CHAPTER I., p. 42.

ATTACHMENT OF THE YOUNG CRAYFISH TO THE MOTHER.

I observe that I had overlooked a passage in the Report on the award
of the Prix Montyon for 1872, Comptes Rendus, LXXV. p. 1341, in which
M. Chantran is stated to have ascertained that the young crayfishes
fix themselves “en saisissant avec un de leurs pinces le filament qui
suspend l’œuf à une fausse patte de la mère.”

In the paper already cited from the Comptes Rendus for 1870, M.
Chantran states that the young remain attached to the mother during
ten days after hatching, that is to say, up to the first moult.
Detached before this period, they die; but after the first moult,
they sometimes leave the {352} mother and return to her again, up to
twenty-eight days, when they become independent.

In a note appended to M. Chantran’s paper, M. Robin states, that
“the young are suspended to the abdomen of the mother by the
intermediation of a chitinous hyaline filament, which extends from
a point of the internal surface of the shell of the egg as far
as the four most internal filaments of each of the lobes of the
median membranous plate of the caudal appendage. The filaments
exist when the embryos have not yet attained three-fourths of their
development.” Is this a larval coat? Rathke does not mention it and I
have seen nothing of it in those recently hatched young which I have
had the opportunity of examining.


NOTE VII., CHAPTER II., p. 64.

THE “SALIVARY” GLANDS AND THE SO-CALLED “LIVER” OF THE CRAYFISH.

Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in Würzburg,
Bd. II. and III.) has described “salivary” glands in the walls of the
œsophagus, in the metastoma, and in the first pair of maxillæ of the
crayfish.

Hoppe-Seyler (Pflügers Archiv, Bd. XIV. 1877) finds that the yellow
fluid ordinarily found in the stomachs of crayfishes always contains
peptone. It dissolves fibrin readily, without swelling it up, at
ordinary temperatures; more quickly at 40° Centigrade. The action
is delayed by even a trace of hydrochloric acid, and is stopped by
the addition of a few drops of water containing 0.2 per cent. of
that acid. By adding alcohol to the yellow fluid, a precipitate is
obtained, which is soluble in water and in glycerine. The aqueous
solution of the precipitate has a strong digestive action on fibrin,
which is arrested by acidulation with hydrochloric acid. These
reactions show that the fluid is very similar to, if not identical
with, the pancreatic fluid of vertebrates.

The secretion of the “liver” taken directly from that gland, has a
more strongly acid reaction than the fluid in the stomach, but has
similar digestive properties. So has an aqueous extract of the gland,
and a watery solution of the alcoholic precipitate. The aqueous
extract also possesses a strong diastatic action on starch, and
breaks up olive oil. There is no more glycogen in the “liver” than is
to be found in other organs, and no constituents of true bile are to
be met with.

{353}


NOTE VIII., CHAPTER II., p. 81.

ANAL RESPIRATION IN CRAYFISH.

Lereboullet (“Note sur une respiration anale observée chez
plusieurs Crustacés;” Mémoires de la Société d’Histoire Naturelle
de Strasbourg, IV. 1850) has drawn attention to what he terms
“anal respiration” in young crayfish, in which he observed water
to be alternately taken into and expelled from the rectum fifteen
to seventeen times in a minute. I have never been able to observe
anything of this kind in the uninjured adult animal, but if the
thoracic ganglia are destroyed, a regular rhythmical dilatation and
closing of the anal end of the rectum at once sets in, and goes
on as long as the hindermost ganglia of the abdomen retain their
integrity. I am much disposed to imagine that the rhythmical movement
is inhibited, when the uninjured crayfish is held in such a position
that the vent can be examined.


NOTE IX., CHAPTER II., p. 82.

THE GREEN GLAND.

The existence of guanin in the green gland rests on the authority
of Will and Gorup-Besanez (Gelehrte Anzeigen, d. k. Baienzschen
Akademie, No. 233, 1848), who say that in this organ and in the organ
of Bojanus of the freshwater mussel, they found “a substance the
reactions of which with the greatest probability indicate guanin,”
but that they had been unable to obtain sufficient material to give
decisive results.

Leydig (Lehrbuch der Histologie, p. 467) long ago stated that the
green gland consists of a much convoluted tube containing granular
cells disposed around a central cavity. Wassiliew (“Ueber die Niere
des Flusskrebses:” Zoologischer Anzeiger, I. 1878) supports the same
view, giving a full account of the minute structure of the organ, and
comparing it with its homologues in the _Copepoda_ and _Phyllopoda_.


NOTE X., CHAPTER III., p. 105.

THE ANATOMY OF THE NERVOUS SYSTEM OF THE CRAYFISH.

The details respecting the origin and the distribution of the nerves
are intentionally omitted. See the memoir by Lemoine of which the
title is given in the “Bibliography.”

{354}


NOTE XI., CHAPTER III., p. 110.

THE FUNCTIONS OF THE NERVOUS SYSTEM OF THE CRAYFISH.

Mr. J. Ward, in his “Observations on the Physiology of the Nervous
System of the Crayfish,” (Proceedings of the Royal Society, 1879) has
given an account of a number of interesting and important experiments
on this subject.

       *       *       *       *       *

NOTE XII., CHAPTER III., p. 124.

THE THEORY OF MOSAIC VISION.

Oscar Schmidt (“Die Form der Krystalkegel im Arthropoden Auge:”
Zeitschrift für Wissenschaftliche Zoologie, XXX. 1878) has pointed
out certain difficulties in the way of the universal application
of the theory of mosaic vision in its present form, which are well
worthy of consideration. I do not think, however, that the substance
of the theory is affected by Schmidt’s objections.


NOTE XIII., CHAPTER III., p. 135.

THE SPERMATOZOA.

Since the discovery of the spermatozoa of the crayfish in 1835–36 by
Henle and von Siebold. the structure and development of these bodies
have been repeatedly studied. The latest discussion of the subject is
contained in a memoir of Dr. C. Grobben (“Beiträge zur Kenntniss der
männlichen Geschlechtsorgane der Dekapoden:” Wien, 1878). There is no
doubt that the spermatozoon consists of a flattened or hemispherical
body, produced at its circumference into a greater or less number
of long tapering curved processes (fig. 34 F). In the interior of
this are two structures, one of which occupies the greater part of
the body, and, when the latter lies flat, looks like a double ring.
This may be called, for distinctness’ sake, the _annulate corpuscle_.
The other is a much smaller _oval corpuscle_, which lies on one
side of the first. The annulate corpuscle is dense, and strongly
refracting; the oval corpuscle is soft, and less sharply defined.
Dr. Grobben describes the annulate corpuscle as “napfartig,” or
cup-shaped; closed below, open above, and with the upper edge turned
inwards, and applied to the inner side of the wall of the cup. It
appeared to me, on the other hand, that the annulate corpuscle is
really a hollow ring, somewhat {355} like one of the ring-shaped
air-cushions one sees, on a very small scale. Dr. Grobben describes
the spermatoblastic cells of the testis and their nuclear spindles;
but his account of the development of the spermatozoa does not
agree with my own observations, which, so far as they have gone,
lead me to infer that the annulate corpuscle of the spermatozoon is
the metamorphosed nucleus of the cell from which the spermatozoon
is developed. For want of material, however, I was unable to bring
my investigations to a satisfactory termination, and I speak with
reserve.


NOTE XIV., CHAPTER IV., p. 174.

THE MORPHOLOGY OF THE CRAYFISH.

The founder of the morphology of the _Crustacea_, M. Milne Edwards,
counts the telson as a somite, and consequently considers that
twenty-one somites enter into the composition of the body in the
_Podophthalmia_. Moreover, he assigns the anterior seven somites
to the head, the middle seven to the thorax, and the hinder seven
to the abdomen. There is a tempting aspect of symmetry about this
arrangement; but as to the limits of the head, the natural line of
demarcation between it and the thorax seems to me to be so clearly
indicated between the somite which bears the second maxillæ and that
which carries the first maxillipedes in the _Crustacea_, and between
the homologous somites in Insects, that I have no hesitation in
retaining the grouping which I have for many years adopted. The exact
nature of the telson needs to be elucidated, but I can find no ground
for regarding it as the homologue of a single somite.

It will be observed that these differences of opinion turn upon
questions of grouping and nomenclature. It would make no difference
to the general argument if it were admitted that the whole body
consists of twenty-one somites and the head of seven.


NOTE XV., CHAPTER IV., p. 199.

THE HISTOLOGY OF THE CRAYFISH.

In dealing with the histology of the crayfish I have been obliged
to content myself with stating the facts as they appear to me. The
discussion of the interpretations put upon these facts by other
observers, especially in the case of those tissues, such as muscle,
on which there is as yet no complete agreement even as to matters of
observation, would require a whole treatise to itself.

{356}


NOTE XVI., CHAPTER IV., p. 221.

THE DEVELOPMENT OF THE CRAYFISH.

The remark made in the last note applies still more strongly to the
history of the development of the crayfish. Notwithstanding the
masterly memoir of Rathke, which constitutes the foundation of all
our knowledge on this subject; the subsequent investigations of
Lereboullet; and the still more recent careful and exhaustive works
of Reichenbach and Bobretsky, a great many points require further
investigation. In all its most important features I have reason to
believe that the account of the process of development given in the
text, is correct.


NOTE XVII., CHAPTER VI., p. 297.

PARASITES OF CRAYFISHES.

In France and Germany crayfishes (apparently, however, only _A.
nobilis_) are infested by parasites, belonging to the genus
_Branchiobdella_. These are minute, flattened, vermiform animals,
somewhat like small leeches, from one-half to one-third of an inch in
length, which attach themselves to the under side of the abdomen (_B.
parasitica_), or to the gills (_B. astaci_), and live on the blood
and on the eggs of the crayfish. A full account of this parasite,
with reference to the literature of the subject, is given by Dormer
(“Ueber die Gattung Branchiobdella:” Zeitschrift für Wiss. Zoologie,
XV. 1865). According to Gay, a similar parasite is found on the
Chilian crayfish. I have never met with it on the English crayfish.
The Lobster has a somewhat similar parasite, _Histriobdella_. Girard,
in the paper cited in the Bibliography, gives a curious account of
the manner in which the little lamellibranchiate mollusk, _Cyclas
fontinalis_, shuts the ends of the ambulatory limbs of crayfishes
which inhabit the same waters, between its valves, so that the
crayfish resembles a cat in walnut shells, and the pinched ends of
the limbs become eroded and mutilated.

{357}



BIBLIOGRAPHY.


The subjoined list indicates the chief books and memoirs, in addition
to those mentioned in the text and in the Appendix, which may be
advantageously consulted by any one who wishes to study more fully
the biology of the crayfishes.


I.—NATURAL HISTORY.

ROESEL VON ROSENHOF. Der Monatlich-herausgegeben Insekten
Belustigung. 1755.

CARBONNIER. L’Écrevisse, Paris, 1869.

BRANDT AND RATZEBURG. Medizinische Zoologie. Bd. II., pp. 58–70.

BELL. British Stalk-eyed Crustacea, 1853.

SOUBEIRAN. Sur l’Histoire naturelle et l’Éducation des Écrevisses.
Comptes Rendus, LX., 1865.

CHANTRAN. Observations sur l’Histoire naturelle des Écrevisses.
Comptes Rendus, LXXI., 1870.

—— Sur la Fécondation des Écrevisses. Ibid., LXXIV., 1872.

—— Expériences sur la Régénération des Yeux chez les Écrevisses.
Ibid., LXXVII., 1873.

—— Observations sur la Formation des Pierres chez les Écrevisses.
Ibid., LXXVIII., 1874.

—— Sur le Mécanisme de la Dissolution intrastomacale des Concrétions
gastriques des Écrevisses. Ibid., LXXVIII., 1874.

STEFFENBERG. Bijdrag til kanne domen on flodkraftens natural
historia, 1872. Abstract in Zoological Record, IX.

VALLOT. Sur l’Écrevisse fluviatile et sur son parasite l’Astacobdelle
branchiale. Comptes Rendus Acad. Sciences, Dijon. Mémoires, 1843–44.
Dijon, 1845.

PUTNAM. On some of the Habits of the Blind Crayfish. Proceedings
Boston Society of Nat. History, XVIII. {358}

HELLER. Ueber einen Flusskrebs-albino. Verhand d. Z. Bot.
Gesellschaft, Wien. Bd. 7, 1857, and Bd. 8, 1858.

LEREBOULLET. Sur les variétés Rouge et Bleue de l’Écrevisse
fluviatile. Comptes Rendus, XXXIII., 1857.

GIRARD. Quelques Remarques sur l’Astacus fluviatilis. Ann. Soc.
Entom. France, T. VII. 1859.


II.—ANATOMY AND PHYSIOLOGY.

BRANDT AND RATZEBURG. _Op. cit._

MILNE EDWARDS. Histoire naturelle des Crustacés. 1834.

ROLLESTON. Forms of Animal Life. 1870.

HUXLEY. Manual of the Anatomy of Vertebrated Animals. 1877.

HUXLEY AND MARTIN. Elementary Biology. 1875.

SUCKOW. Anatomisch-Physiologische Untersuchungen. 1818.

KROHN. Verdauungsorgane des Krebses. Gefässsystem des Flusskrebses.
Isis, 1834.

VON BAER. Ueber die sogenannte Erneuerung des Magens der Krebse und
die Bedeutung der Krebssteine. Müller’s Archiv, 1835.

OESTERLEN. Ueber den Magen des Flusskrebses. Müller’s Archiv, 1840.

T. J. PARKER. On the Stomach of the Freshwater Crayfish. Journal of
Anatomy and Physiology, 1876.

BARTSCH. Die Ernährungs- und Verdauungsorgane des _Astacus
leptodactylus_. Budapester Naturhistor. Hefte II. 1878.

DESZŎ. Ueber das Herz des Flusskrebses und des Hummers. Zoologischer
Anzeiger, I. 1878.

LEREBOULLET. Note sur une Respiration anale observée chez plusieurs
Crustacées. Mém. de la Société d’Histoire Naturelle de Strasbourg,
IV., 1850.

WASSILIEW. Ueber die Niere des Flusskrebses. Zoologischer Anzeiger,
I. 1878.

LEMOINE. Recherches pour servir à l’histoire des systèmes nerveux,
musculaire et glandulaire de l’Écrevisse. Annales des Sciences
Naturelles, Sé. IV. T. 15, 1861.

DIETL. Die Organization des Arthropoden Gehirns. Zeitschrift für
Wiss. Zoologie, XXVII., 1876.

KRIEGER. Ueber das centrale Nervensystem des Flusskrebses.
Zoologischer Anzeiger, I., 1878.

LEYDIG. Das Auge der Gliederthiere. 1864. {359}

MAX SCHULZE. Die Zusammengesetzten Augen der Krebse und Insekten,
1868.

BERGER. Untersuchungen über den Bau des Gehirns und der Retina der
Arthropoden. 1878.

GRENACHER. Untersuchungen über das Sehorgan der Arthropoden. 1879.

O. SCHMIDT. Die Form der Krystalkegel im Arthropoden Auge.
Zeitschrift für Wiss. Zoologie, XXX., 1878.

FARRE. On the organ of hearing in the Crustacea. Phil. Trans. 1843.

LEYDIG. Ueber Geruchs- und Gehörorgane der Krebse und Insekten.
Müller’s Archiv, 1860.

HENSEN. Studien über das Gehörorgan der Decapoden. Zeitschrift für
Wissenschaftliche Zoologie, XIII. 1863.

GROBBEN. Beiträge zur Kenntniss der männlichen Geschlechtsorgane der
Dekapoden. 1878.

BROCCHI. Recherches sur les Organes génitaux mâles des Crustacés
décapodes. Annales des Sciences Naturelles, Sé. VI. ii.

LEYDIG. Zur feineren Bau der Arthropoden. Müller’s Archiv, 1855.

—— Handbuch der Histologie. 1857.

HAECKEL. Ueber die Gewebe des Flusskrebses. Müller’s Archiv, 1857.

BRAUN. Ueber die histologischen Vorgänge bei der Häutung von Astacus
fluviatilis. Würzburg Arbeiten, II.

BAUR. Ueber den Bau der Chitinsehne am Kiefer des Flusskrebses und
ihr Verhalten beim Schalenwechsel. Reichert u. Du Bois Archiv, 1860.

COSTE. Faits pour servir à l’Histoire de la Fécondation chez les
Crustacés. Comptes Rendus, XLVI. 1858.

LEREBOULLET. Recherches sur la mode de Fixation des Œufs aux fausses
pattes abdominales dans les Écrevisses. Annales des Sciences
Naturelles, Sé. IV. T. 14, 1860.


III—DEVELOPMENT.

RATHKE. Ueber die Bildung und Entwickelung des Flusskrebses, 1829.

LEREBOULLET. Recherches d’Embryologie comparée sur le développement
du Brochet, de la Perche et de l’Écrevisse. 1862. {360}

BOBRETSKY. (A Memoir in Russian, of which an abstract is given in
Hofmann and Schwalbe, Jahresbericht für 1873 (1875)).

REICHENBACH. Die Embryonanlage und erste Entwickelung des
Flusskrebses. Zeitschrift für Wiss. Zoologie. 1877.


IV.—TAXONOMY AND DISTRIBUTION OF CRAYFISHES.


A. _General._

MILNE EDWARDS. _Op. cit._

ERICHSON. Uebersicht der Arten der Gattung _Astacus_. Wiegmann’s
Archiv für Naturgeschichte, XII. 1846.

DANA. Crustacea of the United States Exploring Expedition. 1852.

DE SAUSSURE. Note carcinologique sur la Famille des Thalassinides et
sur celle des Astacides. Rev. et Magazin de Zoologie, IX.

HUXLEY. On the Classification and the Distribution of the Crayfishes.
Proceedings of the Zoological Society. 1878.


B. _European and Asiatic._

RATHKE. Zur Fauna der Krym. 1836.

GERSTFELDT AND KESSLER. Cited in the text.

DE HAAN. Fauna Japonica. 1850.

LEREBOULLET. Description de deux nouvelles Espèces d’Écrevisses (_A.
longicornis, A. pallipes_). Mém. Soc. Science Nat. Strasbourg. V.
1858.

HELLER. Crustaceen des südlichen Europa. 1863.

KESSLER. Ein neuer russischer Flusskrebs, _Astacus colchicus_.
Bulletin de la Soc. Imp. des Naturalistes de Moscou, L. 1876.


C. _American._

STIMPSON. Crustacea and Echinodermata of the Pacific shores of North
America. Journal of Boston Society of Natural History VI.; 1857–8.

DE SAUSSURE. Mémoire sur divers Crustacées nouveaux des Antilles et
du Méxique. Mém. de la Société de Physique de Genève T. XIV., 1857.

VON MARTENS. Südbrasilische Süss- und Brackwasser Crustaceen (_A.
pilimanus, A. brasiliensis_), Wiegmann’s Archiv, XXXV., 1869.

——. Ueber Cubansche Crustaceen. _Ibid._ XXXVIII.

HAGEN. Monograph of the North American _Astacidæ_. 1870. {361}


D. _Madagascar._

AUDOUIN AND MILNE EDWARDS. Sur une Espèce nouvelle du genre Écrevisse
(_Astacus_). Écrevisse de Madagascar (_A. Madagascariensis_)., Mém.
du Muséum d’Hist. naturelle, T. II. 1841.


E. _Australia._

VON MARTENS. On a new Species of _Astacus_. Annals & Mag. of Natural
History, 1866.

HELLER. Reise der “Novara.” Zool. Theil. Bd. II. 1865.


F. _New Zealand._

MIERS. Notes on the Genera _Astacoides_ and _Paranephrops_.
Transactions of the New Zealand Institute, IX., 1876.

—— _Paranephrops._ Zoology of “Erebus” and “Terror,” 1874. Catalogue
of New Zealand Crustacea, 1876.

—— Annals of Natural History, 1876.

WOOD-MASON. On the mode in which the Young of the New Zealand
_Astacidæ_ attach themselves to the Mother. Ann. & Mag. Natural
History, 1876.


G. _Fossil Astacomorpha._

OPPEL. Palæontologische Mittheilungen, 1862.

BELL. British Fossil Crustacea. Palæontographical Society.

P. VAN BENEDEN. Sur la Découverte d’un Homard fossile dans l’Argile
de Rupelmonde. Bulletin de l’Acad. Royale de Belgique. XXXIII., 1872.

VON DER MARCK UND SCHLÜTER. Neue Fische und Krebse von der Kreide von
Westphalen. Palæontologica, XV. 1865.

COPE. On three extinct _Astaci_ from the freshwater tertiary of
Idaho. Proceedings of the American Philosophical Society, XI.,
1869–70.

{363}



INDEX.


 A.

 Abdomen, 19, 141
   development of, 213

 Abdominal appendages, 143
   development of, 217

 Abdominal somite, characters of, 142

 Ætiology, 47

 AGASSIZ, 308

 Alimentary canal, 51
   development of, 213, 222

 Ambulatory legs, 168

 American Crayfishes, 243, 247

 _Amœba_, 285

 Amurland Crayfishes, 304

 Antenna, 23, 172
   development of, 214, 218

 Antennule, 23, 173
   development of, 214, 218

 _Anthrapalæmon_, 341

 Anus, 29

 Apodeme, 99, 158, 175

 Appendage, 24, 143, 161, 173
   abdominal, 143
   cephalic, 170
   thoracic, 164

 Archenteron, 211

 Arctogæal province, 314

 Areola, 235

 ARISTOTLE, referred to, 4

 Arteries, 71

 Arteries, development of, 224

 Arthrobranchia, 75

 Arthrophragm, 158

 _Arthropoda_, 279, 284

 Articulations, 95

 Asiatic Crayfishes, 304

 _Astacina_, 254

 _Astacoides_, 250, 313

 _Astacomorpha_, 338

 _Astacopsis_, 250, 264

 _Astacus_, division into sub-genera, 290

 _Astacus angulosus_, 302, 310
   _colchicus_, 302, 310
   _dauricus_, 304, 310
   _fluviatilis_,
     anatomy, general account of, 17–31
     attachment of young to mother, 40, 351
     branchial formula, 266
     development, 205–226
     distribution, geographical 44, 288, 298
     distribution, chronological, 44
     ecdysis, 32, 350
     general characters, 6
     growth, 31, 349
     habits, 8 {364}
     histology, 174
     mortality, 127
     muscular system, 90
     myths concerning, 44
     name, origin of, 13
     nervous system, 101
     newly hatched young, characters of, 219
     nutrition, 48
     occurrence, 5, 8
     organs of alimentation, 51
       circulation, 68
       excretion, 82, 353
       hearing, 116
       reproduction, 128
       respiration, 75, 353
       sight, 118
       smell, 114
       taste, 115
       touch, 113
     prehension of food, 49
     putrid, effect of smell of, 45
     reproduction of lost limbs, 38
     reproduction, sexual, 39, 128, 135, 350
     sexual characters, 7, 20, 32, 145, 241
     somites and appendages, 143
     systematic description, 230
     use as food, 10, 289
     varieties, 289
   _fontinalis_, 290
   _japonicus_, 304
   _klamathensis_, 305
   _leniusculus_, 305
   _leptodactylus_, 299, 302, 303, 310, 320
   _nigrescens_, 244
   _nobilis_, 290, 295, 296, 299, 310
   _oreganus_, 305
   _pachypus_, 302, 310
   _pallipes_, 290
   _politus_, 344
   _saxatilis_, 290
   _Schrenckii_, 304, 310
   _torrentium_, 290, 294, 298, 310, 311
   _tristis_, 290
   _Trowbridgii_, 305

 _Atya_, _Atyidæ_, 331, 336

 Auditory organ, 116
   setæ, 116

 Australian Crayfishes, 306
   province, 314

 Austrocolumbian province, 314

 _Axius_, 271


 B.

 Ball, R., quoted, 36

 Basipodite, 143

 BELL, T., quoted, 37, 42

 Bile-duct, 61, 66

 Biological sciences, scope of, 4

 Blastoderm, 207

 Blastomere, 205

 Blastopore, 209

 Blood, 31, 68, 176
   corpuscles, 69, 176
   development of, 224
   sinuses, 50, 69

 BOBRETSKY, referred to, 356

 BOLIVAR, Dr., 298

 Branchiæ,
   _Astacoides_, 266
   _Astacopsis_, 264 {365}
   _Astacus_, 25, 75, 265
     development of, 224
   _Cancer_, 276
   _Homarus_, 257
   _Palæmon_, 270
   _Palinurus_, 264
   _Penæus_, 267

 Branchial chamber, 25
   formula,
   _Astacoides_, 266
   _Astacopsis_, 264
   _Astacus_, 266
   _Cancer_, 277
   hypothetically complete, 268
   _Palæmon_, 270
   _Palinurus_, 265
   _Penæus_, 267

 _Branchiobdella_, 356

 Branchiostegite, 25
   development of, 217

 BRAUN, quoted, 352

 Brazilian Crayfishes, 306


 C.

 Cæcum, 61

 Calcification of exoskeleton, 197

 Californian Crayfishes, 243

 _Cambarus_, 44, 247, 310, 312

 _Cancer_, 272, 283

 Carapace, 19
   development of, 214

 CARBONNIER, M., quoted, 297, 349, 350

 Cardia, 52

 _Caridina_, 330

 Carpopodite, 165

 Cell, 66, 199

 Cell-aggregate, 190, 199
   division, 200
   theory, 202, 204

 Cephalic appendages, 170
     development of, 217
   flexure, 163
   somites, 154

 Cephalon, 19, 141

 Cephalothorax, 19

 Cervical groove, 19
   spines, 234

 CHANTRAN, M., quoted, 348, 350, 351

 Chelæ, 22

 Chilian Crayfishes, 308

 Chitin, 50
   composition of, 347

 _Chæraps_, 250

 Chorology, 46

 Circulation, 73
   organs of, 68

 Common knowledge and science, 3

 Connective tissue, 178
   development of, 224

 COPE, Prof., quoted, 316

 Cornea, 118

 Coxopodite, 143

 Coxopoditic setæ, 78

 Crab, see _Cancer_

 Crab’s-eye, see Gastrolith

 _Crangon_, 272

 Crayfish, origin of name, 12
   common, see _Astacus fluviatilis_

 Crayfishes, Amurland, 304
   Asiatic, 304
   Australian, 306
   Brazilian, 306
   Californian, 243
   Chilian, 308
   definition of, 254
   Eastern North American, 247, 305
   European, 288, 297
   evolution of, 331
   Figian, 306, 313
   Japanese, 304, 313
   Mascarene, 308, 313
   northern and southern, compared, 252
   Novozelanian, 306, 313
   southern, 249
   Tasmanian, 306
   Western North American, 305, 313

 _Crustacea_, 271, 278

 Crystalline cones, 121

 Cuticle, 33, 50, 175, 192

 _Cyclas_, 356


 D.

 Dactylopodite, 165

 _Daphnia_, asexual reproduction of, 128

 DARWIN, C., referred to, 4

 DE HAAN, quoted, 313

 Development, 205
   abdomen, 213
   abdominal appendages, 217
   alimentary canal, 213, 222
   antennæ, 214, 218
   antennules, 214, 218
   blood and blood vessels, 224
   branchiostegite, 217
   carapace, 214
   cephalic appendages, 217, 219
   connective tissue, 224
   ear, 225
   eye, 225
   eyestalk, 214, 218
   gills, 224
   heart, 224
   kidney, 224
   labrum, 218
   mandibles, 214
   muscles, 224
   nervous system, 213, 224
   reproductive organs, 225
   rostrum, 217
   thoracic appendages, 217, 219

 Digestion, 63

 Distribution, 46
   chronological, of crayfishes, 44, 316, 339
   table of, 345
   geographical, of crayfishes, 44, 288
   causes of, 335
   results of study of, 308, 314

 DORMER, quoted, 356

 DULK, quoted, 349


 E.

 Ear, 116
   development of, 225

 Ecdysis, 32, 350

 Écrevisse à pieds blancs, 289, 297
   à pieds rouges, 289, 297

 Ectoderm, 141

 Ectostracum, 194

 Edelkrebs, 290

 Endoderm, 141

 Endophragmal system, 157

 Endopleurite, 158

 Endopodite, 145

 Endoskeleton, 17

 Endosternite, 158

 Endostracum, 194

 _Engæus_, 250, 306

 _Enoplocytia_, 342

 Epiblast, 211

 Epidermis, 140

 Epimeron, 143

 Epiostracum, 192

 Epipodite, 167 {367}

 Epistoma, 155

 Epithelium, 140, 177

 _Equus excelsus_, occurring with fossil crayfishes, 316

 _Eryma_, 341

 Evolution of crayfishes, 331

 Excretion, organs of, 82

 Exopodite, 145

 Exoskeleton, 17
   chemical composition, 347

 Eye, 118
   compound, 122
   development of, 225

 Eye-stalk, 24, 173
   development of, 214


 F.

 Family, 252

 Fat-cells, 180

 Fibre, muscular, 185

 Fibril, muscular, 185

 Figian Crayfishes, 306

 Filament, muscular, 185

 Filter of stomach, 58

 Flagellum, 167

 Food-yelk, 206

 Foot-jaws, _see_ maxillipedes

 Forceps, 22

 Foregut, 61
   development of, 213, 222

 Fossil crayfishes, 316

 FOSTER, Dr. M., referred to, 110

 France, consumption of crayfish in, 10

 Function, 22


 G.

 _Galaxidæ_, 315

 _Gammarus_, 323

 Ganglion, 103, 105

 Ganglionic corpuscle, 87, 103

 Gastric mill, 53

 Gastrolith, 29, 347
   chemical composition, 349

 Gastrula, 211

 GAY, quoted, 356

 Genus, 249

 Geographical distribution, see Distribution

 GERBE, M., quoted, 350

 Germinal disc, 209
   layer, 206
   spot, 133
   vesicle, 133

 GERSTFELDT, Dr., quoted, 290

 Gills, see Branchiæ

 GIRARD, quoted, 356

 GORUP-BESANEZ, quoted, 353

 Green-gland, 83, 353
   development of, 224

 GROBBEN, Dr., quoted, 354

 Growth of crayfish, 31, 349

 Guanin, 82, 353

 Gullet, see Œsophagus

 GÜNTHER, Dr., quoted, 315


 H.

 HAGEN, Dr., quoted, 305, 312

 _Haplochitonidæ_, 315

 HARVEY, quoted, 5

 Head, see Cephalon

 Hearing, organ of, 116

 Heart, 27, 71
   development of, 224

 HELLER, Dr., quoted, 298, 330

 Hepatic duct, see Bile duct

 Hind gut, 61
   development of, 214, 223

 Histology, 176

 _Histriobdella_, 356

 _Homaridæ_, 263 {368}

 _Homarina_, 261

 _Homarus_, 13, 42, 257, 332

 Homology, homologous, homologue, 148

 _Hoploparia_, 342

 Hypoblast, 211


 I.

 _Idothea_, 323, 334

 Impregnation, 135, 350

 Integument, 50

 Interseptal zone, 183

 Intestine, 29, 61

 Ischiopodite, 165


 J.

 Japanese Crayfishes, 313, 314

 Jaws, 23

 JOHNSTON, J., quoted, 42


 K.

 KESSLER, quoted, 298, 304

 Kidney, see Green gland

 KLUNZINGER, referred to, 330


 L.

 Labrum, 51
   development of, 218

 LAMARCK, referred to, 4

 LEREBOULLET, quoted, 353

 Legs, ambulatory, 168

 LEMOINE, referred to, 353

 LEYDIG, referred to, 115, 353

 Liver, 30, 64
   development of, 223
   nature of secretion, 352

 Lobster, common, see _Homarus_
   Norway, see _Nephrops_
   Rock, see _Palinurus_

 LOVÈN, referred to, 327


 M.

 Machine, living, 128

 M^CINTOSH, Dr. W. C., quoted, 288

 Mandible, 23, 51, 170
   development of, 214

 MARTENS, VON, 306

 _Mastodon mirificus_, occurring with fossil crayfishes, 316

 Maxillæ, 23, 170

 Maxillipedes, 23, 164

 Medullary groove, 213

 Megalopa stage of development, 283

 Meropodite, 165

 Mesoblast, 212

 Mesoderm, 141

 Mesophragm, 158

 Metamere, 143

 Metastoma, 51

 Metope, 278

 Midgut, 61
   development of, 211, 214, 223

 MILNE-EDWARDS, quoted, 13, 289

 _Mollusca_, 284

 Morphology, 46, 138
   comparative, 230

 Mortality of crayfishes, 128

 Morula, 206

 Mosaic vision, 122, 354

 Motor plates, 189

 Mouth, 51

 MÜLLER, JOHANNES, referred to, 122

 Muscle, 57, 90, 175, 181
   development of, 224
   histology of, 90, 181

 Muscles
   of abdomen, 99 {369}
   of chela, 93
   of stomach, 57

 Myosin, 186

 Myotome, 174

 _Mysis_, 281, 323
   _relicta_, origin of, from _M. oculata_, 327

 Mysis stage of development, 280


 N.

 Natural History, 3
   Philosophy, 3

 Nauplius stage of development, 215, 280

 Nearctic province, 314

 _Nephrops_, 259, 332

 Nerve, 101
   auditory, 117
   optic, 118

 Nerve-cells, 103, 187
   fibres, 101, 188

 Nervous system, 105
   development of, 213, 224
   functions of, 354

 Noble crayfish, see _Astacus nobilis_

 Nomenclature, binomial, 13, 15

 Norway lobster, see _Nephrops_

 Novozelanian province, 314

 Nucleated cell, 199

 Nucleolus, 187

 Nucleus, 177, 200
   changes of, in cell-division, 200


 O.

 Œsophagus, 51

 Olfactory organ, 114

 Organ, 22

 Origin of crayfish, evidence as to, 320, 331

 Ovary, 31, 129
   structure of, 131

 Oviduct, 129

 Oviposition, 351

 Ovisac, 132

 Ovum, 129
   structure of, 133


 P.

 Palæarctic province, 314

 _Palæmon_, 268, 328

 _Palinuridæ_, 263

 _Palinurus_, 261, 264

 Palp, 171

 _Paranephrops_, 250, 306, 313

 Paraphragm, 158

 Parasites of crayfish, 356

 _Parastacidæ_, 252, 256, 306, 313

 _Parastacus_, 250, 306

 _Pemphix_, 341

 _Penæus_, 267, 280

 Pericardium, 69

 Perivisceral cavity, 50

 Phyllobranchia, 271

 Physiology, 46

 Pleurobranchia, 79

 Pleuron, 96, 143

 Podobranchia, 75, 165

 _Podophthalmia_, 279

 Pore-canals, 195

 Post-orbital ridge, 233
   spine, 232

 _Potamobiidæ_, 252, 256

 Prawn, see _Palæmon_

 Prehension of food, 49

 Procephalic lobes, 160
   development of, 213

 Propodite, 165

 Protopodite, 143

 _Prototroctes_, 315 {370}

 _Protozoa_, 285

 _Pseudastacus_, 343

 Pylorus, 52


 R.

 Race, 292

 RATHKE, quoted, 356

 RÉAUMUR, quoted, 33

 Reflex action, 108

 REICHENBACH, quoted, 356

 Renal organ, see Green-gland

 Reproduction of lost limbs, 38
   sexual, 39, 128, 135, 350

 Reproductive organs, 128
   development of, 225

 Respiration, anal, 353

 Respiratory organs, see Branchiæ

 _Retropinna_, 315

 ROBIN, quoted, 352

 Rock lobster, see _Palinurus_

 ROESEL VON ROSENHOF, quoted, 41, 43

 RONDOLETIUS, referred to, 4

 Rostrum, 157
   development of, 217


 S.

 Salivary glands, 352

 _Salmonidæ_, parallel between their distribution, and that of _Astacidæ_, 315

 Sarcolemma, 90, 182

 SARS, G. O., referred to, 327

 SARTORIUS VON WALTERHAUSEN, quoted, 322

 Scaphognathite, 80, 170

 Schizopod stage of development, 280

 SCHLÜTER, 317

 SCHMIDT, O., quoted, 354

 SCHRANK, 290

 Science, physical, 3

 Science and common sense, 1

 Segmentation, 174

 Self-causation, 112

 Sensory organs, 113

 Septal line, 183
   zone, 183

 Setæ, 197

 Shrimp, see _Crangon_

 SIEBOLD, VON, referred to, 331

 Sight, organ of, 118

 Sinus, sternal, 69

 Smell, organ of, 114

 Somite, 143, 161, 355
   abdominal, 142
   cephalic, 154
   thoracic, 150

 SOUBEIRAN, M., quoted, 349

 Southern Crayfishes, 249

 Species, 243, 290
   morphological, 291
   physiological, 296

 Spermatozoa, 129, 135, 354

 Spontaneous action, 112

 Squame of antenna, 172

 Steinkrebs, see _Astacus torrentium_

 Sternum, 96, 143

 Stomach, 29, 51

 Stone-crayfish, see _Astacus torrentium_

 Striated spindle, 121

 Swimmeret, 20


 T.

 Taste, organ of, 115

 Teleology, 47, 137

 Tendon, 92, 175 {371}

 Tergum, 96, 143

 Terminal plates, 189

 Terminology, scientific, 14

 Testis, 129
   structure of, 133

 Thoracic appendages, 164
   development of, 217
   somites, 150

 Thorax, 19, 141

 Tissue, 175

 Touch, organ of, 113

 Transformism, 318

 TREVIRANUS, referred to, 4

 Tribe, 252

 Trichobranchiæ, 263

 _Troglocaris_, 337


 V.

 Valves of heart, 73
   of stomach, 59

 VAN HELMONT, quoted, 45

 Variety, 290, 292

 Vas deferens, 130

 Vent, see Anus

 _Vertebrata_, 284
   eye of, 122, 125

 Visual pyramid, 121
   rod, 121

 Vitelline membrane, 133

 Vitellus, 133

 Voluntary action, 112

 VON DER MARCK, 317


 W.

 WARD, J., referred to, 354

 WASSILIEW, quoted, 353

 Whirlpool of life, 84

 WILL, quoted, 353

 WOOD-MASON, quoted, 44


 Y.

 Yelk, 133

 Yelk-division, 205

 Young of _Astacus_, newly hatched, characters of, 219


 Z.

 Zoæa stage of development, 280

PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.



      *      *      *      *      *      *



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