Title: Facts and Arguments for Darwin
Author: Müller, Fritz
Language: English
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FACTS AND ARGUMENTS

FOR

DARWIN.


BY FRITZ MULLER.



WITH ADDITIONS BY THE AUTHOR.



TRANSLATED FROM THE GERMAN

BY W.S. DALLAS, F.L.S.,

ASSISTANT SECRETARY TO THE GEOLOGICAL SOCIETY OF LONDON.



WITH ILLUSTRATIONS.



LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1869.



MR. DARWIN'S WORKS.

A NATURALIST'S VOYAGE ROUND THE WORLD; BEING A JOURNAL OF RESEARCHES
INTO THE NATURAL HISTORY AND GEOLOGY OF COUNTRIES VISITED. Post 8vo. 9
shillings.

THE ORIGIN OF SPECIES, BY MEANS OF NATURAL SELECTION; OR, THE
PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE. WOODCUTS. Post
8vo. 15 shillings.

THE VARIOUS CONTRIVANCES BY WHICH BRITISH AND FOREIGN ORCHIDS ARE
FERTILISED BY INSECTS, AND ON THE GOOD EFFECTS OF INTERCROSSING.
Woodcuts, Post 8vo. 9 shillings.

THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION. Illustrations.
2 volumes, 8vo. 28 shillings.



TRANSLATOR'S PREFACE.

My principal reason for undertaking the translation of Dr. Fritz
Muller's admirable work on the Crustacea, entitled 'Fur Darwin,' was
that it was still, although published as long ago as 1864, and highly
esteemed by the author's scientific countrymen, absolutely unknown to a
great number of English naturalists, including some who have occupied
themselves more or less specially with the subjects of which it treats.
It possesses a value quite independent of its reference to Darwinism,
due to the number of highly interesting and important facts in the
natural history and particularly the developmental history of the
Crustacea, which its distinguished author, himself an unwearied and
original investigator of these matters, has brought together in it. To a
considerable section of English naturalists the tone adopted by the
author in speaking of one of the greatest of their number will be a
source of much gratification.

In granting his permission for the translation of his little book, Dr.
Fritz Muller kindly offered to send some emendations and additions to
certain parts of it. His notes included many corrections of printers'
errors, some of which would have proved unintelligible without his aid,
some small additions and notes which have been inserted in their proper
places, and two longer pieces, one forming a footnote near the close of
Chapter 11, the other at the end of Chapter 12, describing the probable
mode of evolution of the Rhizocephala from the Cirripedia.

Of the execution of the translation I will say but little. My chief
object in this, as in other cases, has been to furnish, as nearly as
possible, a literal version of the original, regarding mere elegance of
expression as of secondary importance in a scientific work. As much of
Dr. Muller's German does not submit itself to such treatment very
readily, I must beg his and the reader's indulgence for any
imperfections arising from this cause.

W.S.D.

LONDON, 15TH FEBRUARY, 1869.



AUTHOR'S PREFACE.

It is not the purpose of the following pages to discuss once more the
arguments deduced for and against Darwin's theory of the origin of
species, or to weigh them one against the other. Their object is simply
to indicate a few facts favourable to this theory, collected upon the
same South American ground, on which, as Darwin tells us, the idea first
occurred to him of devoting his attention to "the origin of
species,--that mystery of mysteries."

It is only by the accumulation of new and valuable material that the
controversy will gradually be brought into a state fit for final
decision, and this appears to be for the present of more importance than
a repeated analysis of what is already before us. Moreover, it is but
fair to leave it to Darwin himself at first to beat off the attacks of
his opponents from the splendid structure which he has raised with such
a master-hand.

F.M.

DESTERRO, 7TH SEPTEMBER, 1863.



CONTENTS.

CHAPTER 1. INTRODUCTORY.

CHAPTER 2. THE SPECIES OF MELITA.

CHAPTER 3. MORPHOLOGY OF CRUSTACEA.

CHAPTER 4. SEXUAL PECULIARITIES AND DIMORPHISM.

CHAPTER 5. RESPIRATION IN LAND CRABS.

CHAPTER 6. STRUCTURE OF THE HEART IN EDRIOPHTHALMA.

CHAPTER 7. DEVELOPMENTAL HISTORY OF PODOPHTHALMA.

CHAPTER 8. DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.

CHAPTER 9. DEVELOPMENTAL HISTORY OF ENTOMOSTRACA,
           CIRRIPEDES, AND RHIZOCEPHALA.

CHAPTER 10. ON THE PRINCIPLES OF CLASSIFICATION.

CHAPTER 11. ON THE PROGRESS OF EVOLUTION.

CHAPTER 12. PROGRESS OF EVOLUTION IN CRUSTACEA.



HISTORY OF CRUSTACEA.


CHAPTER 1. INTRODUCTORY.

When I had read Charles Darwin's book 'On the Origin of Species,' it
seemed to me that there was one mode, and that perhaps the most certain,
of testing the correctness of the views developed in it, namely, to
attempt apply them as specially as possible to some particular group of
animals. such an attempt to establish a genealogical tree, whether for
the families of a class, the genera of a large family, or for the
species of an extensive genus, and to produce pictures as complete and
intelligible as possible of the common ancestors of the various smaller
and larger circles, might furnish a result in three different ways.

1. In the first place, Darwin's suppositions when thus applied might
lead to irreconcilable and contradictory conclusions, from which the
erroneousness of the suppositions might be inferred. If Darwin's
opinions are false, it was to be expected that contradictions would
accompany their detailed application at every step, and that these, by
their cumulative force, would entirely destroy the suppositions from
which they proceeded, even though the deductions derived from each
particular case might possess little of the unconditional nature of
mathematical proof.

2. Secondly, the attempt might be successful to a greater or less
extent. If it was possible upon the foundation and with the aid of the
Darwinian theory, to show in what sequence the various smaller and
larger circles had separated from the common fundamental form and from
each other, in what sequence they had acquired the peculiarities which
now characterise them, and what transformations they had undergone in
the lapse of ages,--if the establishment of such a genealogical tree, of
a primitive history of the group under consideration, free from internal
contradictions, was possible,--then this conception, the more completely
it took up all the species within itself, and the more deeply it enabled
us to descend into the details of their structure, must in the same
proportion bear in itself the warrant of its truth, and the more
convincingly prove that the foundation upon which it is built is no
loose sand, and that it is more than merely "an intellectual dream."

3. In the third place, however, it was possible, and this could not but
appear, prima facie, the most probable case, that the attempt might be
frustrated by the difficulties standing in its way, without settling the
question, either way, in a perfectly satisfactory manner. But if it were
only possible in this way to arrive for oneself at a moderately certain
independent judgment upon a matter affecting the highest questions so
deeply, even this alone could not but be esteemed a great gain.

Having determined to make the attempt, I had in the first place to
decide upon some particular class. The choice was necessarily limited to
those the chief forms of which were easily to be obtained alive in some
abundance. The Crabs and Macrurous Crustacea, the Stomapoda, the
Diastylidae, the Amphipoda and Isopoda, the Ostracoda and Daphnidae, the
Copepoda and Parasita, the Cirripedes and Rhizocephala of our coast,
representing the class of Crustacea with the deficiency only of the
Phyllopoda and Xiphosura, furnished a long and varied, and at the same
time intimately connected series, such as was at my command in no other
class. But even independently of this circumstance the selection of the
Crustacea could hardly have been doubtful. Nowhere else, as has already
been indicated by various writers, is the temptation stronger to give to
the expressions "relationship, production from a common fundamental
form," and the like, more than a mere figurative signification, than in
the case of the lower Crustacea. Among the parasitic Crustacea,
especially, everybody has long been accustomed to speak, in a manner
scarcely admitting of a figurative meaning, of their arrest of
development by parasitism, as if the transformation of species were a
matter of course. It would certainly never appear to any one to be a
pastime worthy of the Deity, to amuse himself with the contrivance of
these marvellous cripplings, and so they were supposed to have fallen by
their own fault, like Adam, from their previous state of perfection.

That a great part of the larger and smaller groups into which this class
is divided, might be regarded as satisfactorily established, was a
further advantage not to be undervalued; whilst in two other classes
with which I was familiar, namely, the Annelida and Acalephae, all the
attempted arrangements could only be considered preliminary revisions.
These undisplaceable groups, like the sharply marked forms of the hard,
many-jointed dermal framework, were not only important as safe starting
points and supports, but were also of the highest value as inflexible
barriers in a problem in which, from its very nature, fancy must freely
unfold her wings.

When I thus began to study our Crustacea more closely from this new
stand-point of the Darwinian theory,--when I attempted to bring their
arrangements into the form of a geological tree, and to form some idea
of the probable structure of their ancestors,--I speedily saw (as indeed
I expected) that it would require years of preliminary work before the
essential problem could be seriously handled. The extant systematic
works generally laid more weight upon the characters separating the
genera, families and orders, than upon those which unite the members of
each group, and consequently often furnished but little employable
material. But above all things a thorough knowledge of development was
indispensable, and every one knows how imperfect is our present
knowledge of this subject. The existing deficiencies were the more
difficult to supply, because, as Van Beneden remarks with regard to the
Decapoda, from the often incredible difference in the development of the
most nearly allied forms, these must be separately studied--usually
family by family, and frequently genus by genus--nay, sometimes, as in
the case of Peneus, even species by species; and because these
investigations, in themselves troublesome and tedious, often depend for
their success upon a lucky chance.

But although the satisfactory completion of the "Genealogical tree of
the Crustacea" appeared to be an undertaking for which the strength and
life of an individual would hardly suffice, even under more favourable
circumstances than could be presented by a distant island, far removed
from the great market of scientific life, far from libraries and
museums--nevertheless its practicability became daily less doubtful in
my eyes, and fresh observations daily made me more favourably inclined
towards the Darwinian theory.

In determining to state the arguments which I derived from the
consideration of our Crustacea in favour of Darwin's views, and which
(together with more general considerations and observations in other
departments), essentially aided in making the correctness of those views
seem more and more palpable to me, I am chiefly influenced by an
expression of Darwin's: "Whoever," says he ('Origin of Species' page
482), "is led to believe that species are mutable, will do a good
service by conscientiously expressing his conviction." To the desire
expressed in these words I respond, for my own part, with the more
pleasure, as this furnishes me with an opportunity of publicly giving
expression in words to the thanks which I feel most deeply to be due
from me to Darwin for the instructions and suggestions for which I am so
deeply indebted to his book. Accordingly I throw this sand-grain with
confidence into the scale against "the load of prejudice by which this
subject is overwhelmed," without troubling myself as to whether the
priests of orthodox science will reckon me amongst dreamers and children
in knowledge of the laws of nature.


CHAPTER 2. THE SPECIES OF MELITA.

A false supposition, when the consequences proceeding from it are
followed further and further, will sooner or later lead to absurdities
and palpable contradictions. During the period of tormenting doubt--and
this was by no means a short one--when the pointer of the scales
oscillated before me in perfect uncertainty between the pro and the con,
and when any fact leading to a quick decision would have been most
welcome to me, I took no small pains to detect some such contradictions
among the inferences as to the class of Crustacea furnished by the
Darwinian theory. But I found none, either then, or subsequently. Those
which I thought I had found were dispelled on closer consideration, or
actually became converted into supports for Darwin's theory.

Nor, so far as I am aware, have any of the NECESSARY consequences of
Darwin's hypotheses been proved by any one else, to stand in clear and
irreconcilable contradiction. And yet, as the most profound students of
the animal kingdom are amongst Darwin's opponents, it would seem that it
ought to have been an easy matter for them to crush him long since
beneath a mass of absurd and contradictory inferences, if any such were
to be drawn from his theory. To this want of demonstrated contradictions
I think we may ascribe just the same importance in Darwin's favour, that
his opponents have attributed to the absence of demonstrated
intermediate forms between the species of the various strata of the
earth. Independently of the reasons which Darwin gives for the
preservation of such intermediate forms being only exceptional, this
last mentioned circumstance will not be regarded as of very great
significance by any one who has traced the development of an animal upon
larvae fished from the sea, and had to seek in vain for months, and even
years, for those transitional forms, which he nevertheless knew to be
swarming around him in thousands.

A few examples may show how contradictions might come forth as necessary
results of the Darwinian hypotheses.

It seems to be a necessity for all crabs which remain for a long time
out of the water (but why is of no consequence to us here), that air
shall penetrate from behind into the branchial cavity. Now these crabs,
which have become more or less estranged from the water, belong to the
most different families--the Raninidae (Ranina), Eriphinae (Eriphia
gonagra), Grapsoidae (Aratus, Sesarma, etc.), Ocypodidae (Gelasimus,
Ocypoda), etc., and the separation of these families must doubtless be
referred to a much earlier period than the habit of leaving the water
displayed by some of their members. The arrangements connected with
aerial respiration, therefore, could not be inherited from a common
ancestor, and could scarcely be accordant in their construction. If
there were any such accordance not referable to accidental resemblance
among them, it would have to be laid in the scale as evidence against
the correctness of Darwin's views. I shall show hereafter how in this
case the result, far from presenting such contradictions, was rather in
the most complete harmony with what might be predicted from Darwin's
theory.

(FIGURE 1. Melita exilii n. sp., male, enlarged five times. The large
branchial lamellae are seen projecting between the legs.)

A second example.--We are already acquainted with four species of Melita
(M. valida, setipes, anisochir, and Fresnelii), and I can add a fifth
(Figure 1), in which the second pair of feet bears upon one side a small
hand of the usual structure, and on the other an enormous clasp-forceps.
This want of symmetry is something so unusual among the Amphipoda, and
the structure of the clasp-forceps differs so much from what is seen
elsewhere in this order, and agrees so closely in the five species, that
one must unhesitatingly regard them as having sprung from common
ancestors belonging to them alone among known species. But one of these
species, M. Fresnelii, discovered by Savigny, in Egypt, is said to want
the secondary flagellum of the anterior antennae, which occurs in the
others. From the trustworthiness of all Savigny's works there can
scarcely be a doubt as to the correctness of this statement. Now, if the
presence or absence of the secondary flagellum possessed the
significance of a distinctive generic character, which is usually
ascribed to it, or if there were other important differences between
Melita Fresnelii and the other species above-mentioned, which would make
it seem natural to separate M. Fresnelii as a distinct genus, and to
leave the others united with the rest of the species of Melita--that is
to say, in the sense of the Darwinian theory, if we assume that all the
other Melitae possessed common ancestors, which were not at the same
time the ancestors of M. Fresnelii--this would stand in contradiction to
the conclusion, derived from the structure of the clasp-forceps, that M.
Fresnelii and the four other species above-mentioned possessed common
ancestors, which were not also the ancestors of the remaining species of
Melita. It would follow:--

1. From the structure of the clasp-forceps: that M. exilii, etc. and M.
Fresnelii would branch off together from a stem which branches off from
M. palmata.

2. From the presence or absence of the secondary flagellum: that M.
palmata, etc. and M. exilii, etc. would branch off together from a stem
which branches off from M. Fresnelii.

As, in the first case, among the Crabs, a typical agreement of
arrangements produced independently of each other would have been a very
suspicious circumstance for Darwin's theory, so also, in the second,
would any difference more profound than that of very nearly allied
species. Now it seems to me that the secondary flagellum can by no means
furnish a reason for doubting the close relationship of M. Fresnelii to
M. exilii, etc., which is indicated by the peculiar structure of the
unpaired clasp-forceps. In the first place we must consider the
possibility that the secondary flagellum, which is not always easy to
detect, may only have been overlooked by Savigny, as indeed Spence Bate
supposes to have been the case. If it is really deficient it must be
remarked that I have found it in species of the genera Leucothoe,
Cyrtophium and Amphilochus, in which genera it was missed by Savigny,
Dana and Spence Bate--that a species proved by the form of the Epimera
(Coxae Sp. B.) of the caudal feet (uropoda Westw.), etc., to be a true
Amphithoe* possesses it (* I accept this and all the other genera of
Amphipoda here mentioned, with the limits given to them by Spence Bate
('Catalogue of Amphipodous Crustacea').)--that in many species of
Cerapus it is reduced to a scarcely perceptible rudiment--nay, that it
is sometimes present in youth and disappears (although perhaps not
without leaving some trace) at maturity, as was found by Spence Bate to
be the case in Acanthonotus Owenii and Atylus carinatus, and I can
affirm with regard to an Atylus of these seas, remarkable for its
plumose branchiae--and that from all this, at the present day when the
increasing number of known Amphipoda and the splitting of them into
numerous genera thereby induced, compels us to descend to very minute
distinctive characters, we must nevertheless hesitate before employing
the secondary flagellum as a generic character. The case of Melita
Fresnelii therefore cannot excite any doubts as to Darwin's theory.


CHAPTER 3. MORPHOLOGY OF CRUSTACEA--NAUPLIUS-LARVAE.

If the absence of contradictions among the inferences deduced from them
for a narrow and consequently easily surveyed department must prepossess
us in favour of Darwin's views, it must be welcomed as a positive
triumph of his theory if far-reaching conclusions founded upon it should
SUBSEQUENTLY be confirmed by facts, the existence of which science, in
its previous state, by no means allowed us to suspect. From many results
of this kind upon which I could report, I select as examples, two, which
were of particular importance to me, and relate to discoveries the great
significance of which in the morphology and classification of the
Crustacea will not be denied even by the opponents of Darwin.

Considerations upon the developmental history of the Crustacea had led
me to the conclusion that, if the higher and lower Crustacea were at all
derivable from common progenitors, the former also must once have passed
through Nauplius-like conditions. Soon afterwards I discovered
Naupliiform larvae of Shrimps ('Archiv fur Naturgeschichte' 1860 1 page
8), and I must admit that this discovery gave me the first decided turn
in Darwin's favour.

(FIGURE 2. Tanais dubius (?) Kr. female, magnified 25 times, showing the
orifice of entrance (x) into the cavity overarched by the carapace, in
which an appendage of the second pair of maxillae (f) plays. On four
feet (i, k, l, m) are the rudiments of the lamellae which subsequently
form the brood-cavity.)

The similar number of segments* occurring in the Crabs and Macrura,
Amphipoda and Isopoda, in which the last seven segments are always
different from the preceding ones in the appendages with which they are
furnished, could only be regarded as an inheritance from the same
ancestors.

(* Like Claus I do not regard the eyes of the Crustacea as limbs, and
therefore admit no ocular segment; on the other hand I count in the
median piece of the tail, to which the character of a segment is often
denied. In opposition to its interpretation as a segment of the body,
only the want of limbs can be cited; in its favour we have the relation
of the intestine, which usually opens in this piece, and sometimes even
traverses its whole length, as in Microdeutopus and some other
Amphipoda. In Microdeutopus, as Spence Bate has already pointed out, one
is even led to regard small processes of this tubular caudal piece as
rudimentary members. Bell also ('British Stalk-eyed Crustacea' page 20),
states that he observed limbs of the last segment in Palaemon serratus
in the form of small moveable points.

The attempt has often been made to divide the body of the higher
Crustacea into small sections composed of equal numbers of segments,
these sections consisting of 3, 5 or 7 segments. None of these attempts
has ever met with general acceptance; my own investigations lead me to a
conception which nearly approaches Van Beneden's. I assume four sections
of 5 segments each--the primitive body, the fore-body, the hind-body,
and the middle-body. The primitive body includes the segments which the
naupliiform larva brings with it out of the egg; it is afterwards
divided, by the younger sections which become developed in its middle,
into the head and tail. To this primitive body belong the two pairs of
antennae, the mandibles and the caudal feet ("posterior pair of
pleopoda," Sp. B.). Even in the mature animal the fact that these
terminal sections belong to one another is sometimes betrayed by the
resemblance of their appendages, especially that of the outer branch of
the caudal feet, with the outer branch (the so-called scale) of the
second pair of antennae. Like the antennae, the caudal feet may also
become the bearers of high sensorial apparatus, as is shown by the ear
of Mysis.

The sequence of the sections of the body in order of time seems
originally to have been, that first the fore-body, then the hind-body,
and finally the middle-body was formed. The fore-body appears, in the
adult animal, to be entirely or partially amalgamated with the head; its
appendages (siagonopoda Westw.) are all or in part serviceable for the
reception of food, and generally sharply distinguished from those of the
following group. The segments of the middle-body seem always to put
forth limbs immediately after their own appearance, whilst the segments
of the hind-body often remain destitute of feet through long portions of
the larval life or even throughout life (as in many female Diastylidae),
a reason, among many others, for not, as is usual, regarding the
middle-body of the Crustacea as equivalent to the constantly footless
abdomen of Insects. The appendages of the middle-body (pereiopoda) seem
never, even in their youngest form, to possess two equal branches, a
peculiarity which usually characterises the appendages of the hind-body.
This is a circumstance which renders very doubtful the equivalence of
the middle-body of the Malacostraca with the section of the body which
in the Copepoda bears the swimming feet and in the Cirripedia the cirri.

The comprehension of the feet of the hind-body and tail in a single
group (as "fausses pattes abdominales," or as "pleopoda") seems not to
be justifiable. When there is a metamorphosis, they are probably always
produced at different periods, and they are almost always quite
different in structure and function. Even in the Amphipoda, in which the
caudal feet usually resemble in appearance the last two pairs of
abdominal feet, they are in general distinguished by some sort of
peculiarity, and whilst the abdominal feet are reproduced in wearisome
uniformity throughout the entire order, the caudal feet are, as is
well-known, amongst the most variable parts of the Amphipoda.)

And if at the present day the majority of the Crabs and Macrura, and
indeed the Stalk-eyed Crustacea in general, pass through Zoea-like
developmental states, and the same mode of transformation was to be
ascribed to their ancestors, the same thing must also apply, if not to
the immediate ancestors of the Amphipoda and Isopoda, at least to the
common progenitors of these and the Stalk-eyed Crustacea. Any such
assumption as this was, however, very hazardous, so long as not a single
fact properly relating to the Edriophthalma could be adduced in its
support, as the structure of this very coherent group seemed to be
almost irreconcilable with many peculiarities of the Zoea. Thus, in my
eyes, this point long constituted one of the chief difficulties in the
application of the Darwinian views to the Crustacea, and I could
scarcely venture to hope that I might yet find traces of this passage
through the Zoea-form among the Amphipoda or Isopoda, and thus obtain a
positive proof of the correctness of this conclusion. At this point Van
Beneden's statement that a cheliferous Isopod (Tanais Dulongii),
belonging, according to Milne-Edwards, to the same family as the common
Asellus aquaticus, possesses a carapace like the Decapoda, directed my
attention to these animals, and a careful examination proved that these
Isopods have preserved, more truly than any other adult Crustacea, many
of the most essential peculiarities of the Zoeae, especially their mode
of respiration. Whilst in all other Oniscoida the abdominal feet serve
for respiration, these in our cheliferous Isopod (Figure 2) are solely
motory organs, into which no blood-corpuscle ever enters, and the chief
seat of respiration is, as in the Zoeae, in the lateral parts of the
carapace, which are abundantly traversed by currents of blood, and
beneath which a constant stream of water passes, maintained, as in Zoeae
and the adult Decapoda, by an appendage of the second pair of maxillae,
which is wanting in all other Edriophthalma.

For both these discoveries, it may be remarked in passing, science is
indebted less to a happy chance than immediately to Darwin's theory.

Species of Peneus live in the European seas, as well as here, and their
Nauplius-brood has no doubt repeatedly passed unnoticed through the
hands of the numerous naturalists who have investigated those seas, as
well as through my own,* for it has nothing which could attract
particular attention amongst the multifarious and often wonderful
Nauplius-forms. (* Mecznikow has recently found Naupliiform
shrimp-larvae in the sea near Naples.) When I, fancying from the
similarity of its movements that it was a young Peneus-Zoea, had for the
first time captured such a larva, and on bringing it under the
microscope found a Nauplius differing toto coelo from this Zoea, I might
have thrown it aside as being completely foreign to the developmental
series which I was tracing, if the idea of early Naupliiform stages of
the higher Crustacea, which indeed I did not believe to be still extant,
had not at the moment vividly occupied my attention.

And if I had not long been seeking among the Edriophthalma for traces of
the supposititious Zoea-state, and seized with avidity upon everything
that promised to made this refractory Order serviceable to me, Van
Beneden's short statement could hardly have affected me so much in the
manner of an electric shock, and impelled me to a renewed study of the
Tanaides, especially as I had once before plagued myself with them in
the Baltic, without getting any further than my predecessors, and I have
not much taste for going twice over the same ground.


CHAPTER 4. SEXUAL PECULIARITIES AND DIMORPHISM.

Our Tanais, which in nearly all the particulars of its structure is an
extremely remarkable animal, furnished me with a second fact worthy of
notice in connection with the theory of the origin of species by natural
selection.

When hand-like or cheliform structures occur in the Crustacea, these are
usually more strongly developed in the males than in the females, often
becoming enlarged in the former to quite a disproportionate size, as we
have already seen to be the case in Melita. A better known example of
such gigantic chelae is presented by the males of the Calling Crabs
(Gelasimus), which are said in running to carry these claws "elevated,
as if beckoning with them"--a statement which, however, is not true of
all the species, as a small and particularly large-clawed one, which I
have seen running about by thousands in the cassava-fields at the mouth
of the Cambriu, always holds them closely pressed against its body.

A second peculiarity of the male Crustacea consists not unfrequently in
a more abundant development on the flagellum of the anterior antennae of
delicate filaments which Spence Bate calls "auditory cilia," and which I
have considered to be olfactory organs, as did Leydig before me,
although I was not aware of it. Thus they form long dense tufts in the
males of many Diastylidae, as Van Beneden also states with regard to
Bodotria, whilst the females only possess them more sparingly. In the
Copepoda, Claus called attention to the difference of the sexes in this
respect. It seems to me, as I may remark in passing, that this stronger
development in the males is greatly in favour of the opinion maintained
by Leydig and myself, as in other cases male animals are not
unfrequently guided by the scent in their pursuit of the ardent females.

Now, in our Tanais, the young males up to the last change of skin
preceding sexual maturity resemble the females, but then they undergo an
important metamorphosis. Amongst other things they lose the moveable
appendages of the mouth even to those which serve for the maintenance of
the respiratory current; their intestine is always found empty, and they
appear only to live for love. But what is most remarkable is, that they
now appear under two different forms. Some (Figure 3) acquire powerful,
long-fingered, and very mobile chelae, and, instead of the single
olfactory filament of the female, have from 12 to 17 of these organs,
which stand two or three together on each joint of the flagellum. The
others (Figure 5) retain the short thick form of the chelae of the
females; but, on the other hand, their antennae (Figure 6) are equipped
with a far greater number of olfactory filaments, which stand in groups
of from five to seven together.

(FIGURE 3. Head of the ordinary form of the male of Tanais dubius (?)
Kr. magnified 90 times. The terminal setae of the second pair of
antennae project between the cheliferous feet.

FIGURE 4. Buccal region of the same from below; lambda, labrum.

FIGURE 5. Head of the rarer form of the male, magnified 25 times.

FIGURE 6. Flagellum of the same, with olfactory filaments, magnified 90
times.)

In the first place, and before inquiring into its significance, I will
say a word upon this fact itself. It was natural to consider whether two
different species with very similar females and very different males
might not perhaps live together, or whether the males, instead of
occurring in two sharply defined forms, might not be only variable
within very wide limits. I can admit neither of these suppositions. Our
Tanais lives among densely interwoven Confervae, which form a coat of
about an inch in thickness upon stones in the neighbourhood of the
shore. If a handful of this green felt is put into a large glass with
clear sea-water, the walls of the glass are soon seen covered with
hundreds, nay with thousands, of these little, plump, whitish Isopods.
In this way I have examined thousands of them with the simple lens, and
I have also examined many hundreds with the microscope, without finding
any differences among the females, or any intermediate forms between the
two kinds of males.

To the old school this occurrence of two kinds of males will appear to
be merely a matter of curiosity. To those who regard the "plan of
creation" as the "free conception of an Almighty intellect, matured in
the thoughts of the latter before it is manifested in palpable, external
forms," it will appear to be a mere caprice of the Creator, as it is
inexplicable either from the point of view of practical adaptation, or
from the "typical plan of structure." From the side of Darwin's theory,
on the contrary, this fact acquires meaning and significance, and it
appears in return to be fitted to throw light upon a question in which
Bronn saw "the first and most material objection against the new
theory," namely, how it is possible that from the accumulation in
various directions of the smallest variations running out of one
another, varieties and species are produced, which stand out from the
primary form clearly and sharply like the petiolated leaf of a
Dicotyledon, and are not amalgamated with the primary form and with each
other like the irregular curled lobes of a foliaceous Lichen.

Let us suppose that the males of our Tanais, hitherto identical in
structure, begin to vary, in all directions as Bronn thinks, for aught I
care. If the species was adapted to its conditions of existence, if the
BEST in this respect had been attained and secured by natural selection,
fresh variations affecting the species as a species would be
retrogressions, and thus could have no prospect of prevailing. They must
rather have disappeared again as they arose, and the lists would remain
open to the males under variation, only in respect of their sexual
relations. In these they might acquire advantages over their rivals by
their being enabled either to seek or to seize the females better. The
best smellers would overcome all that were inferior to them in this
respect, unless the latter had other advantages, such as more powerful
chelae, to oppose to them. The best claspers would overcome all less
strongly armed champions, unless these opposed to them some other
advantage, such as sharper senses. It will be easily understood how in
this manner all the intermediate steps less favoured in the development
of the olfactory filaments or of the chelae would disappear from the
lists, and two sharply defined forms, the best smellers and the best
claspers, would remain as the sole adversaries. At the present day the
contest seems to have been decided in favour of the latter, as they
occur in greatly preponderating numbers, perhaps a hundred of them to
one smeller.

To return to Bronn's objection. When he says that "for the support of
the Darwinian theory, and in order to explain why many species do not
coalesce by means of intermediate forms, he would gladly discover some
external or internal principle which should compel the variations of
each species to advance in ONE direction, instead of merely permitting
them in all directions," we may, in this as in many other cases, find
such a principle in the fact that actually only a few directions stand
open in which the variations are at the same time improvements, and in
which therefore they can accumulate and become fixed; whilst in all
others, being either indifferent or injurious, they will go as lightly
as they come.

(FIGURE 7. Orchestia Darwinii, n. sp. male.)

The occurrence of two kinds of males in the same species may perhaps not
be a very rare phenomenon in animals in which the males differ widely
from the females in structure. But only in those which can be procured
in sufficient abundance, will it be possible to arrive at a conviction
that we have not before us either two different species, or animals of
different ages. From my own observation, although not very extensive, I
can give a second example. It relates to a shore-hopper (Orchestia). The
animal (Figure 7) lives in marshy places in the vicinity of the sea,
under decaying leaves, in the loose earth which the Marsh Crabs
(Gelasimus, Sesarma, Cyclograpsus, etc.) throw up around the entrance to
their borrows, and even under dry cow-dung and horse-dung. If this
species removes to a greater distance from the shore than the majority
of its congeners (although some of them advance very far into the land
and even upon mountains of a thousand feet in height, such as O.
tahitensis, telluris, and sylvicola), its male differs still more from
all known species by the powerful chelae of the second pair of feet.
Orchestia gryphus, from the sandy coast of Monchgut, alone presents a
somewhat similar structure, but in a far less degree; elsewhere the form
of the hand usual in the Amphipoda occurs. Now there is a considerable
difference between the males of this species, especially in the
structure of these chelae--a different so great that we can scarcely
find a parallel to it elsewhere between two species of the genus--and
yet, as in Tanais, we do not meet with a long series of structures
running into one another, but only two forms united by no intermediate
terms (Figures 8 and 9). The males would be unhesitatingly regarded as
belonging to two well-marked species if they did not live on the same
spot, with undistinguishable females. That the two forms of the chelae
of the males occur in this species is so far worthy of notice, because
the formation of the chelae, which differs widely from the ordinary
structure in the other species, indicates that it has quite recently
undergone considerable changes, and therefore such a phenomenon was to
be expected in it rather than in other species.

(FIGURES 8 AND 9. The two forms of the chelae of the male of Orchestia
Darwinii, magnified 45 times.)

I cannot refrain from taking this opportunity of remarking that (so far
as appears from Spence Bate's catalogue), for two different kinds of
males (Orchestia telluris and sylvicola) which live together in the
forests of New Zealand, only one form of female is known, and hazarding
the supposition that we have here a similar case. It does not seem to me
to be probable that two nearly allied species of these social Amphipoda
should occur mixed together under the same conditions of life.

(FIGURE 10. Coxal lamella of the penultimate pair of feet of the male
(a), and coxal lamella, with the three following joints of the same pair
of feet of the female (b) of Melita Messalina, magnified 45 diam.

FIGURE 11. Coxal lamella of the same pair of feet of the female of M.
insatiabilis.)

As the males of several species of Melita are distinguished by the
powerful unpaired clasp-forceps, the females of some other species of
the same genus are equally distinguished from all other Amphipoda by the
circumstance that in them a peculiar apparatus is developed which
facilitates their being held by the male. The coxal lamellae of the
penultimate pair of feet are produced into hook-like processes, of which
the male lays hold with the hands of the first pair of feet. The two
species in which I am acquainted with this structure are amongst the
most salacious animals of their order, even females which are laden with
eggs in all stages of development, not unfrequently have their males
upon their backs. The two species are nearly allied to Melita palmata
Leach (Gammarus Dugesii, Edw.), which is widely distributed on the
European coasts, and has been frequently investigated; unfortunately,
however, I can find no information as to whether the females of this or
any other European species possess a similar contrivance. In M. exilii
all the coxal lamellae are of the ordinary formation. Nevertheless, be
this as it will, whether they exist in two or in twenty species, the
occurrence of these peculiar hook-like processes is certainly very
limited.

Now our two species live sheltered beneath slightly tilted stones in the
neighbourhood of the shore: one of them, Melita Messalina, so high that
it is but rarely covered by the water; the other, Melita insatiabilis, a
little lower; both species live together in numerous swarms. We cannot
therefore suppose that the loving couples are threatened with
disturbance more frequently than those of other species, nor would it be
more difficult for the male, than for those of other species, in case of
his losing his female, to find a new one. Nor is it any more easy to see
how the contrivance on the body of the female for insuring the act of
copulation could be injurious to other species. But so long as it is not
demonstrated that our species are particularly in want of this
contrivance, or that the latter would rather be injurious than
beneficial to other species, its presence only in these few Amphipoda
will have to be regarded not as the work of far-seeing wisdom, but as
that of a favourable chance made use of by Natural Selection. Under the
latter supposition its isolated occurrence is intelligible, whilst we
cannot perceive why the Creator blessed just these few species with an
apparatus which he found to be quite compatible with the "general plan
of structure" of the Amphipoda, and yet denied it to others which live
under the same external conditions, and equal them even in their
extraordinary salacity. Associated with, or in the immediate vicinity of
the two species of Melita, live two species of Allorchestes, the pairs
of which are met with almost more numerously than the single animals,
and yet their females show no trace of the above-mentioned processes of
the coxal lamellae.

These cases, I think, must be brought to bear against the conception
supported with so much genius and knowledge by Agassiz, that species are
embodied thoughts of the Creator; and, with these, all similar instances
in which arrangements which would be equally beneficial to all the
species of a group are wanting in the majority and only conferred upon a
few special favourites, which do not seem to want them any more than the
rest.


CHAPTER 5. RESPIRATION IN LAND CRABS.

Among the numerous facts in the natural history of the Crustacea upon
which a new and clear light is thrown by Darwin's theory, besides the
two forms of the males in our Tanais and in Orchestia Darwinii, there is
one which appears to me of particular importance, namely, the character
of the branchial cavity in the air-breathing Crabs, of which,
unfortunately, I have been unable to investigate some of the most
remarkable (Gecarcinus, Ranina). As this character, namely, the
existence of an entrance behind the branchiae, has hitherto been
noticed, even as a fact, only in Ranina, I will go into it in some
detail. I have already mentioned that, as indeed is required by Darwin's
theory, this entrant orifice is produced in different manners in the
different families.

In the Frog-crab (Ranina) of the Indian Ocean, which, according to
Rumphius, loves to climb up on the roofs of the houses, the ordinary
anterior entrant orifice is entirely wanting according to Milne-Edwards,
and the entrance of a canal opening into the hindmost parts of the
branchial cavity is situated beneath the commencement of the abdomen.

The case is most simple in some of the Grapsoidae, as in Aratus Pisonii,
a charming, lively Crab which ascends the mangrove bushes (Rhizophora)
and gnaws their leaves. By means of its short but remarkably acute
claws, which prick like pins when it runs over the hand, this Crab
climbs with the greatest agility upon the thinnest twigs. Once, when I
had one of these animals sitting upon my hand, I noticed that it
elevated the hinder part of its carapace, and that by this means a wide
fissure was opened upon each side above the last pair of feet, through
which I could look far into the branchial cavity. I have since been
unable to procure this remarkable animal again, but on the other hand, I
have frequently repeated the same observation upon another animal of the
same family (apparently a true Grapsus), which lives abundantly upon the
rocks of our coast. Whilst the hinder part of the carapace rises and the
above-mentioned fissure is formed, the anterior part seems to sink, and
to narrow or entirely close the anterior entrant orifice. Under water
the elevation of the carapace never takes place. The animal therefore
opens its branchial cavity in front or behind, according as it has to
breathe water or air. How the elevation of the carapace is effected I do
not know, but I believe that a membranous sac, which extends from the
body cavity far into the branchial cavity beneath the hinder part of the
carapace, is inflated by the impulsion of the fluids of the body, and
the carapace is thereby raised.

I have also observed the same elevation of the carapace in some species
of the allied genera Sesarma and Cyclograpsus, which dig deep holes in
marshy ground, and often run about upon the wet mud, or sit, as if
keeping watch, before their burrows. One must, however, wait for a long
time with these animals, when taken out of the water, before they open
their branchial cavity to the air, for they possess a wonderful
arrangement, by means of which they can continue to breathe water for
some time when out of the water. The orifices for the egress of the
water which has served for respiration, are situated in these, as in
most Crabs, in the anterior angles of the buccal frame ("cadre buccal,"
M.-Edw.), whilst the entrant fissures of the branchial cavity extend
from its hinder angles above the first pair of feet. Now that portion of
the carapace which extends at the sides of the mouth between the two
orifices ("regions pterygostomiennes"), appears in our animals to be
divided into small square compartments. Milne-Edwards has already
pointed this out as a particularly remarkable peculiarity. This
appearance is caused partly by small wart-like elevations, and partly
and especially by curious geniculated hairs, which to a certain extent
constitute a fine net or hair-sieve extended immediately over the
surface of the carapace. Thus when a wave of water escapes from the
branchial cavity, it immediately becomes diffused in this network of
hairs and then again conveyed back to the branchial cavity by vigorous
movements of the appendage of the outer maxilliped which works in the
entrant fissure. Whilst the water glides in this way over the carapace
in the form of a thin film, it will again saturate itself with oxygen,
and may then serve afresh for the purposes of respiration. In order to
complete this arrangement the outer maxillipeds, as indeed has long been
known, bear a projecting ridge furnished with a dense fringe of hairs,
which commences in front near their median line and passes backwards and
outwards to the hinder angle of the buccal frame. Thus the two ridges of
the right and left sides form together a triangle with the apex turned
forwards,--a breakwater by which the water flowing from the branchial
cavity is kept away from the mouth and reconducted to the branchial
cavity. In very moist air the store of water contained in the branchial
cavity may hold out for hours, and it is only when this is used up that
the animal elevates its carapace in order to allow the air to have
access to its branchiae from behind.

In Eriphia gonagra the entrant orifices of the respiratory cavity
serving for aerial respiration are situated, not, as in the Grapsoidae,
above, but behind the last pair of feet at the sides of the abdomen.

(FIGURE 12. Posterior entrance to the branchial cavity of Ocypoda
rhombea, Fab., natural size. The carapace and the fourth foot of the
right side are removed.

FIGURE 13. Points of some of the hairs of the basal joints of the foot,
magnified 45 diam.)

The swift-footed Sand-Crabs (Ocypoda) are exclusively terrestrial
animals, and can scarcely live for a single day in water; in a much
shorter period a state of complete relaxation occurs and all voluntary
movements cease.* (* As this was not observed in the sea, but in glass
vessels containing sea-water, it might be supposed that the animals
become exhausted and die, not because they are under water but because
they have consumed all the oxygen which it contained. I therefore put
into the same water from which I had just taken an unconscious Ocypoda,
with its legs hanging loosely down, a specimen of Lupea diacantha which
had been reduced to the same state by being kept in the air, and this
recovered in the water just as the Ocypoda did in the air.) In these a
peculiar arrangement on the feet of the third and fourth pairs (Figure
12) has long been known, although its connexion with the branchial
cavity has not been suspected. These two pairs of feet are more closely
approximated than the rest; the opposed surfaces of their basal joints
(therefore the hinder surface on the third, and the anterior surface on
the fourth feet) are smooth and polished, and their margins bear a dense
border of long, silky, and peculiarly formed hairs (Figure 13).
Milne-Edwards who rightly compares these surfaces, as to their
appearance, with articular surfaces, thinks that they serve to diminish
the friction between the two feet. In considering this interpretation,
the question could not but arise why such an arrangement for the
diminution of friction should be necessary in these particular Crabs and
between these two feet, leaving out of consideration the fact that the
remarkable brushes of hair, which on the other hand must increase
friction, also remain unexplained. But as I was bending the feet of a
large Sand-Crab to and fro in various directions, in order to see in
what movements of the animal friction occurred at the place indicated,
and whether these might, perhaps, be movements of particular importance
to it and such as would frequently recur, I noticed, when I had
stretched the feet widely apart, in the hollow between them a round
orifice of considerable size, through which air could easily be blown
into the branchial cavity, and a fine rod might even be introduced into
it. The orifice opens into the branchial cavity behind a conical lobe,
which stands above the third foot in place of a branchia which is
wanting in Ocypoda. It is bounded laterally by ridges, which rise above
the articulation of the foot, and to which the lower margin of the
carapace is applied. Exteriorly, also, it is overarched by these ridges
with the exception of a narrow fissure. This fissure is overlaid by the
carapace, which exactly at this part projects further downwards than
elsewhere, and in this way a complete tube is formed. Whilst in Grapsus
the water is allowed to reach the branchiae only from the front, I saw
it in Ocypoda flow in also through the orifice just described.

In the position of posterior entrant orifice and the accompanying
peculiarities of the third and fourth pairs of feet, two other
non-aquatic species of the same family, which I have had the opportunity
of examining, agree with Ocypoda. One of these, perhaps Gelasimus
vocans, which lives in the mangrove swamps, and likes to furnish the
mouth of its burrow with a thick, cylindrical chimney of several inches
in height, has the brushes on the basal joints of the feet in question
composed of ordinary hairs. The other, a smaller Gelasimus, not
described in Milne-Edwards' 'Natural History of Crustacea,' which
prefers drier places and is not afraid to run about on the burning sand
under the vertical rays of the noonday sun in December, but can also
endure being in water at least for several weeks, resembles Ocypoda in
having these brushes composed of non-setiform, delicate hairs, indeed
even more delicate and more regularly constructed than in Ocypoda.* (*
This smaller Gelasimus is also remarkable because the chameleon-like
change of colour exhibited by many Crabs occurs very strikingly in it.
The carapace of a male which I have now before me shone with a dazzling
white in its hinder parts five minutes since when I captured it, at
present it shows a dull gray tint at the same place.) What may be the
significance of these peculiar hairs,--whether they only keep foreign
bodies from the branchial cavity,--whether they furnish moisture to the
air flowing past them,--or whether, as their aspect, especially in the
small Gelasimus, reminds one of the olfactory filaments of the Crabs,
they may also perform similar functions,--are questions the due
discussion of which would lead us too far from our subject. Nevertheless
it may be remarked that in both species, especially in Ocypoda, the
olfactory filaments in their ordinary situation are very much reduced,
and when they are in the water their flagella never perform the peculiar
beating movements which may be observed in other Crabs, and even in the
larger Gelasimus; moreover, the organ of smell must probably be sought
in these air-breathing Crabs, as in the air-breathing Vertebrata, at the
entrance to the respiratory cavity.

So much for the facts with regard to the aerial respiration of the
Crabs. It has already been indicated why Darwin's theory requires that
when any peculiar arrangements exist for aerial respiration, these will
be differently constructed in different families. That experience is in
perfect accordance with this requirement is the more in favour of
Darwin, because the schoolmen far from being able to foresee or explain
such profound differences, must rather regard them as extremely
surprising. If, in the nearly allied families of the Ocypodidae and
Grapsoidae, the closest agreement prevails in all the essential
conditions of their structure; if the same plan of structure is
slavishly followed in everything else, in the organs of sense, in the
articulation of the limbs, in every trabecula and tuft of hairs in the
complicated framework of the stomach, and in all the arrangements
subserving aquatic respiration, even to the hairs of the flagella
employed in cleaning the branchiae,--why have we suddenly this
exception, this complete difference, in connection with aerial
respiration?

The schoolmen will scarcely have an answer for this question, except by
placing themselves on the theologico-teleological stand-point which has
justly fallen into disfavour amongst us, and from which the mode of
production of an arrangement is supposed to be explained, if its
"adaptation" to the animal can be demonstrated. From this point of view
we might certainly say that a widely gaping fissure which had nothing
prejudicial in it to Aratus Pisonii among the foliage of the mangrove
bushes, was not suitable to the Ocypoda living in sand; that in the
latter, in order to prevent the penetration of the sand, the orifice of
the branchial cavity must be placed at its lowest part, directed
downwards, and concealed between broad surfaces fringed with protective
brushes of hair. It is far from the intention of these pages to enter
upon a general refutation of this theory of adaptation. Indeed there is
scarcely anything essential to be added to the many admirable remarks
that have been made upon this subject since the time of Spinoza. But
this may be remarked, that I regard it as one of the most important
services of the Darwinian theory that it has deprived those
considerations of usefulness which are still undeniable in the domain of
life, of their mystical supremacy. In the case before us it is
sufficient to refer to the Gelasimus of the mangrove swamps, which
shares the same conditions of life with various Grapsoidae and yet does
not agree with them, but with the arenicolous Ocypoda.


CHAPTER 6. STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA.

Scarcely less striking than the example of the air-breathing Crabs, is
the behaviour of the heart in the great section Edriophthalma, which may
advantageously be divided, after the example of Dana and Spence Bate,
only into two orders, the Amphipoda and the Isopoda.

In the Amphipoda, to which the above-mentioned naturalists correctly
refer the Caprellidae and Cyamidae (Latreille's Laemodipoda), the heart
has always the same position; it extends in the form of a long tube
through the six segments following the head, and has three pairs of
fissures, furnished with valves, for the entrance of the blood, situated
in the second, third, and fourth of these segments. It was found to be
of this structure by La Valette in Niphargus (Gammarus puteanus), and by
Claus in Phronima; and I have found it to be the same in a considerable
number of species belonging to the most different families.* (* The
young animals in the egg, a little before their exclusion, are usually
particularly convenient for the observation of the fissures in the
heart; they are generally sufficiently transparent, the movements of the
heart are less violent than at a later period, and they lie still even
without the pressure of a glass cover. Considering the common opinion as
to the distribution of the Amphipoda, namely, that they increase in
multiplicity towards the poles, and diminish towards the equator, it may
seem strange that I speak of a considerable number of species on a
subtropical coast. I therefore remark that in a few months and without
examining any depths inaccessible from the shore, I obtained 38
different species, of which 34 are new, which, with the previously known
species (principally described by Dana) gives 60 Brazilian Amphipoda,
whilst Kroyer in his 'Gronlands Amfipoder' was acquainted with only 28
species, including 2 Laemodipoda, from the Arctic Seas, although these
had been investigated by a far greater number of Naturalists.)

The sole unimportant exception which I have hitherto met with is
presented by the genus Brachyscelus,* (*According to Milne-Edwards'
arrangement the females of this genus would belong to the "Hyperines
ordinaires" and the previously unknown males to the "Hyperines
anormales," the distinguishing character of which, namely the curiously
zigzagged inferior antennae, is only a sexual peculiarity of the male
animals. In systematising from single dead specimens, as to the sex,
age, etc. of which nothing is known, similar errors are unavoidable.
Thus, in order to give another example of very recent date, a celebrated
Ichthyologist, Bleeker, has lately distinguished two groups of the
Cyprinodontes as follows: some, the Cyprinodontini, have a "pinna analis
non elongata," and the others, the Aplocheilini, a "pinna analis
elongata": according to this the female of a little fish which is very
abundant here would belong to the first, and the male to the second
group. Such mistakes, as already stated, are unavoidable by the
"dry-skin" philosopher, and therefore excusable; but they nevertheless
prove in how random a fashion the present systematic zoology frequently
goes on, without principles or sure foundations, and how much it is in
want of the infallible touchstone for the value of the different
characters, which Darwin's theory promises to furnish.) in which the
heart possesses only two pairs of fissures, as it extends forward only
into the second body-segment, and is destitute of the pair of fissures
situated in this segment in other forms.* (* I find, in Milne-Edwards'
'Lecons sur la Physiol. et l'Anat. comp.' 3 page 197, the statement
that, according to Frey and Leuckart, the heart of Caprella linearis
possesses FIVE pairs of fissures. I have examined perfectly transparent
young Caprellae (probably the young of Caprella attenuata, Dana, with
which they occurred), but can only find the usual three pairs.)

Considering this uniformity presented by the heart in the entire order
of the Amphipoda, it cannot but seem very remarkable, that in the very
next order of the Isopoda, we find it to be one of the most changeable
organs.

In the cheliferous Isopods (Tanais) the heart resembles that of the
Amphipoda in its elongated tubular form, as well as in the number and
position of the fissures, but with this difference, that the two
fissures of each pair do not lie directly opposite each other.

(FIGURE 14. Heart of a young Cassidina.

FIGURE 15. Heart of a young Anilocra.

FIGURE 16. Abdomen of the male of Entoniscus Cancrorum. h. Heart. l.
Liver.)

In all other Isopoda the heart is removed towards the abdomen. In the
wonderfully deformed parasitic Isopods of the Porcellanae (Entoniscus
porcellanae), the spherical heart of the female is confined to a short
space of the elongated first abdominal segment, and seems to possess
only a single pair of fissures. In the male of Entoniscus Cancrorum (n.
sp.), the heart (Figure 16) is situated in the third abdominal segment.
In the Cassidinae, the heart (Figure 14) is likewise short and furnished
with two pairs of fissures, situated in the last segment of the thorax
and the first segment of the abdomen. Lastly, in a young Anilocra, I
find the heart (Figure 15) extending through the whole length of the
abdomen and furnished with four (or five?) fissures, which are not
placed in pairs but alternately to the right and left in successive
segments. In other animals of this order, which I have as yet only
cursorily examined, further differences will no doubt occur. But why, in
two orders so nearly allied to each other, should we find in the one
such a constancy, in the other such a variability, of the same highly
important organ? From the schoolmen we need expect no explanation, they
will either decline the discussion of the "wherefore" as foreign to
their province, as lying beyond the boundaries of Natural History, or
seek to put down the importunate question by means of a sounding
paraphrase of the facts, abundantly sprinkled with Greek words. As I
have unfortunately forgotten my Greek, the second way out of the
difficulty is closed to me; but as I luckily reckon myself not amongst
the incorporated masters, but, to use Baron von Liebig's expression,
amongst the "promenaders on the outskirts of Natural History," this
affected hesitation of the schoolmen cannot dissuade me from seeking an
answer, which indeed presents itself most naturally from Darwin's point
of view.

As not only the Tanaides (which reasons elsewhere stated (vide supra)
justify us in regarding as particularly nearly related to the primitive
Isopod) and the Amphipoda, but also the Decapod Crustacea, possess a
heart with three pairs of fissures essentially in the same position; and
as the same position of the heart recurs (vide infra) even in the
embryos of the Mantis-Shrimps (Squilla), in which the heart of the adult
animal, and even, as I have elsewhere shown, that of the larvae when
still far from maturity, extends in the form of a long tube with
numerous openings far into the abdomen, we must unhesitatingly regard
the heart of the Amphipoda as the primitive form of that organ in the
Edriophthalma. As, moreover, in these animals the blood flows from the
respiratory organs to the heart without vessels, it is very easy to see
how advantageous it must be to them to have these organs as much
approximated as possible. We have reason to regard as the primitive mode
of respiration, that occurring in Tanais (vide supra). Now, where, as in
the majority of the Isopoda, branchiae were developed upon the abdomen,
the position and structure of the heart underwent a change, as it
approached them more nearly, but without the reproduction of a common
plan for these earlier modes of structure, either because this
transformation of the heart took place only after the division of the
primary form into subordinate groups, or because, at least at the time
of this division, the varying heart had not yet become fixed in any new
form. Where, on the contrary, respiration remained with the anterior
part of the body,--whether in the primitive fashion of Zoea, as in the
Tanaides, or by the development of branchiae on the thorax, as in the
Amphipoda,--the primitive form of the heart was inherited unchanged,
because any variations which might make their appearance were rather
injurious than advantageous, and disappeared again immediately.

I close this series of isolated examples with an observation which
indeed only half belongs to the province of the Crustacea to which these
pages ought to be confined, and which also has no further connexion with
the preceding circumstances than that of being an "intelligible and
intelligence-bringing fact" only from the point of view of Darwin's
theory. To-day as I was opening a specimen of Lepas anatifera in order
to compare the animal with the description in Darwin's 'Monograph on the
Subclass Cirripedia,' I found in the shell of this Cirripede, a
blood-red Annelide, with a short, flat body, about half an inch long and
two lines in breadth, with twenty-five body-segments, and without
projecting setigerous tubercles or jointed cirri. The small cephalic
lobe bore four eyes and five tentacles; each body-segment had on each
side at the margin a tuft of simple setae directed obliquely upwards,
and at some distance from this, upon the ventral surface, a group of
thicker setae with a strongly uncinate bidentate apex. There was above
EACH of the lateral tufts of bristles a branchia, simple on a few of the
foremost segments, and then strongly arborescent to the end of the body.
The animal, a female filled with ova, evidently, from these characters,
belongs to the family of the Amphinomidae; the only family the members
of which, being excellent swimmers, live in the open sea.

That this animal had not strayed accidentally into the Lepas, but
appertained to it as a regular and permanent guest, is evidenced by its
considerable size in proportion to the narrow entrance of the test of
the Lepas, by the complete absence of the iridescence which usually
distinguishes the skin of free Annelides and especially of the
Amphinomidae, by the formation and position of the inferior setae, etc.
But that a worm belonging to this particular family Amphinomidae living
in the high sea, occurs as a guest in the Lepas, which also floats in
the sea attached to wood, etc., is at once intelligible from the
stand-point of the Darwinian theory, whilst the relationship of this
parasite to the free-living worms of the open sea remains perfectly
unintelligible under the supposition that it was independently created
for dwelling in the Lepas.

But however favourable the examples hitherto referred to may be for
Darwin, the objection may be raised against them, and that with perfect
justice, that they are only isolated facts, which, when the
considerations founded upon them are carried far beyond what is
immediately given, may only too easily lead us from the right path, with
the deceptive glimmer of an ignis fatuus. The higher the structure to be
raised, the wider must be the assuring base of well-sifted facts.

Let us turn then to a wider field, that of the developmental history of
the Crustacea, upon which science has already brought together a varied
abundance of remarkable facts, which, however, have remained a barren
accumulation of unmanageable raw-material, and let us see how, under
Darwin's hand, these scattered stones unite to form a well-jointed
structure, in which everything, bearing and being borne, finds its
significant place. Under Darwin's hand! for I shall have nothing to do
except just to place the building stones in the position which his
theory indicates for them. "When kings build, the carters have to work."


CHAPTER 7. DEVELOPMENTAL HISTORY OF PODOPHTHALMA.

Let us first glance over the extant facts.

Among the Stalk-eyed Crustacea (Podophthalma) we know only a very few
species which quit the egg in the form of their parents, with the full
number of well-jointed appendages to the body. This is the case
according to Rathke* in the European fresh-water Crayfish, and according
to Westwood in a West Indian Land Crab (Gecarcinus). (* Authorities are
cited only for facts which I have had no opportunity of confirming.)
Both exceptions therefore belong to the small number of Stalk-eyed
Crustacea which live in fresh water or on the land, as indeed in many
other cases fresh-water and terrestrial animals undergo no
transformations, whilst their allies in the sea have a metamorphosis to
undergo. I may refer to the Earthworms and Leeches among the Annelida,
which chiefly belong to the land and to fresh water,--to the Planariae
of the fresh waters and the Tetrastemma of the sparingly saline Baltic
among the Turbellaria,--to the Pulmonate Gasteropoda, and to the
Branchiferous Gasteropoda of the fresh waters, the young of which
(according to Troschel's 'Handb. der Zoologie') have no ciliated buccal
lobes, although such organs are possessed by the very similar
Periwinkles (Littorina).

All the marine forms of this section appear to be subject to a more or
less considerable metamorphosis. This appears to be only inconsiderable
in the common Lobster, the young of which, according to Van Beneden, are
distinguished from the adult animal, by having their feet furnished,
like those of Mysis, with a swimming branch projecting freely outwards.
From a figure given by Couch the appendages of the abdomen and tail also
appear to be wanting.

Far more profound is the difference of the youngest brood from the
sexually mature animal in by far the greater majority of the
Podophthalma, which quit the egg in the form of Zoea. This young form
occurs, so far as our present observations go, in all the Crabs, with
the sole exception of the single species investigated by Westwood. I say
SPECIES, and not GENUS, for in the same genus, Gecarcinus, Vaughan
Thompson found Zoea-brood,* which is also met with in other terrestrial
Crabs (Ocypoda, Gelasimus, etc.). (* Bell ('Brit. Stalk-eyed Crust.'
page 45) considers himself justified in "eliminating" Thompson's
observation at once, because he could only have examined ovigerous
females preserved in alcohol. But any one who had paid so much attention
as Thompson to the development of these animals, must have been well
able to decide with certainty upon eggs, if not too far from maturity or
badly preserved, whether a Zoea would be produced from them. Moreover,
the mode of life of the Land-Crabs is in favour of Thompson. "Once in
the year," says Troschel's 'Handbuch der Zoologie,' "they migrate in
great crowds to the sea in order to deposit their eggs, and afterwards
return much exhausted towards their dwelling places, which are reached
only by a few." For what purpose would be these destructive migrations
in species whose young quit the egg and the mother as terrestrial
animals?) All the Anomura seem likewise to commence their lives as
Zoeae: witness the Porcellanae, the Tatuira (Hippa emerita) and the
Hermit Crabs. Among the Macrura we are acquainted with the same earliest
form principally in several Shrimps and Prawns, such as Crangon (Du
Cane), Caridina (Joly), Hippolyte, Palaemon, Alpheus, etc. Lastly, it is
not improbable, that the youngest brood of the Mantis-Shrimps (Squilla)
is also in the same case.

The most important peculiarities which distinguish this Zoea-brood from
the adult animal, are as follows:--

The middle-body with its appendages, those five pairs of feet to which
these animals owe their name of Decapoda, is either entirely wanting, or
scarcely indicated; the abdomen and tail are destitute of appendages,
and the latter consists of a single piece. The mandibles, as in the
Insecta, have no palpi. The maxillipedes, of which the third pair is
often still wanting, are not yet brought into the service of the mouth,
but appear in the form of biramose natatory feet. Branchiae are wanting,
or where their first rudiments may be detected as small verruciform
prominences, these are dense cell-masses, through which the blood does
not yet flow, and which therefore have nothing to do with respiration.
An interchange of the gases of the water and blood may occur all over
the thin-skinned surface of the body; but the lateral parts of the
carapace may unhesitatingly be indicated as the chief seat of
respiration. They consist, exactly as described by Leydig in the
Daphniae, of an outer and inner lamina, the space between which is
traversed by numerous transverse partitions dilated at their ends; the
spaces between these partitions are penetrated by a more abundant flow
of blood than occurs anywhere else in the body of the Zoea. To this may
be added that a constant current of fresh water passes beneath the
carapace in a direction from behind forwards, maintained as in the adult
animal, by a foliaceous or linguiform appendage of the second pair of
maxillae (Figure 18). The addition of fine coloured particles to the
water allows this current of water to be easily detected even in small
Zoeae.

(FIGURE 17. Zoea of a Marsh Crab (Cyclograpsus ?), magnified 45 diam.

FIGURE 18. Maxilla of the second pair in the same species, magnified 180
diam.)

The Zoeae of the Crabs (Figure 17) are usually distinguished by long,
spiniform processes of the carapace. One of these projects upwards from
the middle of the back, a second downwards from the forehead, and
frequently there is a shorter one on each side near the posterior
inferior angles of the carapace. All these processes are, however,
wanting in Maia according to Couch, and in Eurynome according to
Kinahan; and in a third species of the same group of the Oxyrhynchi
(belonging or nearly allied to the genus Achaeus) I also find only an
inconsiderable dorsal spine, whilst the forehead and sides are unarmed.
This is another example warning us to be cautious in deductions from
analogy. Nothing seemed more probable than to refer back the beak-like
formation of the forehead in the Oxyrhynchi to the frontal process of
the Zoea, and now it appears that the young of the Oxyrhynchi are really
quite destitute of any such process. The following are more important
peculiarities of the Zoeae of the Crabs, although less striking than
these processes of the carapace which, in combination with the large
eyes, often give them so singular an appearance:--the anterior (inner)
antennae are simple, not jointed, and furnished at the extremity with
from two to three olfactory filaments; the posterior (outer) antennae
frequently run out into a remarkably long spine-like process ("styliform
process," Spence Bate), and bear, on the outside, an appendage, which is
sometimes very minute ("squamiform process" of Spence Bate),
corresponding with the antennal scale of the Prawns,* (* In a memoir on
the metamorphoses of the Porcellanae I have erroneously described this
appendage as the "flagellum.") and the first rudiment of the future
flagellum is often already recognisable. Of natatory feet (afterwards
maxillipeds) only two pairs are present; the third (not, as Spence Bate
thinks, the first) is entirely wanting, or, like the five following
pairs of feet, present only as a minute bud. The tail, of very variable
form, always bears THREE pairs of setae at its hinder margin. The Zoeae
of the Crabs usually maintain themselves in the water in such a manner
that the dorsal spine stands upwards, the abdomen is bent forwards, the
inner branch of the natatory feet is directed forwards, and the outer
one outwards and upwards.

(FIGURES 19 TO 23. Tails of the Zoeae of various Crabs.

FIGURE 19. Pinnotheres.

FIGURE 20. Sesarma.

FIGURE 21. Xantho.

FIGURES 22 AND 23 of unknown origin.)

It is further to be remarked that the Zoeae of the Crabs, as also of the
Porcellanae, of the Tatuira and of the Shrimps and Prawns, are
enveloped, on escaping from the egg, by a membrane veiling the spinous
processes of the carapace, the setae of the feet, and the antennae, and
that they cast this in a few hours. In Achaeus I have observed that the
tail of this earliest larval skin resembles that of the larvae of
Shrimps and Prawns, and the same appears to be the case in Maia (see
Bell, 'Brit. Stalk-eyed Crust.' page 44).

Widely as they seem to differ from them at the first glance, the Zoeae
of the Porcellanae (Figure 24) approach those of the true Crabs very
closely. The antennae, organs of the mouth, and natatory feet, exhibit
the same structure. But the tail bears FIVE pairs of setae, and the
dorsal spine is wanting, whilst, on the contrary, the frontal process
and the lateral spines are of extraordinary length, and directed
straight forward and backward.

(FIGURE 24. Zoea of Porcellana stellicola, F. Mull. Magnified 15 diam.

FIGURE 25. Zoea of the Tatuira (Hippa emerita), magnified 45 diam.

FIGURE 26. Zoea of a small Hermit Crab, magnified 45 diam.)

The Zoea of the Tatuira (Figure 25) also appears to differ but little
from those of the true Crabs, which it likewise resembles in its mode of
locomotion. The carapace possesses only a short, broad frontal process;
the posterior margin of the tail is edged with numerous short setae.

The Zoea of the Hermit Crabs (Figure 26) possesses the simple inner
antennae of the Zoea of the true Crabs; the outer antennae bear upon the
outside on a short stalk a lamella of considerable size analogous to the
scale of the antennae of the Prawns; on the inside, a short, spine-like
process; and between the two the flagellum, still short, but already
furnished with two apical setae. As in the Crabs, there are only two
pairs of well-developed natatory feet (maxillipedes), but the third pair
is also present in the form of a two-jointed stump of considerable size,
although still destitute of setae. The tail bears five pairs of setae.
The little animal usually holds itself extended straight in the water,
with the head directed downwards.

This is also the position in which we usually see the Zoeae of the
Shrimps and Prawns (Figure 27), which agree in their general appearance
with those of the Hermit Crabs. Between the large compound eyes there is
in them a small median eye. The inner antennae bear, at the end of a
basal joint sometimes of considerable length, on the inside a plumose
seta, which also occurs in the Hermit Crabs, and on the outside a short
terminal joint with one or more olfactory filaments. The outer antennae
exhibit a well-developed and sometimes distinctly articulated scale, and
within this usually a spiniform process; the flagellum appears generally
to be still wanting. The third pair of maxillipedes seems to be always
present, at least in the form of considerable rudiments. The spatuliform
caudal lamina bears from five to six pairs of setae on its hinder
margin.

The development of the Zoea-brood to the sexually mature animal was
traced by Spence Bate in Carcinus maenas. He proved that the
metamorphosis is a perfectly gradual one, and that no sharply separated
stages of development, like the caterpillar and pupa of the Lepidoptera,
could be defined in it. Unfortunately we possess only this single
complete series of observations, and its results cannot be regarded at
once as universally applicable; thus the young Hermit Crabs retain the
general aspect and mode of locomotion of Zoeae, whilst the rudiments of
the thoracic and abdominal feet are growing, and then, when these come
into action, appear at once in a perfectly new form, which differs from
that of the adult animal chiefly by the complete symmetry of the body
and by the presence of four pairs of well-developed natatory feet on the
abdomen.* (* Glaucothoe Peronii, M.-Edw., may be a young and still
symmetrical Pagurus of this kind.)

(FIGURE 27. Zoea of a Palaemon residing upon Rhizostoma cruciatum,
Less., magnified 45 diam.)

The development of the Palinuridiae seems to be very peculiar. Claus
found in the ova of the Spiny Lobster (Palinurus), embryos with a
completely segmented body, but wanting the appendages of the tail,
abdomen, and last two segments of the middle-body; they possess a single
median and considerably compound eye; the anterior antennae are simple,
the posterior furnished with a small secondary branch; the mandibles
have no palpi; the maxillipedes of the third pair, like the two
following pairs of feet, are divided into two branches of nearly equal
length; whilst the last of the existing pairs of feet and the second
pair of maxillipedes bear only an inconsiderable secondary branch.
Coste, as is well known, asserts that he has bred young Phyllosomata
from the ova of this lobster--a statement that requires further proof,
especially as the more recent investigations of Claus upon Phyllosoma by
no means appear to be in its favour.

The large compound eyes, which usually soon become moveable, and
sometimes stand upon long stalks even in the earliest period, as well as
the carapace, which covers the entire fore-body, indicate at once that
the position of the larvae hitherto considered, notwithstanding all
their differences, is under the Podophthalma. But not a single
characteristic of this section is retained by the brood of some Prawns
belonging to the genus Peneus or in its vicinity. These quit the egg
with an unsegmented ovate body, a median frontal eye, and three pairs of
natatory feet, of which the anterior are simple, and the other two
biramose--in fact, in the larval form, so common among the lower
Crustacea, to which O.F. Muller gave the name of Nauplius. No trace of a
carapace! no trace of the paired eyes! no trace of masticating organs
near the mouth which is overarched by a helmet-like hood!

(FIGURE 28. Nauplius of a Prawn, magnified 45 diam.

FIGURE 29. Young Zoea of the same Prawn, magnified 45 diam.

FIGURE 30. Older Zoea of the same Prawn, magnified 45 diam.

FIGURE 31. Mysis-form of the same Prawn, magnified 45 diam.)

In the case of one of these species the intermediate forms which lead
from the Nauplius to the Prawn, have been discovered in a nearly
continuous series.

The youngest Nauplius (Figure 28) is immediately followed by forms in
which a fold of skin runs across the back behind the third pair of feet,
and four pairs of stout processes (rudiments of new limbs) sprout forth
on the ventral surface. Within the third pair of feet, powerful
mandibles are developed.

In a subsequent moult the new limbs (maxillae, and anterior and
intermediate maxillipedes) come into action, and in this way the
Nauplius becomes a Zoea (Figure 29), agreeing perfectly with the Zoea of
the Crabs in the number of the appendages of the body, although very
different in form and mode of locomotion and even in many particulars of
internal structure. The chief organs of motion are still the two
anterior pairs of feet, which are slender and furnished with long setae;
the third pair of feet loses its branches, and becomes converted into
mandibles destitute of palpi. The labrum acquires a spine directed
forward and of considerable size, which occurs in all the Zoeae of
allied species. The biramose maxillipedes appear to assist but slightly
in locomotion. The forked tail reminds us rather of the forms occurring
in the lower Crustacea, especially the Copepoda, than of the spatuliform
caudal plate which characterises the Zoeae of Alpheus, Palaemon,
Hippolyte, and other Prawns, of the Hermit Crabs, the Tatuira and the
Porcellanae. The heart possesses only one pair of fissures, and has no
muscles traversing its interior like trabeculae, whilst in other Zoeae
two pairs of fissures and an interior apparatus of trabeculae are always
distinctly recognisable.

During this Zoeal period the paired eyes, the segments of the
middle-body and abdomen, the posterior maxillipedes, the lateral caudal
appendages and the stump-like rudiments of the feet of the middle-body
are formed (Figure 30). The caudal appendages sprout forth like other
limbs freely on the ventral surface, whilst in other Prawns, the
Porcellanae, etc., they are produced in the interior of the spatuliform
caudal plate.

As the feet of the middle-body come into action, simultaneously with
other profound changes, the Zoea passes into the Mysis- or
Schizopod-form (Figure 31). The antennae cease to serve for locomotion,
their place is taken by the thoracic feet, furnished with long setae,
and by the long abdomen which just before was laboriously dragged along
as a useless burden, but now, with its powerful muscles, jerks the
animal through the water in a series of lively jumps. The anterior
antennae have lost their long setae, and by the side of the last
(fourth) joint, endowed with olfactory filaments, there appears a second
branch, which is at first of a single joint. The previously
multi-articulate outer branch of the posterior antennae has become a
simple lamella, the antennal scale of the Prawn; beside this appears the
stump-like rudiment of the flagellum, probably as a new formation, the
inner branch disappearing entirely. The five new pairs of feet are
biramose, the inner branch short and simple, the outer one longer,
annulated at the end, furnished with long setae, and kept, as in Mysis,
in constant whirling motion. The heart acquires new fissures, and
interior muscular trabeculae.

During the Mysis-period, the auditory organs in the basal joint of the
anterior antennae are formed; the inner branches of the first three
pairs of feet are developed into chelae and the two hinder pairs into
ambulatory feet; palpi sprout from the mandibles, branchiae on the
thorax, and natatory feet on the abdomen. The spine on the labrum
becomes reduced in size. In this way the animal gradually approaches the
Prawn-form, in which the median eye has become indistinct, the spine of
the labrum, and the outer branches of the cheliferous and ambulatory
feet have been lost, the mandibular palpi and the abdominal feet have
acquired distinct joints and setae, and the branchiae come into action.

In another Prawn, the various larval states of which may be easily
recognised as belonging to the same series by the presence of a
dark-yellow, sharply-defined spot surrounding the median eye, the
youngest Zoea (Figure 32), probably produced from the Nauplius, agrees
in all essential particulars with the species just described; its
further development is, however, very different, especially in that
neither the feet of the middle, nor those of the hind-body are formed
simultaneously, and that a stage of development comparable to Mysis in
the number and structure of the limbs does not occur.

(FIGURE 32. Youngest (observed) Zoea of another Prawn. The minute buds
of the third pair of maxillipedes are visible. The formation of the
abdominal segments has commenced. Paired eyes still wanting. Magnified
45 diam.)

Traces of the outer maxillipedes make their appearance betimes. Then
feet appear upon four segments of the middle-body, and these are
biramose on the three anterior segments, and simple, the inner branch
being deficient, on the fourth segment. On the inner branches the chelae
are developed; the outer branches are lost before an inner branch has
made its appearance on the fourth segment (Figure 32). The latter again
becomes destitute of appendages, so that in this case at an early period
four, and at a later only three, segments of the middle-body bear limbs.
The fifth segment is still entirely wanting, whilst all the abdominal
segments have also acquired limbs, and this one after the other, from
before backwards. The adult animal, as shown by the three pairs of
chelae, will certainly be very nearly allied to the preceding species.*
(* The oldest observed larvae (see Figure 33) are characterised by the
extraordinary length of the flagella of the outer antennae, and in this
respect resemble the larva of Sergestes found by Claus near Messina
(Zeitschr. fur Wiss. Zool. Bd. 13 Taf 27 Figure 14). This unusual length
of the antennae leads to the supposition that they belong to our
commonest Prawn, which is very frequently eaten, and is most nearly
allied to Peneus setiferus of Florida. Claus's Acanthosoma (l.c. Figure
13) is like the younger Mysis-form of the larva figured by me in the
'Archiv fur Naturgeschichte,' 1836, Taf 2, Figure 18, and which I am
inclined to refer to Sicyonia carinata.)

The youngest larva of the Schizopod genus Euphausia observed by Claus,
stands very near the youngest Zoea of our Prawns; but whilst its
anterior antennae are already biramose, and it therefore appears to be
more advanced, it still wants the middle maxillipedes. In it also Claus
found the heart furnished with only a single pair of fissures. Do not
Nauplius-like states in this case also precede the Zoea?

(FIGURE 33. Older larva produced from the Zoea represented in Figure 32.
The last segment and the last two pairs of feet of the middle-body are
wanting. Magnified 20 diam.)

The developmental history of Mysis, the near relationship of which with
the Shrimps and Prawns has recently again been generally recognised, has
been described in detail by Van Beneden. So far as I have tested them I
can only confirm his statements. The development of the embryo commences
with the formation of the tail! This makes its appearance as a simple
lobe, the dorsal surface of which is turned towards and closely applied
to that of the embryo. (The young of other Stalk-eyed Crustacea are, as
is well known, bent in the egg in such a manner that the ventral
surfaces of the anterior and posterior halves of the body are turned
towards each other,--in these, therefore, the dorsal, and in Mysis the
ventral surface appears convex.) The tail soon acquires the furcate form
with which we made acquaintance in the last Prawn-Zoea described. Then
two pairs of thick ensiform appendages make their appearance at the
opposite end of the body, and behind these a pair of tubercles which are
easily overlooked. These are the antennae and mandibles. The
egg-membrane now bursts, before any internal organ, or even any tissue,
except the cells of the cutaneous layer, is formed. The young animal
might be called a Nauplius; but essentially there is nothing but a rough
copy of a Nauplius-skin, almost like a new egg-membrane, within which
the Mysis is developed. The ten pairs of appendages of the fore-
(maxillae, maxillipedes) and middle-body make their appearance
simultaneously, as do the five pairs of abdominal feet at a later
period. Soon after the young Mysis casts the Nauplius-envelope it quits
the brood-pouch of the mother.* (* Van Beneden, who regards the
eye-peduncles as limbs, cannot however avoid remarking upon Mysis: "Ce
pedicule n'apparait aucunement comme les autres appendices, et parait
avoir une autre valeur morphologique.")

For some time, owing to an undue importance being ascribed to the want
of a particular branchial cavity, Mysis, Leucifer, and Phyllosoma were
referred to the Stomapoda, which are now again limited, as originally by
Latreille, to the Mantis-shrimps (Squilla), the Glass-shrimps
(Erichthus) and their nearest allies. Of the developmental history of
these we have hitherto been acquainted with only isolated fragments. The
tracing of the development in the egg is rendered difficult by the
circumstance, that the Mantis-shrimps do not, like the Decapoda, carry
their spawn about with them, but deposit it in the subterranean passages
inhabited by them in the form of thin, round, yellow plates. The spawn
is consequently exceedingly difficult to procure, and unfortunately it
becomes spoilt in a day when it is removed from its natural hatching
place, whilst on the contrary the progress of development may be
followed for weeks together in the eggs of a single Crab kept in
confinement. The eggs of Squilla, like those removed from the body of
the Crab, die because they are deprived of the rapid stream of fresh
water which the mother drives through her hole for the purpose of her
own respiration.

The accompanying representation of the embryo of Squilla shows that it
possesses a long, segmented abdomen without appendages, a bilobate tail,
six pairs of limbs, and a short heart; the latter only pulsates weakly
and slowly. If it acquires more limbs before exclusion, the youngest
larva must stand on the same level as the youngest larva of Euphausia
observed by Claus.

(FIGURE 34. Embryo of a Squilla, magnified 45 diam. a. heart.

FIGURE 35. Older larva (Zoea) of a Stomapod, magnified 15 diam.)

Of the two larval forms at present known which are with certainty to be
ascribed, if not to Squilla, at least to a Stomapod, I pass over the
younger one* (* 'Archiv fur Naturgeschichte' 1863 Taf 1.) as its limbs
cannot be positively interpreted, and will only mention that in it the
last three abdominal segments are still destitute of appendages. The
older larva (Figure 35), which resembles the mature Squilla especially
in the structure of the great raptorial feet and of the preceding pair,
still wants the six pairs of feet following the raptorial feet. The
corresponding body-segments are already well developed, an unpaired eye
is still present, the anterior antennae are already biramose, whilst the
flagellum is wanting in the posterior, and the mandibles are destitute
of palpi; the four anterior abdominal segments bear biramose natatory
feet, without branchiae; the fifth abdominal segment has no appendages,
and this is also the case with the tail, which still appears as a simple
lamina, fringed on the hinder margin with numerous short teeth. It is
evident that the larva stands essentially in the grade of Zoea.


CHAPTER 8. DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.

Less varied than that of the Stalk-eyed Crustacea is the mode of
development of the Isopoda and Amphipoda, which Leach united in the
section Edriophthalma, or Crustacea with sessile eyes.

(FIGURE 36. Embryo of Ligia in the egg, magnified 15 diam. D. yelk; L.
liver.)

The Rock-Slaters (Ligia) may serve as an example of the development of
the Isopoda. In these, as in Mysis, the caudal portion of the embryo is
bent not downwards, but upwards; as in Mysis also, a larval membrane is
first of all formed, within which the Slater is developed. In Mysis this
first larval skin may be compared to a Nauplius; in Ligia it appears
like a maggot quite destitute of appendages, but produced into a long
simple tail (Figure 37). The egg-membrane is retained longer than in
Mysis; it bursts only when the limbs of the young Slater are already
partially developed in their full number. The dorsal surface of the
Slater is united to the larval skin a little behind the head. At this
point, when the union has been dissolved a little before the change of
skin, there is a foliaceous appendage, which exists only for a short
time, and disappears before the young Slater quits the brood-pouch of
the mother.

(FIGURE 37. Maggot-like larva of Ligia, magnified 15 diam. R remains of
the egg-membrane. We see on the lower surface, from before
backwards:--the anterior and posterior antennae, the mandibles, the
anterior and posterior maxillae, maxillipedes, six ambulatory feet, the
last segment of the middle-body destitute of appendages, five abdominal
feet, and the caudal feet.)

The young animal, when it begins to take care of itself, resembles the
old ones in almost all parts, except one important difference; it
possesses only six, instead of seven pairs of ambulatory feet; and the
last segment of the middle-body is but slightly developed and destitute
of appendages. It need hardly be mentioned that the sexual peculiarities
are not yet developed, and that in the males the hand-like enlargements
of the anterior ambulatory feet and the copulatory appendages are still
deficient.

(FIGURE 38. Embryo of a Philoscia in the egg, magnified 25 diam.)

To the question, how far the development of Ligia is repeated in the
other Isopoda, I can only give an unsatisfactory answer. The curvature
of the embryo upwards instead of downwards was met with by me as well as
by Rathke in Idothea, and likewise in Cassidina, Philoscia, Tanais, and
the Bopyridae,--indeed, I failed to find it in none of the Isopoda
examined for this purpose. In Cassidina also the first larval skin
without appendages is easily detected; it is destitute of the long tail,
but is strongly bent in the egg, as in Ligia, and consequently cannot be
mistaken for an "inner egg-membrane." This, however, might happen in
Philoscia, in which the larval skin is closely applied to the
egg-membrane (Figure 38), and is only to be explained as the larval skin
by a reference to Ligia and Cassidina. The foliaceous appendage on the
back has long been known in the young of the common Water Slater
(Asellus).* (* Leydig has compared this foliaceous appendage of the
Water Slaters with the "green gland" or "shell-gland" of other
crustacea, assuming that the green gland has no efferent duct and
appealing to the fact that the two organs occur "in the same place."
This interpretation is by no means a happy one. In the first place we
may easily ascertain in Leucifer, as was also found to be the case by
Claus, that the "green gland" really opens at the end of the process
described by Milne-Edwards as a "tubercule auditif" and by Spence Bate
as an "olfactory denticle." And, secondly, the position is about as
different as it can well be. In the one case a paired gland, opening at
the base of the posterior antennae, and therefore on the lower surface
of the SECOND segment; in the other an unpaired structure rising in the
median line of the back BEHIND THE SEVENTH SEGMENT, ("behind the
boundary line of the first thoracic segment," Leydig).) That the last
pair of feet of the thorax is wanting in the young of the Wood-lice
(Porcellionides, M.-Edw.) and Fish-lice (Cymothoadiens, M.-Edw.) has
already been noticed by Milne-Edwards. This applies also to the
Box-Slaters (Idothea), to the viviparous Globe-Slaters (Sphaeroma) and
Shield-Slaters (Cassidina), to the Bopyridae (Bopyrus, Entoniscus,
Cryptoniscus, n.g.), and to the Cheliferous Slaters (Tanais), and
therefore probably to the great majority of the Isopoda. All the other
limbs are usually well developed in the young Isopoda. In Tanais alone,
all the abdominal feet are wanting (but not those of the tail); they are
developed simultaneously with the last pair of feet of the thorax.

(FIGURE 39. Embryo of Cryptoniscus planarioides, magnified 90 diam.

FIGURE 40. Last foot of the middle-body of the larva of Entoniscus
Porcellanae, magnified 180 diam.)

The last pair of feet on the middle-body of the larva, consequently the
penultimate pair in the adult animal, is almost always similar in
structure to the preceding pair. A remarkable exception is, however,
presented in this respect by Cryptoniscus and Entoniscus,--remarkable as
a confirmation of Darwin's proposition that "parts developed in an
unusual manner are very variable," for in the peculiarly-formed pair of
feet there exists the greatest possible difference between the three
species hitherto observed. In Cryptoniscus (Figure 39) this last foot is
thin and rod-like; in Entoniscus Cancrorum remarkably long and furnished
with a strongly thickened hand and a peculiarly constructed chela; in
Entoniscus Porcellanae very short, imperfectly jointed, and with a large
ovate terminal joint (Figure 40).

Some Isopods undergo a considerable change immediately before the
attainment of sexual maturity. This is the case with the males of Tanais
which have already been noticed, and, according to Hesse, with the
Pranizae, in which both sexes are said to pass into the form known as
Anceus. But Spence Bate, a careful observer, states that he has seen
females of the form of Praniza laden with eggs far advanced in their
development.

(FIGURE 41. Entoniscus Cancrorum, female, magnified 3 times.

FIGURE 42. Cryptoniscus planarioides, female, magnified 3 times.

FIGURE 43. Embryo of a Corophium, magnified 90 diam.)

In this order we meet for the first time with an extensive retrograde
metamorphosis as a consequence of a parasitic mode of life. Even in some
Fish-lice (Cymothoa) the young are lively swimmers, and the adults
stiff, stupid, heavy fellows, whose short clinging feet are capable of
but little movement. In the Bopyridae (Bopyrus, Phryxus, Kepone, etc.,
which might have been conveniently left in a single genus), which are
parasitic on Crabs, Lobsters, etc., taking up their abode chiefly in the
branchial cavity, the adult females are usually quite destitute of eyes;
the antennae are rudimentary; the broad body is frequently
unsymmetrically developed in consequence of the confined space; its
segments are more or less amalgamated with each other; the feet are
stunted, and the appendages of the abdomen transformed from natatory
feet with long setae into foliaceous or tongue-shaped and sometimes
ramified branchiae. In the dwarfish males the eyes, antennae, and feet,
are usually better preserved than in the females; but on the other hand
all the appendages of the abdomen have not unfrequently disappeared, and
sometimes every trace of segmentation. In the females of Entoniscus,
which are found in the body-cavity of Crabs and Porcellanae, the eyes,
antennae, and buccal organs, the segmentation of the vermiform body, and
in one species (Figure 41) the whole of the limbs, disappear almost
without leaving a trace; and Cryptoniscus planarioides would almost be
regarded as a Flatworm rather than an Isopod, if its eggs and young did
not betray its Crustacean nature. Among the males of these various
Bopyridae, that of Entoniscus Porcellanae occupies the lowest place; it
is confined all its life to six pairs of feet, which are reduced to
shapeless rounded lumps.

The Amphipoda are distinguishable from the Isopoda at an early period in
the egg by the different position of the embryo, the hinder extremity of
which is bent downwards. In all the animals of this order which have
been examined for it,* (* In the genera Orchestoidea, Orchestia,
Allorchestes, Montagua, Batea n.g., Amphilochus, Atylus, Microdeutopus,
Leucothoe, Melita, Gammarus (according to Meissner and La Valette),
Amphithoe, Cerapus, Cyrtophium, Corophium, Dulichia, Protella and
Caprella.) a peculiar structure makes its appearance very early on the
anterior part of the back, by which the embryo is attached to the "inner
egg-membrane," and which has been called the "micropylar apparatus," but
improperly as it seems to me.* (* Little as a name may actually affect
the facts, we ought certainly to confine the name "micropyle" to canals
of the egg-membrane, which serve for the entrance of the semen. But the
outer egg-membrane passes over the "micropylar apparatus" of the
Amphipoda without any perforation, according to Meissner's and La
Valette's own statements; it appears never to be present before
fecundation, attains its greatest development at a subsequent period of
the ovular life, and the delicate canals which penetrate it do not even
seem to be always present, indeed it seems to belong to the embryo
rather than to the egg-membrane. I have never been able to convince
myself that the so-called "inner egg-membrane" is really of this nature,
and not perhaps the earliest larva skin, not formed until after
impregnation, as might be supposed with reference to Ligia, Cassidina
and Philoscia.) It will remind us of the union of the young Isopoda with
the larval membrane and of the unpaired "adherent organ" on the nape of
the Cladocera, which is remarkably developed in Evadne and persists
throughout life; but in Daphnia pulex, according to Leydig, although
present in the young animals, disappears without leaving a trace in the
adults.

The young animal, whilst still in the egg, acquires the full number of
its segments and limbs. In cases where segments are amalgamated
together, such as the last two segments of the thorax in Dulichia, the
last abdominal segments and the tail in Gammarus ambulans and Corophium
dentatum, n. sp., and the last abdominal segments and the tail in
Brachyscelus,* or where one or more segments are deficient, as in
Dulichia and the Caprellae, we find the same fusion and the same
deficiencies in young animals taken out of the brood-pouch of their
mother. (* According to Spence Bate, in Brachyscelus crusculum the fifth
abdominal segment is not amalgamated with the sixth (the tail) but with
the fourth, which I should be inclined to doubt, considering the close
agreement which this species otherwise shows with the two species that I
have investigated.) Even peculiarities in the structure of the limbs, so
far as they are common to both sexes, are usually well-marked in the
newly hatched young, so that the latter generally differ from their
parents only by their stouter form, the smaller number of the antennal
joints and olfactory filaments, and also of the setae and teeth with
which the body or feet are armed, and perhaps by the comparatively
larger size of the secondary flagellum. An exception to this rule is
presented by the Hyperinae which usually live upon Acalephae. In these
the young and adults often have a remarkably different appearance; but
even in these there is no new formation of body-segments and limbs, but
only a gradual transformation of these parts.*

(* In the young of Hyperia galba Spence Bate did not find any of the
abdominal feet, or the last two pairs of thoracic feet, but this very
remarkable statement required confirmation the more because he examined
these minute animals only in the dried state. Subsequently I had the
wished-for opportunity of tracing the development of a Hyperia which is
not uncommon upon Ctenophora, especially Beroe gilva, Eschsch. The
youngest larva from the brood-pouch of the mother already possess THE
WHOLE of the thoracic feet; on the other hand, like Spence Bate, I
cannot find those of the abdomen. At first simple enough, all these feet
soon become converted, like the anterior feet, into richly denticulated
prehensile feet, and indeed of three different forms, the anterior feet
(Figure 44) the two following pairs (Figure 45) and finally the three
last pairs (Figure 46) being similarly constructed and different from
the rest. In this form the feet remain for a very long time, whilst the
abdominal appendages grow into powerful natatory organs, and the eyes,
which at first seemed to me to be wanting, into large hemispheres. In
the transition to the form of the adult animal the last three pairs of
feet (Figure 49) especially undergo a considerable change. The
difference between the two sexes is considerable; the females are
distinguished by a very broad thorax, and the males (Lestrigonus) by
very long antennae, of which the anterior bear an unusual abundance of
olfactory filaments.

Their youngest larvae of course cannot swim; they are helpless little
animals which firmly cling especially to the swimming laminae of their
host; the adult Hyperiae, which are not unfrequently met with free in
the sea, are, as is well known, the most admirable swimmers in their
order. ("Il nage avec une rapidite extreme," says Van Beneden of H.
Latreillii M.-Edw.)

The transformation of the Hyperiae is evidently to be regarded as
ACQUIRED and not INHERITED, that is to say the late appearance of the
abdominal appendages and the peculiar structure of the feet in the young
are not to be brought into unison with the historical development of the
Amphipoda, but to be placed to the account of the parasitic mode of life
of the young.

As in Brachyscelus, free locomotion has been continued to the adult and
not to the young, contrary to the usual method among parasites. Still
more remarkable is a similar circumstance in Caligus, among the
parasitic Copepoda. The young animal, described by Burmeister as a
peculiar genus, Chalimus, lies at anchor upon a fish by means of a cable
springing from its forehead, and having its extremity firmly seated in
the skin of the fish. When sexual maturity is attained, the cable is
cut, and the adult Caligi, which are admirable swimmers, are not
unfrequently captured swimming freely in the sea. (See 'Archiv. fur
Naturgeschichte' 1852 1 page 91).)

(FIGURES 44 TO 46. Feet of a half-grown Hyperia Martinezii, n. sp.
(Named after my valued friend the amiable Spanish zoologist, M.
Francisco de Paula Martinez y Saes, at present on a voyage round the
world.)

FIGURES 47 TO 49. Feet of a nearly adult male of the same species; 44
and 47 from the first pair of anterior feet (gnathopoda); 44 and 48 from
the first, and 46 and 49 from the last pair of thoracic feet. Magnified
90 diam.)

Thus, in order to give a few examples, the powerful chelae of the
antepenultimate pair of feet, of Phromina sedentaria, are produced,
according to Pagenstecher, from simple feet of ordinary structure; and
vice versa, the chelae on the penultimate pair of feet of the young
Brachyscelus, become converted into simple feet. In the young of the
last-mentioned genus the long head is drawn out into a conical point and
bears remarkably small eyes; in course of growth, the latter, as in most
of the Hyperinae, attain an enormous size, and almost entirely occupy
the head, which then appears spherical, etc.

The difference of the sexes which, in the Gammarinae is usually
expressed chiefly in the structure of the anterior feet (gnathopoda, Sp.
Bate) and in the Hyperinae in the structure of the antennae, is often so
great that males and females have been described as distinct species,
and even repeatedly placed in different genera (Orchestia and Talitrus,
Cerapus and Dercothoe, Lestrigonus and Hyperia) or even families
(Hyperines anormales and Hyperines ordinaires). Nevertheless it is only
developed when the animals are nearly full-grown. Up to this period the
young resemble the females in a general way, even in some cases in which
these differ more widely than the males from the "Type" of the order.
Thus in the male Shore-hoppers (Orchestia) the second pair of the
anterior feet is provided with a powerful hand, as in the majority of
the Amphipoda, but very differently constructed in the females. The
young, nevertheless, resemble the female. Thus also,--and this is an
extremely rare case,* (* "I know of no case in which the inferior
(antennae) are obsolete, when the superior are developed," Dana.
(Darwin, 'Monograph on the Subclass Cirripedia, Lepadidae' page
15.)--the females of Brachyscelus are destitute of the posterior (or
inferior) antennae; the male possesses them like other Amphipodae; in
the young I, like Spence Bate, can find no trace of them.

It is, however, to be particularly remarked, that the development of the
sexual peculiarities does not stand still on the attainment of sexual
maturity.

(FIGURE 50. Foot of the second pair ("second pair of gnathopoda") of the
male of Orchestia Tucurauna, magnified 15 diam.

FIGURE 51. Foot of the second pair ("second pair of gnathopoda") of the
female of Orchestia Tucurauna, magnified 15 diam.)

For example, the younger sexually mature males of Orchestia Tucurauna,
n. sp., have slender inferior antennae, with the joints of the flagellum
not fused together, the clasping margin ("palm," Sp. Bate) of the hand
in the second pair of feet is uniformly convex, the last pair of feet is
slender and similar to the preceding. Subsequently the antennae become
thickened, two, three, or four of the first joints of the flagellum are
fused together, the palm of the hand acquires a deep emargination near
its inferior angle, and the intermediate joints of the last pair of feet
become swelled into a considerable incrassation. No museum-zoologist
would hesitate about fabricating two distinct species, if the oldest and
youngest sexually mature males were sent to him without the uniting
intermediate forms. In the younger males of Orchestia Tucuratinga,
although the microscopic examination of their testes showed that they
were already sexually mature, the emargination of the clasping margin of
the hand (represented in Figure 50) and the corresponding process of the
finger, are still entirely wanting. The same may be observed in Cerapus
and Caprella, and probably in all cases where hereditary sexual
differences occur.

(FIGURE 52. Male of a Bodotria, magnified 10 diam. Note the long
inferior antennae, which are closely applied to the body, and of which
the apex is visible beneath the caudal appendages.)

Next to the extensive sections of the Stalk-eyed and Sessile-eyed
Crustacea, but more nearly allied to the former than to the latter,
comes the remarkable family of the Diastylidae or Cumacea. The young,
which Kroyer took out of the brood-pouch of the female, and which
attained one-fourth of the length of their mother, resembled the adult
animals almost in all parts. Whether, as in Mysis and Ligia, a
transformation occurs within the brood-pouch, which is constructed in
the same way as in Mysis, is not known.* (* A trustworthy English
Naturalist, Goodsir, described the brood-pouch and eggs of Cuma as early
as 1843. Kroyer, whose painstaking care and conscientiousness is
recognised with wonder by every one who has met him on a common field of
work, confirmed Goodsir's statements in 1846, and, as above mentioned,
took out of the brood-pouch embryos advanced in development and
resembling their parents. By this the question whether the Diastylidae
are full-grown animals or larvae, is completely and for ever set at
rest, and only the famous names of Agassiz, Dana and Milne-Edwards, who
would recently reduce them again to larvae (see Van Beneden, 'Rech. sur
la Fauna littor. de Belgique' Crustacees pages 73 and 74), induce me, on
the basis of numerous investigations of my own, to declare in Van
Beneden's words; "Parmi toutes les formes embryonnaires de podophthalmes
ou d'edriophthalmes que nous avons observees sur nos cotes, nous n'en
avons pas vu une seule qui eut meme la moindre resemblance avec un Cuma
quelconque." The ONLY THING that suits the larvae of Hippolyte, Palaemon
and Alpheus, in the family character of the Cumacea as given by Kroyer
which occupies three pages (Kroyer, 'Naturh. Tidsskrift, Ny Raekke,' Bd.
2 pages 203 to 206) is: "Duo antennarum paria." And this, as is well
known, applies to nearly all Crustacea. How well warranted are we
therefore in identifying the latter with the former. However, it is
sufficient for any one to glance at the larva of Palaemon (Figure 27)
and the Cumacean (Figure 52) in order to be convinced of their
extraordinary similarity!) The caudal portion of the embryo in the
Diastylidae, as I have recently observed, is curved upwards as in the
Isopoda, and the last pair of feet of the thorax is wanting.

Equally scanty is our knowledge of the developmental history of the
Ostracoda. We know scarcely anything except that their anterior limbs
are developed before the posterior one (Zenker). The development of
Cypris has recently been observed by Claus:--"The youngest stages are
shell-bearing Nauplius-forms."


CHAPTER 9. DEVELOPMENTAL HISTORY OF ENTOMOSTRACA, CIRRIPEDES, AND
RHIZOCEPHALA.

The section of the Branchiopoda includes two groups differing even in
their development,--the Phyllopoda and the Cladocera. The latter minute
animals, provided with six pairs of foliaceous feet, which chiefly
belong to the fresh waters, and are diffused under similar forms over
the whole world, quit the egg with their full number of limbs. The
Phyllopoda, on the contrary, in which the number of feet varies between
10 and 60 pairs, and some of which certainly live in the saturated lie
of salterns and natron-lakes, but of which only one rather divergent
genus (Nebalia) is found in the sea,* have to undergo a metamorphosis.
(* If the Phyllopoda may be regarded as the nearest allies of the
Trilobites, they would furnish, with Lepidosteus and Polypterus,
Lepidosiren and Protopterus, a further example of the preservation in
fresh waters of forms long since extinguished in the sea. The occurrence
of the Artemiae in supersaline water would at the same time show that
they do not escape destruction by means of the fresh water, but in
consequence of the less amount of competition in it.) Mecznikow has
recently observed the development of Nebalia, and concludes from his
observations "that Nebalia, during its embryonal life, passes through
the Nauplius- and Zoea-stages, which in the Decapoda occur partly (in
Peneus) in the free state." "Therefore," says he, "I regard Nebalia as a
Phyllopodiform Decapod." The youngest larvae [of the Phyllopoda] are
Nauplii, which we have already met with exceptionally in some Prawns,
and which we shall now find reproduced almost without exception. The
body-segments and feet, which are sometimes so numerous, are formed
gradually from before backwards, without the indication of any
sharply-discriminated regions of the body either by the time of their
appearance or by their form. All the feet are essentially constructed in
the same manner and resemble the maxillae of the higher Crustacea.* (*
"The maxilla of the Decapod-larva (Krebslarve) is a sort of Phyllopodal
foot" (Claus).) We might regard the Phyllopoda as Zoeae which have not
arrived at the formation of a peculiarly endowed abdomen or thorax, and
instead of these have repeatedly reproduced the appendages which first
follow the Nauplius-limbs.

Of the Copepoda--some of which, living in a free state, people the fresh
waters, and in far more multifarious forms the sea, whilst others, as
parasites, infest animals of the most various classes and often become
wonderfully deformed--the developmental history, like their entire
natural history, was, until lately, in a very unsatisfactory state. It
is true, that we long ago knew that the Cyclopes of our fresh waters
were excluded in the Nauplius-form, and that we were acquainted with
some others of their young states; we had learnt, through Nordmann, that
the same earliest form belonged to several parasitic Crustacea, which
had previously passed, almost universally, as worms; but the connecting
intermediate forms which would have permitted us to refer the regions of
the body and the limbs of the larvae to those of the adult animal, were
wanting. The comprehensive and careful investigations of Claus have
filled up this deficiency in our knowledge, and rendered the section of
the Copepoda one of the best known in the whole class. The following
statements are derived from the works of this able naturalist. From the
abundance of valuable materials which they contain I select only those
which are indispensable for the comprehension of the development of the
Crustacea in general, because, in what relates to the Copepoda in
particular, the facts have already been placed in the proper light by
the representation of their most recent investigator, and must appear to
any one whose eyes are open, as important evidence in favour of the
Darwinian theory.* (* I am still unacquainted with Claus' latest and
larger work, but no doubt the same may be said of it.)

(FIGURES 53 AND 54. Nauplii of Copepoda, the former magnified 90, the
latter 180 diam.)

All the larvae of the free Copepoda investigated by Claus, have, at the
earliest period, three pairs of limbs (the future antennae and
mandibles), the anterior with a single, and the two following ones with
a double series of joints, or branchiae. The unpaired eye, labrum, and
mouth, already occupy their permanent positions. The posterior portion,
which is usually short and destitute of limbs, bears two terminal setae,
between which the anus is situated. The form in this Nauplius-brood is
extremely various,--it is sometimes compressed laterally, sometimes
flat,--sometimes elongated, sometimes oval, sometimes round or even
broader than long, and so forth. The changes which the first larval
stages undergo during the progress of growth, consist essentially in an
extension of the body and the sprouting forth of new limbs. "The
following stage already displays a fourth pair of extremities, the
future maxillae." Then follow at once three new pairs of limbs (the
maxillipedes and the two anterior pairs of natatory feet). The larva
still continues like a Nauplius, as the three anterior pairs of limbs
represent rowing feet; at the next moult it is converted into the
youngest Cyclops-like state, when it resembles the adult animal in the
structure of the antennae and buccal organs, although the number of
limbs and body segments is still much less, for only the rudiments of
the third and fourth pairs of natatory feet have made their appearance
in the form of cushions fringed with setae, and the body consists of the
oval cephalothorax, the second, third, and fourth thoracic segments, and
an elongated terminal joint. In the Cyclopidae the posterior antennae
have lost their secondary branch, and the mandibles have completely
thrown off the previously existing natatory feet, whilst in the other
families these appendages persist, more or less altered. "Beyond this
stage of free development, many forms of the parasitic Copepoda, such as
Lernanthropus and Chondracanthus, do not pass, as they do not acquire
the third and fourth pairs of limbs, nor does a separation of the fifth
thoracic segment from the abdomen take place; others (Achtheres) even
fall to a lower grade by the subsequent loss of the two pairs of
natatory feet. But all free Copepoda, and most of the parasitic
Crustacea, pass through a longer or shorter series of stages of
development, in which the limbs acquire a higher degree of division into
joints in continuous sequence, the posterior pairs of feet are
developed, and the last thoracic segment and the different abdominal
segments are successively separated from the common terminal portion."

(FIGURE 55. Nauplius of Tetraclita porosa after the first moult,
magnified 90 diam. The brain is seen surrounding the eye, and from it
the olfactory filaments issue; behind it are some delicate muscles
passing to the buccal hood.)

There is only one thing more to be indicated in the developmental
history of the parasitic Crustacea, namely that some of them, such as
Achtheres percarum, certainly quit the egg like the rest in a
Nauplius-like form, inasmuch as the plump, oval, astomatous body bears
two pairs of simple rowing feet, and behind these, as traces of the
third pair, two inflations furnished each with a long seta, but that
beneath this Nauplius-skin a very different larva lies ready prepared,
which in a few hours bursts its clumsy envelope and then makes its
appearance in a form "which agrees in the segmentation of the body and
in the development of the extremities with the first Cyclops-stage"
(Claus). The entire series of Nauplius-stages which are passed through
by the free Copepoda, are in this case completely over-leapt.

A final and very peculiar section of the Crustacea is formed by the two
orders of the Cirripedia and Rhizocephala.* (* The most various opinions
prevail as to the position of the Cirripedia. Some ascribe to them a
very subordinate position among the Copepoda; as Milne-Edwards (1852).
In direct opposition to this notion of his father's, Alph. Milne-Edwards
places them (as Basinotes) opposite to all the other Crustacea
(Eleutheronotes). Darwin regards them as forming a peculiar sub-class
equivalent to the Podophthalma, Edriophthalma, etc. This appears to me
to be most convenient. I would not combine the Rhizocephala with the
Cirripedia, as Liljeborg has done, but place them in opposition as
equivalent, like the Amphipoda and Isopoda. The near relationship of the
Cirripedia to the Ostracoda is also spoken of, but the similarity of the
so-called "Cypris-like larvae," or Cirriped-pupae as Darwin calls them,
to Cypris is so purely external, even as regards the shell, that the
relationship appears to me to be scarcely greater than that of
Peltogaster socialis (Figure 59) with the family of the sausages.)

In these also the brood bursts out in the Nauplius-form, and speedily
strips off its earliest larva-skin which is distinguished by no
peculiarities worth noticing. Here also we find again the same pyriform
shape of the unsegmented body, the same number and structure of the
feet, the same position of the median eye (which, however, is wanting in
Sacculina purpurea, and according to Darwin in some species of Lepas),
and the same position of the "buccal hood," as in the Nauplii of the
Prawns and Copepoda. From the latter the Nauplii of the Cirripedia and
Rhizocephala are distinguished by the possession of a dorsal shield or
carapace, which sometimes (Sacculina purpurea) projects far beyond the
body all round; and they are distinguished not only from other Nauplii,
but as far as I know from all other Crustacea, by the circumstance that
structures which are elsewhere combined with the two anterior limbs
(antennae), here occur separated from them.

The anterior antennae of the Copepoda, Cladocera, Phyllopoda (Leydig,
Claus), Ostracoda (at least the Cypridinae), Diastylidae, Edriophthalma,
and Podophthalma, with few exceptions relating to terrestrial animals or
parasites, bear peculiar filaments which I have already repeatedly
mentioned as "olfactory filaments." A pair of similar filaments spring,
in the larvae of the Cirripedia and Rhizocephala, directly from the
brain.

(FIGURE 56. Nauplius of Sacculina purpurea, shortly before the second
moult, magnified 180 diam. We may recognise in the first pair of feet
the future adherent feet, and in the abdomen six pairs of natatory feet
with long setae.)

At the base of the inferior antennae in the Decapoda the so-called
"green-gland" has its opening; in the Macrura at the end of a conical
process. A similar conical process with an efferent duct traversing it
is very striking in most of the Amphipoda. In the Ostracoda, Zenker
describes a gland situated in the base of the inferior antennae, and
opening at the extremity of an extraordinarily long "spine." In the
Nauplii of Cyclops and Cyclopsine, Claus finds pale "shell-glands,"
which commence in the intermediate pair of limbs (the posterior
antennae). On the other hand in the Nauplii of the Cirripedia and
Rhizocephala the "shell-glands" open at the ends of conical processes,
sometimes of most remarkable length, which spring from the angles of the
broad frontal margin, and have been interpreted sometimes as antennae
(Burmeister, Darwin) and sometimes as mere "horns of the carapace"
(Krohn). The connexion of the "shell-glands" with the frontal horns has
been recognised unmistakably in the larvae of Lepas, and indeed the
resemblance of the frontal horns with the conical processes on the
inferior antennae of the Amphipoda, is complete throughout.* (* In
connexion with this it may be mentioned that, in the females of
Brachyscelus, in which the posterior antennae are deficient, the conical
processes with the canal permeating them are nevertheless retained.)

(FIGURE 57. Pupa of a Balanide (Chthamalus ?), magnified 50 diam. The
adherent feet are retracted within the rather opaque anterior part of
the shell.

FIGURE 58. Pupa of Sacculina purpurea, magnified 180 diam. The filaments
on the adherent feet may be the commencements of the future roots.)

Notwithstanding their agreement in this important peculiarity, the
Nauplii of these two orders present material differences in many other
particulars. The abdomen of the young Cirripede is produced beneath the
anus into a long tail-like appendage which is furcate at the extremity,
and over the anus there is a second long, spine-like process; the
abdomen in the Rhizocephala terminates in two short points,--in a
"moveable caudal fork, as in the Rotatoria," (O. Schmidt). The young
Cirripedes have a mouth, stomach, intestine, and anus, and their two
posterior pairs of limbs are beset with multifarious teeth, setae, and
hooks, which certainly assist in the inception of nourishment. All this
is wanting in the young Rhizocephala. The Nauplii of the Cirripedia have
to undergo several moults whilst in that form; the Nauplii of the
Rhizocephala, being astomatous, cannot of course live long as Nauplii,
and in the course of only a few days they become transformed into
equally astomatous "pupae," as Darwin calls them.

The carapace folds itself together, so that the little animal acquires
the aspect of a bivalve shell, the foremost limbs become transformed
into very peculiar adherent feet ("prehensile antennae," Darwin), and
the two following pairs are cast off; like the frontal horns. On the
abdomen six pairs of powerful biramose natatory feet with long setae
have been formed beneath the Nauplius-skin, and behind these are two
short, setigerous caudal appendages (Figure 58).

The pupae of the Cirripedia (Figure 57), which are likewise astomatous,
agree completely in all these parts with those of the Rhizocephala, even
to the minutest details of the segmentation and bristling of the
natatory feet;* they are especially distinguished from them by the
possession of a pair of composite eyes. (* Compare the figure given by
Darwin (Balanidae Plate 30 Figure 5) of the first natatory foot of the
pupa of Lepas australis, with that of Lernaeodiscus Porcellanae
published in the 'Archiv fur Naturgeschichte' (1863 Taf 3 Figure 5). The
sole distinction, that in the latter there are only 3 setae at the end
of the outer branch, whilst in the Cirripedia there are 4 on the first
and 5 on the following natatory feet, may be due to an error on my
part.) Sometimes also traces of the frontal horns seem to persist.* (*
Darwin describes as "acoustic orifices" small apertures in the shell of
the pupae of the Cirripedia, which, frequently surrounded by a border,
are situated, in Lepas pectinata, upon short, horn-like processes. I
feel scarcely any hesitation in regarding the apertures as those of the
"shell-glands," and the horn-like processes as remains of the frontal
horns.)

As the Cirripedia and Rhizocephala now in general resemble each other
far more than in their Nauplius-state, this is also the case with the
individual members of each of the two orders.

The pupae in both orders attach themselves by means of the adherent
feet; those of the Cirripedes to rocks, shells, turtles, drift-wood,
ships, etc.,--those of the Rhizocephala to the abdomen of Crabs,
Porcellanae, and Hermit Crabs. The carapace of the Cirripedes becomes
converted, as is well-known, into a peculiar test, on account of which
they were formerly placed among the Mollusca, and the natatory feet grow
into long cirri, which whirl nourishment towards the mouth, which is now
open. The Rhizocephala remain astomatous; they lose all their limbs
completely, and appear as sausage-like, sack-shaped or discoidal
excrescences of their host, filled with ova (Figures 59 and 60); from
the point of attachment closed tubes, ramified like roots, sink into the
interior of the host, twisting round its intestine, or becoming diffused
among the sac-like tubes of its liver. The only manifestations of life
which persist in these non plus ultras in the series of retrogressively
metamorphosed Crustacea, are powerful contractions of the roots, and an
alternate expansion and contraction of the body, in consequence of which
water flows into the brood-cavity and is again expelled, through a wide
orifice.* (* The roots of Sacculina purpurea (Figure 60) which is
parasitic upon a small Hermit Crab, are made use of by two parasitic
Isopods, namely a Bopyrus and the before mentioned Cryptoniscus
planarioides (Figure 42). These take up their abode beneath the
Sacculina and cause it to die away by intercepting the nourishment
conveyed by the roots; the roots, however, continue to grow, even
without the Sacculina, and frequently attain an extraordinary extension,
especially when a Bopyrus obtains its nourishment from them.)

(FIGURE 59. Young of Peltogaster socialis on the abdomen of a small
Hermit Crab; in one of them the fasciculately ramified roots in the
liver of the Crab are shown. Animal and roots deep yellow.

FIGURE 60. Young Sacculina purpurea with its roots; the animal
purple-red, the roots dark grass-green. Magnified 5 diam.)

Out of several Cirripedes, which are anomalous both in structure and
development, Cryptophialus minutus must be mentioned here; Darwin found
it in great quantities together in the shell of Concholepas peruviana on
the Chonos Islands. The egg, which is at first elliptical, soon,
according to Darwin, becomes broader at the anterior extremity, and
acquires three club-shaped horns, one at each anterior angle and one
behind; no internal parts can as yet be detected. Subsequently the
posterior horn disappears, and the adherent feet may be recognised
within the anterior ones. From this "egg-like larva"--(Darwin says of
it, "I hardly know what to call it")--the pupa is directly produced. Its
carapace is but slightly compressed laterally and hairy, as in Sacculina
purpurea; the adherent feet are of considerable size, and the natatory
feet are wanting, as, in the adult animal, are the corresponding cirri.
As I learn from Mr. Spence Bate, the Nauplius-stage appears to be
overleaped and the larvae to leave the egg in the pupa-form, in the case
of a Rhizocephalon (Peltogaster ?) found by Dr. Powell in the Mauritius.

(FIGURES 61 TO 63. Eggs of Tetraclita porosa in segmentation, magnified
90 diam. The larger of the two first-formed spheres of segmentation is
always turned towards the pointed end of the egg.

FIGURE 64. Egg of Lernaeodiscus Porcellanae, in segmentation, magnified
90 diam.)

I will conclude this general view with a few words upon the earliest
processes in the development of the Crustacea. Until recently it was
regarded as a general rule that, by the partial segmentation of the
vitellus a germinal disc was formed, and in this, corresponding to the
ventral surface of the embryo, a primitive band. We now know that in the
Copepoda (Claus), in the Rhizocephala (Figure 64), and, as I can add, in
the Cirripedia (Figures 61 to 63) the segmentation is complete, and the
embryos are sketched out in their complete form without any preceding
primitive band. Probably the latter will always be the case where the
young are hatched as true Nauplii (and not merely with a Nauplius-skin,
as in Achtheres). The two modes of development may occur in very closely
allied animals, as is proved by Achtheres among the Copepoda.* (* I have
not mentioned the Pycnogonidae, because I do not regard them as
Crustacea; nor the Xiphosura and Trilobites, because, having never
investigated them myself, I knew too little about them, and especially
because I am unacquainted with the details of the explanations given by
Barrande of the development of the latter. According to Mr. Spence Bate
"the young of Trilobites are of the Nauplius-form.")


CHAPTER 10. ON THE PRINCIPLES OF CLASSIFICATION.

Perhaps some one else, more fortunate than myself, may be able, even
without Darwin, to find the guiding clue through the confusion of
developmental forms, now so totally different in the nearest allies, now
so surprisingly similar in members of the most distant groups, which we
have just cursorily reviewed. Perhaps a sharper eye may be able, with
Agassiz, to make out "the plan established from the beginning by the
Creator,"* (* "A plan fully matured in the beginning and undeviatingly
pursued;" or "In the beginning His plan was formed and from it He has
never swerved in any particular" (Agassiz and Gould, 'Principles of
Zoology').) who may have written here, as a Portuguese proverb says
"straight in crooked lines."* (* "Deos escrive direito em linhas
tortas." To read this remarkable writing we need the spectacles of
Faith, which seldom suit eyes accustomed to the Microscope.) I cannot
but think that we can scarcely speak of a general plan, or typical mode
of development of the Crustacea, differentiated according to the
separate Sections, Orders, and Families, when, for example, among the
Macrura, the River Crayfish leaves the egg in its permanent form; the
Lobster with Schizopodal feet; Palaemon, like the Crabs, as a Zoea; and
Peneus, like the Cirripedes, as a Nauplius,--and when, still, within
this same sub-order Macrura, Palinurus, Mysis and Euphausia again
present different young forms,--when new limbs sometimes sprout forth as
free rudiments on the ventral surface, and are sometimes formed beneath
the skin which passes smoothly over them, and both modes of development
are found in different limbs of the same animal and in the same pair of
limbs in different animals,--when in the Podophthalma the limbs of the
thorax and abdomen make their appearance sometimes simultaneously, or
sometimes the former and sometimes the latter first, and when further in
each of the two groups the pairs sometimes all appear together, and
sometimes one after the other,--when, among the Hyperina, a simple foot
becomes a chela in Phronima and a chela a simple foot in Brachyscelus,
etc.

And yet, according to the teaching of the school, it is precisely in
youth, precisely in the course of development, that the "Type" is mostly
openly displayed. But let us hear what the Old School has to tell us as
to the significance of developmental history, and its relation to
comparative anatomy and systematic zoology.

Let two of its most approved masters speak.

"Whilst comparative anatomy," said Johannes Muller, in 1844, in his
lectures upon this science (and the opinions of my memorable teacher
were for many years my own), "whilst comparative anatomy shows us the
infinitely multifarious formation of the same organ in the Animal
Kingdom, it furnishes us at the same time with the means, by the
comparison of these various forms, of recognising the truly essential,
the type of these organs, and separating therefrom everything
unessential. In this, developmental history serves it as a check or
test. Thus, as the idea of development is not that of mere increase of
size, but that of progress from what is not yet distinguished, but which
potentially contains the distinction in itself, to the actually
distinct,--it is clear, that the less an organ is developed, so much the
more does it approach the type, and that, during its development, it
more and more acquires peculiarities. The types discovered by
comparative anatomy and developmental history must therefore agree."

Then, after Johannes Muller has combated the idea of a graduated scale
of animals, and of the passage through several animal grades during
development, he continues:--"What is true in this idea is, that every
embryo at first bears only the type of its section, from which the type
of the Class, Order, etc., is only afterwards developed."

In 1856, in an elementary work,* (* 'Principles of Zoology' Part 1
Comparative Physiology. By Louis Agassiz and A.A. Gould Revised Edition
Boston 1856.) in which it is usual to admit only what are regarded as
the assured acquisitions of science, Agassiz expresses himself as
follows:--

"The ovarian eggs of all animals are perfectly identical, small cells
with a vitellus, germinal vesicle and germinal spot" (paragraph 278).
"The organs of the body are formed in the sequence of their organic
importance; the most essential always appear first. Thus the organs of
vegetative life, the intestine, etc., appear later than those of animal
life, the nervous system, skeleton, etc., and these in turn are preceded
by the more general phenomena belonging to the animal as such"
(paragraph 318). "Thus, in Fishes, the first changes consist in the
segmentation of the vitellus and the formation of a germ, processes
which are common to all classes of animals. Then the dorsal furrow,
characteristic of the Vertebrate, appears--the brain, the organs of the
senses; at a later period are formed the intestine, the limbs, and the
permanent form of the respiratory organs, from which the class is
recognised with certainty. It is only after exclusion that the
peculiarities of the structure of the teeth and fins indicate the genus
and species" (paragraph 319). "Hence the embryos of different animals
resemble each other the more, the younger they are" (paragraph 320).
"Consequently the high importance of developmental history is
indubitable. For, if the formation of the organs takes place in the
order corresponding to their importance, this sequence must of itself be
a criterion of their comparative value in classification. The
peculiarities which appear earlier should be considered of higher value
than those which appear subsequently" (paragraph 321). "A system, in
order to be true and natural, must agree with the sequence of the organs
in the development of the embryo" (paragraph 322).

I do not know whether any one at the present day will be inclined to
subscribe to this proposition in its whole extent.* (* Agassiz' own
views have lately become essentially different, so far as can be made
out from Rud. Wagner's notice of his 'Essay on Classification.' Agassiz
himself does not attempt any criticism of the above cited older views,
which, however, are still widely diffused. With his recent conception I
am unfortunately acquainted only from R. Wagner's somewhat confused
report, and have therefore thought it better not to attempt any critical
remarks upon it.) It is certain, however, that views essentially similar
are still to be met with everywhere in discussions on classification,
and that even within the last few years, the very sparingly successful
attempts to employ developmental history as the foundation of
classification have been repeated.

But how do these propositions agree with our observations on the
developmental history of the Crustacea? That these observations relate
for the most part to their "free metamorphosis" after their quitting the
egg, cannot prejudice their application to the propositions enunciated
especially with regard to "embryonal development" in the egg; for
Agassiz himself points out (paragraph 391) that both kinds of change are
of the same nature and of equal importance and that no "radical
distinction" is produced by the circumstance that the former take place
before and the latter after birth.

"The ovarian eggs of all animals are identical, small cells with
vitellus, germinal vesicle and germinal spot." Yes, somewhat as all
Insects are identical, small animals with head, thorax, and abdomen;
that is to say if, only noticing what is common to them, we leave out of
consideration the difference of their development, the presence or
absence and the multifarious structure of the vitelline membrane, the
varying composition of the vitellus, the different number and formation
of the germinal spots, etc. Numerous examples, which might easily be
augmented, of such profound differences, are furnished by Leydig's
'Lehrbuch der Histologie.' In the Crustacea the ovarian eggs actually
sometimes furnish excellent characters for the discrimination of species
of the same genus; thus, for example, in one Porcellana of this country
they are blackish-green, in a second deep blood-red, and in a third dark
yellow; and within the limits of the same order they present
considerable differences in size, which, as Van Beneden and Claus have
already pointed out, stands in intimate connexion with the subsequent
mode of development.

"The organs of the body are formed in the sequence of their organic
importance; the most essential always appear first." This proposition
might be characterised a priori as undemonstrable, since it is
impossible either in general, or for any particular animal, to establish
a sequence of importance amongst equally indispensable parts. Which is
the more important, the lung or the heart--the liver or the kidney?--the
artery or the vein? Instead of giving the preference, with Agassiz, to
the organs of animal life, we might with equal justice give it to those
of vegetative life, as the latter are conceivable without the former,
but not the former without the latter. We might urge that, according to
this proposition, provisional organs as the first produced must exceed
the later-formed permanent organs in importance.

But let us stick to the Crustacea. In Polyphemus Leydig finds the first
traces of the intestinal tube even during segmentation. In Mysis a
provisional tail is first formed, and in Ligia a maggot-like larva-skin.
The simple median eye appears earlier, and would therefore be more
important than the compound paired eyes; the scale of the antennae in
the Prawns would be more important than the flagellum; the maxillipedes
of the Decapoda would be more important than the chelae and ambulatory
feet, and the anterior six pairs of feet in the Isopoda, than the
precisely similarly formed seventh pair; in the Amphipoda the most
important of all organs would be the "micropylar apparatus," which
disappears without leaving a trace soon after hatching; in Cyclops the
setae of the tail would be more important than all the natatory feet; in
the Cirripedia the posterior antennae, as to which we do not know what
becomes of them, would be more important than the cirri, and so forth.
The most unimportant of all organs would be the sexual organs, and the
most essential peculiarity would consist in colour, which is to be
referred back to the ovarian egg.

"The embryos, or young states of different animals, resemble each other
the more, the younger they are," or, as Johannes Muller expresses it,
"they approach the more closely to the common type." Different as may be
the ideas connected with the word "type," no one will dispute that the
typical form of the penultimate pair of feet in the Amphipoda is that of
a simple ambulatory foot, and not that of a chela, for the latter occurs
in no single adult Amphipod; we know it only in the young of the genus
Brachyscelus, which therefore in this respect undoubtedly depart more
widely than the adults from the type of their order. This applies also
to the young males of the Shore-hoppers (Orchestia) with regard to the
second pair of anterior feet (gnathopoda). In like manner no one will
hesitate to accept the possession of seven pairs of feet as a "typical"
peculiarity of the Edriophthalma, which Agassiz, on this account, names
Tetradecapoda; the young Isopoda, which are Dodecapoda, are also in this
respect further from the "type" than the adults.

It is certainly a rule, and this Darwin's theory would lead us to
expect, that in the progress of development those forms which are at
first similar gradually depart further from each other; but here, as in
other classes, the exceptions, for which the Old School has no
explanation, are numerous. Not unfrequently we might indeed directly
reverse the proposition and assert that the difference becomes the
greater, the further we go back in the development, and this not only in
those cases in which one of two nearly allied species is directly
developed, and the other passes through several larval stages, such as
the common Crayfish and the Prawns which are produced from
Nauplius-brood. The same may be said, for example, of the Isopoda and
Amphipoda. In the adult animals the number of limbs is the same; at the
first sight of a Cyrtophium or a Dulichia, and even after the careful
examination of a Tanais, we may be in doubt whether we have an Isopod or
an Amphipod before us; in the newly-hatched young the number of limbs is
different, and if we go back to their existence in the egg, the most
passing glance to see whether the curvature is upwards or downwards
suffices to distinguish even the youngest embryos of the two orders.

In other instances, the courses which lead from a similar starting-point
to a similar goal, separate widely in the middle of the development, as
in the Prawns with Nauplius-brood already described.

Finally, so that even the last possibility may be exhausted, it
sometimes happens that the greatest similarity occurs in the middle of
the development. The most striking example of this is furnished by the
Cirripedia and Rhizocephala, whether we compare the two orders or the
members of each with one another; from a segmentation quite different in
its course (see Figures 61 to 64) proceed different forms of Nauplius,
these become converted into exceedingly similar pupae, and from the
pupae again proceed sexually mature animals, differing from each other
toto coelo.

"If the formation of the organs occurs in the order corresponding to
their importance, this sequence must of itself be a criterion of their
comparative value in classification." THAT IS TO SAY, SUPPOSING THE
PHYSIOLOGICAL AND CLASSIFICATIONAL VALUE OF AN ORGAN TO COINCIDE! Just
as in Christian countries there is a catechismal morality, which every
one has upon his lips, but no one considers himself bound to follow, or
expects to see followed by anybody else, so also has Zoology its dogmas,
which are as universally acknowledged, as they are disregarded in
practice. Such a dogma as this is the supposition tacitly made by
Agassiz. Of a hundred who feel themselves compelled to give their
systematic confession of faith as the introduction to a Manual or
Monographic Memoir, ninety-nine will commence by saying that a natural
system cannot be founded upon a single character, but that it has to
take into account all characters, and the general structure of the
animal, but that we must not simply sum up these characters like
equivalent magnitudes, that we must not count but weigh them, and
determine the importance to be ascribed to each of them according to its
physiological significance. This is probably followed by a little jingle
of words in general terms on the comparative importance of animal and
vegetative organs, circulation, respiration, and the like. But when we
come to the work itself, to the discrimination and arrangement of the
species, genera, families, etc., in all probability not one of the
ninety-nine will pay the least attention to these fine rules, or
undertake the hopeless attempt to carry them out in detail. Agassiz, for
example, like Cuvier, and in opposition to the majority of the German
and English zoologists, regards the Radiata as one of the great primary
divisions of the Animal Kingdom, although no one knows anything about
the significance of the radiate arrangement in the life of these
animals, and notwithstanding that the radiate Echinodermata are produced
from bilateral larvae. The "true Fishes" are divided by him into
Ctenoids and Cycloids, according as the posterior margin of their scales
is denticulated or smooth, a circumstance the importance of which to the
animal must be infinitely small, in comparison to the peculiarities of
the dentition, formation of the fins, number of vertebrae, etc.

And, to return to our Class of the Crustacea, has any particular
attention been paid in their classification to the distinctions
prevailing in the "most essential organs"? For instance, to the nervous
system? In the Corycaeidae, Claus found all the ventral ganglia fused
together into a single broad mass, and in the Calanidae a long ventral
chain of ganglia,--the former, therefore, in this respect resembling the
Spider Crabs and the latter the Lobster; but no one would dream on this
account of supposing that there was a relationship between the
Corycaeidae and the Crabs, or the Calanidae and the Lobsters.--Or to the
organs of circulation? We have among the Copepoda, the Cyclopidae and
Corycaeidae without a heart, side by side with the Calanidae and
Pontellidae with a heart. And in the same way among the Ostracoda, the
Cypridinae, which I find possess a heart, place themselves side by side
with Cypris and Cythere which have no such organ.--Or to the respiratory
apparatus? Milne-Edwards did this when he separated Mysis and Leucifer
from the Decapoda, but he himself afterwards saw that this was an error.
In one Cypridina I find branchiae of considerable size, which are
entirely wanting in another species, but this does not appear to me to
be a reason for separating these species even generically.

On the other hand, what do we know of the physiological significance of
the number of segments, and all the other matters which we are
accustomed to regard as typical peculiarities of the different organs,
and to which we usually ascribe the highest systematic value?

"Those peculiarities which first appear, should be more highly estimated
than those which appear subsequently. A system, in order to be true and
natural, must agree with the sequence of the organs in the development
of the embryo." If the earlier manifested peculiarities are to be
estimated more highly than those which afterwards make their appearance,
then in those cases in which the structure of the adult animal requires
one position in the system, and that of the larva another, the latter
and not the former must decide the point. As the Lernaeae and
Cirripedes, on account of their Nauplius-brood, were separated from
their previous connexions and referred to the Crustacea, we shall, for
the same reason, have to separate Peneus from the Prawns and unite it
with the Copepoda and Cirripedia. But the most zealous embryomaniac
would probably shrink from this course.

A "true and natural system" of the Crustacea to be in accordance with
the sequence of the phenomena would have to take into account in the
first place the various modes of segmentation, then the position of the
embryo, next the number of limbs produced within the egg and so forth,
and might be represented somewhat as follows:--

CLASSIS CRUSTACEA.

Sub-class I. HOLOSCHISTA.--Segmentation complete. No primitive band.
Nauplius-brood.

Ord. 1. Ceratometopa.--Nauplius with frontal horns. (Cirripedia,
Rhizocephala.)

Ord. 2. LEIOMETOPA.--Nauplius without frontal horns. (Copepoda, without
Achtheus, etc., Phyllopoda, Peneus.)

Sub-class II. HEMISCHISTA.--Segmentation not complete.

   A. Nototropa.--Embryo bent upwards.

Ord. 3. Protura.--The tail is first formed. (Mysis.)

Ord. 3. Saccomorpha.--A maggot-like larva-skin is first formed.
(Isopoda.)

B. Gasterotropa.--Embryo bent ventrally.

Ord. 5. Zoeogona.--Full number of limbs not produced in the egg.
Zoea-brood. (The majority of the Podophthalmata.)

Ord. 6. Ametabola.--Full number of limbs produced in the egg. (Astacus,
Gecarcinus, Amphipoda less Hyperia ?)

This sample may suffice. The farther we go into details in this
direction, the more brilliantly, as may easily be imagined, does the
naturalness of such an arrangement as this force itself upon us.

All things considered, we may apply the judgment which Agassiz
pronounced upon Darwin's theory, with far greater justice to the
propositions just examined:--"No theory," says he, "however plausible it
may be, can be admitted in science, unless it is supported by facts."


CHAPTER 11. ON THE PROGRESS OF EVOLUTION.

From this scarcely unavoidable but unsatisfactory side-glance upon the
old school, which looks down with so great an air of superiority upon
Darwin's "intellectual dream" and the "giddy enthusiasm" of its friends,
I turn to the more congenial task of considering the developmental
history of the Crustacea from the point of view of the Darwinian theory.

Darwin himself, in the thirteenth chapter of his book, has already
discussed the conclusions derived from his hypotheses in the domain of
developmental history. For a more detailed application of them, however,
it is necessary in the first place to trace these general conclusions a
little further than he has there done.

The changes by which young animals depart from their parents, and the
gradual accumulation of which causes the production of new species,
genera, and families, may occur at an earlier or later period of
life,--in the young state, or at the period of sexual maturity. For the
latter is by no means always, as in the Insecta, a period of repose;
most other animals even then continue to grow and to undergo changes.
(See above, the remarks on the males of the Amphipoda.) Some variations,
indeed, from their very nature, can only occur when the young animal has
attained the adult stage of development. Thus the Sea Caterpillars
(Polynoe) at first possess only a few body-segments, which, during
development, gradually increase to a number which is different in
different species, but constant in the same species; now before a young
animal could exceed the number of segments of its parents, it must of
course have attained that number. We may assume a similar supplementary
progress wherever the deviation of the descendants consists in an
addition of new segments and limbs.

Descendants therefore reach a new goal, either by deviating sooner or
later whilst still on the way towards the form of their parents, or by
passing along this course without deviation, but then, instead of
standing still, advance still farther.

The former mode will have had a predominant action where the posterity
of common ancestors constitutes a group of forms standing upon the same
level in essential features, as the whole of the Amphipoda, Crabs, or
Birds. On the other hand we are led to the assumption of the second mode
of progress, when we seek to deduce from a common original form, animals
some of which agree with young states of others.

In the former case the developmental history of the descendants can only
agree with that of their ancestors up to a certain point at which their
courses separate,--as to their structure in the adult state it will
teach us nothing. In the second case the entire development of the
progenitors is also passed through by the descendants, and, therefore,
so far as the production of a species depends upon this second mode of
progress, the historical development of the species will be mirrored in
its developmental history. In the short period of a few weeks or months,
the changing forms of the embryo and larvae will pass before us, a more
or less complete and more or less true picture of the transformations
through which the species, in the course of untold thousands of years,
has struggled up to its present state.

(FIGURES 65 TO 67. Young Tubicolar worms, magnified with the simple lens
about 6 diam.:

FIGURE 65.* Without operculum, Protula-stage. (* Figure 65 is drawn from
memory, as the little animals, which I at first took for young Protulae,
only attracted my attention when I remarked the appearance of the
operculum, which induced me to draw them.)

FIGURE 66. With a barbate opercular peduncle, Filograna-stage;

FIGURE 67. With a naked opercular peduncle, Serpula-stage.)

One of the simplest examples is furnished by the development of the
Tubicolar Annelids; but from its very simplicity it appears well adapted
to open the eyes of many who, perhaps, would rather not see, and it may
therefore find a place here. Three years ago I found on the walls of one
of my glasses some small worm-tubes (Figure 65), the inhabitants of
which bore three pairs of barbate branchial filaments, and had no
operculum. According to this we should have been obliged to refer them
to the genus Protula. A few days afterwards one of the branchial
filaments had become thickened at the extremity into a clavate operculum
(Figure 66), when the animals reminded me, by the barbate opercular
peduncle, of the genus Filograna, only that the latter possesses two
opercula. In three days more, during which a new pair of branchial
filaments had sprouted forth, the opercular peduncle had lost its
lateral filaments (Figure 67), and the worms had become Serpulae. Here
the supposition at once presents itself that the primitive tubicolar
worm was a Protula,--that some of its descendants, which had already
become developed into perfect Protulae, subsequently improved themselves
by the formation of an operculum which might protect their tubes from
inimical intruders,--and that subsequent descendants of these latter
finally lost the lateral filaments of the opercular peduncle, which
they, like their ancestors, had developed.

What say the schools to this case? Whence and for what purpose, if the
Serpulae were produced or created as ready-formed species, these lateral
filaments of the opercular peduncle? To allow them to sprout forth
merely for the sake of an invariable plan of structure, even when they
must be immediately retracted again as superfluous, would certainly be
an evidence rather of childish trifling or dictatorial pedantry, than of
infinite wisdom. But no, I am mistaken; from the beginning of all things
the Creator knew, that one day the inquisitive children of men would
grope about after analogies and homologies, and that Christian
naturalists would busy themselves with thinking out his Creative ideas;
at any rate, in order to facilitate the discernment by the former that
the opercular peduncle of the Serpulae is homologous with a branchial
filament, He allowed it to make a detour in its development, and pass
through the form of a barbate branchial filament.

The historical record preserved in developmental history is gradually
EFFACED as the development strikes into a constantly straighter course
from the egg to the perfect animal, and it is frequently SOPHISTICATED
by the struggle for existence which the free-living larvae have to
undergo.

Thus as the law of inheritance is by no means strict, as it gives room
for individual variations with regard to the form of the parents, this
is also the case with the succession in time of the developmental
processes. Every father of a family who has taken notice of such
matters, is well aware that even in children of the same parents, the
teeth, for example, are not cut or changed, either at the same age, or
in the same order. Now in general it will be useful to an animal to
obtain as early as possible those advantages by which it sustains itself
in the struggle for existence. A precocious appearance of peculiarities
originally acquired at a later period will generally be advantageous,
and their retarded appearance disadvantageous; the former, when it
appears accidentally, will be preserved by natural selection. It is the
same with every change which gives to the larval stages, rendered
multifarious by crossed and oblique characters, a more straightforward
direction, simplifies and abridges the process of development, and
forces it back to an earlier period of life, and finally into the life
of the egg.

As this conversion of a development passing through different young
states into a more direct one, is not the consequence of a mysterious
inherent impulse, but dependent upon advances accidentally presenting
themselves, it may take place in the most nearly allied animals in the
most various ways, and require very different periods of time for its
completion. There is one thing, however, that must not be overlooked
here. The historical development of a species can hardly ever have taken
place in a continuously uniform flow; periods of rest will have
alternated with periods of rapid progress. But forms, which in periods
of rapid progress were severed from others after a short duration, must
have impressed themselves less deeply upon the developmental history of
their descendants, than those which repeated themselves unchanged,
through a long series of successive generations in periods of rest.
These more fixed forms, less inclined to variation, will present a more
tenacious resistance in the transition to direct development, and will
maintain themselves in a more uniform manner and to the last, however
different may be the course of this process in other respects.

In general, as already stated, it will be advantageous to the young to
commence the struggle for existence in the form of their parents and
furnished with all their advantages--in general, but not without
exceptions. It is perfectly clear that a brood capable of locomotion is
almost indispensable to attached animals, and that the larvae of
sluggish Mollusca, or of worms burrowing in the ground, etc., by
swarming briskly through the sea perform essential services by
dispersing the species over wider spaces. In other cases a metamorphosis
is rendered indispensable by the circumstance that a division of labour
has been set up between the various periods of life; for example, that
the larvae have exclusively taken upon themselves the business of
nourishment. A further circumstance to be taken into consideration is
the size of the eggs,--a simpler structure may be produced with less
material than a more compound one,--the more imperfect the larva, the
smaller may the egg be, and the larger is the number of these that the
mother can furnish with the same expenditure of material. As a rule, I
believe indeed, this advantage of a more numerous brood will not by any
means outweigh that of a more perfect brood, but it will do so in those
cases in which the chief difficulty of the young animals consists in
finding a suitable place for their development, and in which, therefore,
it is of importance to disperse the greatest possible number of germs,
as in many parasites.

As the conversion of the original development with metamorphosis into
direct development is here under discussion, this may be the proper
place to say a word as to the already indicated absence of metamorphosis
in fresh-water and terrestrial animals the marine allies of which still
undergo a transformation. This circumstance seems to be explicable in
two ways. Either species without a metamorphosis migrated especially
into the fresh waters, or the metamorphosis was more rapidly got rid of
in the emigrants than in their fellows remaining in the sea.

Animals without a metamorphosis would naturally transfer themselves more
easily to a new residence, as they had only themselves and not at the
same time multifarious young forms to adapt to the new conditions. But
in the case of animals with a metamorphosis, the mortality among the
larvae, always considerable, must have become still greater under new
than under accustomed conditions, every step towards the simplification
of the process of development must therefore have given them a still
greater preponderance over their fellows, and the effacing of the
metamorphosis must have gone on more rapidly. What has taken place in
each individual case, whether the species has immigrated after it had
lost the metamorphosis, or lost the metamorphosis after its immigration,
will not always be easy to decide. When there are marine allies without,
or with only a slight metamorphosis, like the Lobster as the cousin of
the Cray-fish, we may take up the former supposition; when allies with a
metamorphosis still live upon the land or in fresh water, as in the case
of Gecarcinus, we may adopt the latter.

That besides this gradual extinction of the primitive history, a
FALSIFICATION of the record preserved in the developmental history takes
place by means of the struggle for existence which the free-living young
states have to undergo, requires no further exposition. For it is
perfectly evident that the struggle for existence and natural selection
combined with this, must act in the same way, in change and development,
upon larvae which have to provide for themselves, as upon adult animals.
The changes of the larvae, independent of the progress of the adult
animal, will become the more considerable, the longer the duration of
the life of the larva in comparison to that of the adult animal, the
greater the difference in their mode of life, and the more sharply
marked the division of labour between the different stages of
development. These processes have to a certain extent an action opposed
to the gradual extinction of the primitive history; they increase the
differences between the individual stages of development, and it will be
easily seen how even a straightforward course of development may be
again converted by them into a development with metamorphosis. By this
means many, and it seems to me valid reasons may be brought up in favour
of the opinion that the most ancient Insects approached more nearly to
the existing Orthoptera, and perhaps to the wingless Blattidae, than to
any other order, and that the "complete metamorphosis" of the Beetles,
Lepidoptera, etc., is of later origin. There were, I believe, perfect
Insects before larvae and pupae; but, on the contrary, Nauplii and Zoeae
far earlier than perfect Prawns. In contradistinction to the INHERITED
metamorphosis of the Prawns, we may call that of the Coleoptera,
Lepidoptera, etc. an ACQUIRED metamorphosis.*

(* I will here briefly give my reasons for the opinion that the
so-called "complete metamorphosis" of Insects, in which these animals
quit the egg as grubs or caterpillars, and afterwards become quiescent
pupae incapable of feeding, was not inherited from the primitive
ancestor of all Insects, but acquired at a later period.

The order Orthoptera, including the Pseudoneuroptera (Ephemera,
Libellula, etc.) appears to approach nearest to the primitive form of
Insects. In favour of this view we have:--

1. The structure of their buccal organs, especially the formation of the
labium, "which retains, either perfectly or approximately, the original
form of a second pair of maxillae" (Gerstacker).

2. The segmentation of the abdomen; "like the labium, the abdomen also
very generally retains its original segmentation, which is shown in the
development of eleven segments" (Gerstacker). The Orthoptera with eleven
segments in the abdomen, agree perfectly in the number of their
body-segments with the Prawn-larva represented in Figure 33, or indeed,
with the higher Crustacea (Podophthalma and Edriophthalma) in general,
in which the historically youngest last thoracic segment (see page 123),
which is sometimes late-developed, or destitute of appendages, or even
deficient, is still wanting.

3. That, as in the Crustacea, the sexual orifice and anus are placed
upon different segments; "whilst the former is situated in the ninth
segment, the latter occurs in the eleventh" (Gerstacker).

4. Their palaeontological occurrence; "in a fossil state the Orthoptera
make their appearance the earliest of all Insects, namely as early as
the Carboniferous formation, in which they exceed all others in number"
(Gerstacker).

5. The absence of uniformity of habit at the present day in an order so
small when compared with the Coleoptera, Hymenoptera, etc. For this also
is usually a phenomenon characteristic of very ancient groups of forms
which have already overstepped the climax of their development, and is
explicable by extinction in mass. A Beetle or a Butterfly is to be
recognised as such at the first glance, but only a thorough
investigation can demonstrate the mutual relationships of Termes,
Blatta, Mantis, Forficula, Ephemera, Libellula, etc. I may refer to a
corresponding remarkable example from the vegetable world: amongst Ferns
the genera Aneimia, Schizaea and Lygodium, belonging to the group
Schizaeaceae which is very poor in species, differ much more from each
other than any two forms of the group Polypodiaceae which numbers its
thousands of species.

If, from all this, it seems right to regard the Orthoptera as the order
of Insects approaching most nearly to the common primitive form, we must
also expect that their mode of development will agree better with that
of the primitive form, than, for example, that of the Lepidoptera, in
the same way that some of the Prawns (Peneus) approaching most closely
the primitive form of the Decapoda, have most truly preserved their
original mode of development. Now, the majority of the Orthoptera quit
the egg in a form which is distinguished from that of the adult Insect
almost solely by the want of wings; these larvae then soon acquire
rudiments of wings, which appear more strongly developed after every
moult. Even this perfectly gradual transition from the youngest larva to
the sexually mature Insect, preserves in a far higher degree the picture
of an original mode of development, than does the so-called complete
metamorphosis of the Coleoptera, Lepidoptera, or Diptera, with its
abruptly separated larva-, pupa- and imago-states.

The most ancient Insects would probably have most resembled these
wingless larvae of the existing Orthoptera. The circumstance that there
are still numerous wingless species among the Orthoptera, and that some
of these (Blattidae) are so like certain Crustacea (Isopods) in habit
that both are indicated by the same name ("Baratta") by the people in
this country, can scarcely be regarded as of any importance.

The contrary supposition that the oldest Insects possessed a "complete
metamorphosis," and that the "incomplete metamorphosis" of the
Orthoptera and Hemiptera is only of later origin, is met by serious
difficulties. If all the classes of Arthropoda (Crustacea, Insecta,
Myriopoda and Arachnida) are indeed all branches of a common stem (and
of this there can scarcely be a doubt), it is evident that the
water-inhabiting and water-breathing Crustacea must be regarded as the
original stem from which the other terrestrial classes, with their
tracheal respiration, have branched off. But nowhere among the Crustacea
is there a mode of development comparable to the "complete
metamorphosis" of the Insecta, nowhere among the young or adult
Crustacea are there forms which might resemble the maggots of the
Diptera or Hymenoptera, the larvae of the Coleoptera, or the
caterpillars of the Lepidoptera, still less any bearing even a distant
resemblance to the quiescent pupae of these animals. The pupae, indeed,
cannot at all be regarded as members of an original developmental
series, the individual stages of which represent permanent ancestral
states, for an animal like the mouthless and footless pupa of the
Silkworm, enclosed by a thick cocoon, can never have formed the final,
sexually mature state of an Arthropod.

In the development of the Insecta we never see new segments added to
those already present in the youngest larvae, but we do see segments
which were distinct in the larva afterwards become fused together or
disappear. Considering the parallelism which prevails throughout organic
nature between palaeontological and embryonic development, it is
therefore improbable that the oldest Insects should have possessed fewer
segments than some of their descendants. But the larva of the
Coleoptera, Lepidoptera, etc., never have more than nine abdominal
segments, it is therefore not probable that they represent the original
young form of the oldest Insects, and that the Orthoptera, with an
abdomen of eleven segments, should have been subsequently developed from
them.

Taking into consideration on the one hand these difficulties, and on the
other the arguments which indicate the Orthoptera as the order most
nearly approaching the primitive form, it is my opinion that the
"incomplete metamorphosis" of the Orthoptera is the primitive one,
INHERITED from the original parents of all Insects, and the "complete
metamorphosis" of the Coleoptera, Diptera, etc., a subsequently ACQUIRED
one.)

Which of the different modes of development at present occurring in a
class of animals may claim to be that approaching most nearly to the
original one, is easy to judge from the above statements.

The primitive history of a species will be preserved in its
developmental history the more perfectly, the longer the series of young
states through which it passes by uniform steps; and the more truly, the
less the mode of life of the young departs from that of the adults, and
the less the peculiarities of the individual young states can be
conceived as transferred back from later ones in previous periods of
life, or as independently acquired.

Let us apply this to the Crustacea.


CHAPTER 12. PROGRESS OF EVOLUTION IN CRUSTACEA.

According to all the characters established in the last paragraph, the
Prawn that we traced from the Nauplius through states analogous to Zoea
and Mysis to the form of a Macrurous Crustacean appears at present to be
the animal, which in the section of the higher Crustacea (Malacostraca)
furnishes the truest and most complete indications of its primitive
history. That it is the most complete is at once evident. That it is the
truest must be assumed, in the first place, because the mode of life of
the various ages is less different than in the majority of the other
Podophthalma; for from the Nauplius to the young Prawn they were found
swimming freely in the sea, whilst Crabs, Porcellanae, the Tatuira,
Squilla, and many Macrura, when adult usually reside under stones, in
the clefts of rocks, holes in the earth, subterranean galleries, sand,
etc., not to mention other deviations in habits such as are presented by
the Hermit Crabs, Pinnotheres, etc.,--and secondly and especially
because the peculiarities which distinguish the Zoea of this species
particularly from other Zoeae (the employment of the anterior limbs for
swimming, the furcate tail, the simple heart, the deficiency of the
paired eyes and abdomen at first, etc.) are neither to be deduced from a
retro-transfer of late-acquired advantages to this early period of life,
nor to be regarded at all as advantages over other Zoeae which the larva
might have acquired in the struggle for existence.

A similar development must have been once passed through by the
primitive ancestor of all Malacostraca, probably differing from that of
our Prawn, especially in the circumstance that it would go on more
uniformly without the sudden change of form and mode of locomotion
produced in the latter by the simultaneous sprouting forth and entering
into action in the Nauplius of four and in the Zoea of five pairs of
limbs. It is to be supposed that, not only originally but even still, in
the larvae of the first Malacostraca, the new body-segments and pairs of
limbs are formed singly,--first of all the segments of the fore-body,
then those of the abdomen, and finally those of the middle-body,--and,
moreover, that in each region of the body the anterior segments were
formed earlier than the posterior ones, and therefore last of all the
hindermost segment of the middle-body. Of this original mode more or
less distinct traces still remain, even in species in which, in other
respects, the course of development of their ancestors is already nearly
effaced. Thus the abdominal feet of the Prawn-larva represented in
Figure 33, are formed singly from before backwards, and after these the
last feet of the middle-body; thus, in Palinurus, the last two pairs of
feet of the middle-body are formed later than the rest; thus in the
young larvae of the Stomapoda the last three abdominal segments are
destitute of limbs, which are still wanting on the last of them in older
larvae; and thus, in the Isopoda, the historically newest pair of feet
is produced later than all the rest. In the Copepoda this formation of
new segments and limbs, gradually advancing from before backwards, is
more perfectly preserved than in any of the higher Crustacea.* (* It is
well known that, in many cases, even in adult animals the last segment
of the middle-body, or some of its last segments, either want their
limbs or are themselves deficient (Entoniscus Porcellanae male,
Leucifer, etc.). This might be due to the animals having separated from
the common stem before these limbs were formed at all. But in those
cases with which I am best acquainted, it seems to me more probable that
the limbs have been subsequently lost again. That these particular limbs
and segments are more easily lost than others is explained by the
circumstance that, as the youngest, they have been less firmly fixed by
long-continued inheritance. ("Mr. Dana believes, that in ordinary
Crustaceans, the abortion of the segments with their appendages almost
always takes place at the posterior end of the cephalothorax."--Darwin,
Balanidae, page 111.))

The original development of the Malacostraca starting from the Nauplius,
or the lowest free-living grade with which we are acquainted in the
class of Crustacea, is now-a-days nearly effaced in the majority of
them. That this extinction has actually taken place in the way already
deduced as a direct consequence from Darwin's theory, will be the more
easily demonstrated, the more this process is still included in the
course of life, and the less completely it is already worn out. We may
hope to obtain the most striking examples in the still unknown
developmental history of the various Schizopoda, Peneidae, and, indeed,
of the Macrura in general. At present the multifarious Zoea-forms appear
to be particularly instructive. Almost all the peculiarities by which
they depart from the primitive form of the Zoea of Peneus (Figures 29,
30 and 32), may in fact be conceived as transferred back from a later
period into this early period of life. This is the case with the large
compound eyes,--with the structure of the heart,--with the raptorial
feet in Squilla,--and with the powerful, muscular, straightly-extended
abdomen in Palaemon, Alpheus, Hippolyte, and the Hermit Crabs. (In the
latter, indeed, the abdomen of the adult animal is a shapeless sac
filled with the liver and generative organs, but it is still tolerably
powerful in the Glaucothoe-stage, and was certainly still more powerful
when this stage was still the permanent form of the animal.) It is also
the case with the abdomen of the Zoeae of the Crabs, the Porcellanae,
and the Tatuira, which is still powerful, although usually bent under
the breast; the two last swim tolerably by means of the abdomen, even
when adult, as do the true Crabs in the young state known as Megalops.
It is the case, lastly, with the conversion of the two anterior pairs of
limbs into antennae. The second pair of antennae, which, in the various
Zoeae always remains a step behind that of the adult animal, is
particularly remarkable. In the Crabs the "scale" is entirely wanting;
their Zoeae have it indicated in the form of a moveable appendage, which
is often exceedingly minute. In the Hermit Crabs a similar, usually
moveable, spiniform process occurs as the remains of the scale; their
Zoeae have a well-developed but inarticulate scale. A precisely similar
scale is possessed by the adult Prawns, in the Zoeae of which it exists
still in a jointed form, like the outer branch of the second pair of
feet of the Nauplius or Peneus-Zoea.

The long, spiniform processes on the carapace of the Zoeae of the Crabs
and Porcellanae are not to be explained in this way, but their advantage
to the larvae is evident. Thus, for example, if the body of the Zoea of
Porcellana stellicola (Figure 24), without the processes of the carapace
and without the abdomen, which however is not rigidly extensible, is
scarcely half a line in length, whilst with the processes it is four
lines long, a mouth of eight times the width is necessary in order to
swallow the little animal when thus armed.* (* Persephone, a rare Crab,
belonging to the family Leucosiidae, is served in the same manner by its
long chelate feet. If we seize the animal, it extends them most
obstinately straight downwards, so that in all probability we should
more easily break than bend them.) Consequently these processes of the
carapace may be regarded as acquired by the Zoea itself in the struggle
for existence.

The formation of new limbs beneath the skin of the larvae is also to be
referred to an earlier occurrence of processes which originally took
place at a later period. The original course must have been that they
sprouted forth in a free form upon the ventral surface of the larva in
the next stage after the change of skin; whilst now they are developed
before the change of skin, and thus only come into action a stage
earlier. In larvae which, for other reasons, must be regarded as more
nearly approaching the primitive form, the original mode usually
prevails in this particular also. Thus the caudal feet (the "lateral
caudal lamellae") are formed freely on the ventral surface in Euphausia
and the Prawns with Nauplius-brood, and within the caudal lamellae in
the Prawns with Zoea-brood, in Pagurus and Porcellana.

A compression of several stages into one, and thereby an abridgement and
simplification of the course of development, is expressed in the
simultaneous appearance of several new pairs of limbs.

How earlier young states may gradually be completely lost, is shown by
Mysis and the Isopoda. In Mysis there is still a trace of the
Nauplius-stage; being transferred back to a period when it had not to
provide for itself, the Nauplius has become degraded into a mere skin;
in Ligia (Figures 36 and 37) this larva-skin has lost the last traces of
limbs, and in Philoscia (Figure 38) it is scarcely demonstrable.

Like the spinous processes of the Zoeae, the chelae on the penultimate
pair of feet of the young Brachyscelus are to be regarded as acquired by
the larva itself. The adult animals swim admirably and are not confined
to their host; as soon as the specimens of Chrysaora Blossevillei,
Less., or Rhizostoma cruciatum, Less., on which they are seated, become
the sport of the waves in the neighbourhood of the shore, they escape
from them, and are only to be obtained from lively Acalephs. The young
are helpless creatures and bad swimmers; a special apparatus for
adhesion must be of great service to them.

To review the developmental history of the different Malacostraca in
detail would furnish no results at all correspondent to the time
occupied by it,--if our knowledge was more complete it would be more
profitable. I therefore abandon it, but will not omit to mention that in
it many difficulties which cannot at present be satisfactorily solved
would present themselves. To these isolated difficulties I ascribe the
less importance, however, because even a little while ago, before the
discovery of the Prawn-Nauplius, this entire domain of the development
of the Malacostraca was almost inaccessible to Darwin's theory.

Nor will I dwell upon the contradictions which appear to result from the
application of the Darwinian theory to this department. I leave it to
our opponents to find them out. Most of them may easily be proved to be
only apparent. There are two of these objections, however, which lie so
much on the surface that they can hardly escape being brought forward,
and these, I think, I must get rid of.

"The peculiarities in which the Zoeae of the Crabs, the Porcellanae, the
Tatuira, the Hermit Crabs, and the Prawns with Zoea-brood agree, and by
which they are in common distinguished from the larvae of Peneus
produced from Nauplii, forces us (it might be said) to the supposition
that the common ancestor of these various Decapods quitted the egg in a
similar Zoea-form. But then neither Peneus with its Nauplius-brood, nor
even apparently the Palinuri could be referred back to this ancestor.
The mode of development of Peneus and Palinurus, as also several
peculiar larvae of unknown origin, but which are in all probability to
be attributed to Macrurous Crustacea, necessitate on the contrary the
opposite supposition, namely, that the different groups of the Macrura
have passed from their original to their present mode of development
independently of each other and also independently of the Crabs." To
this we may answer that the occurrence of the Zoea-form in all the
above-mentioned Decapoda, its existence in Peneus during the whole of
that period of life which is richest in progress and in which the wide
gap between the Nauplius and the Decapod is filled up, its recurrence
even in the development of the Stomapoda, the occurrence of a larval
form closely approaching the youngest Zoea of Peneus in the Schizopod
genus Euphausia, and the reminiscence of the structure of Zoea, which
even the adult Tanais has preserved in its mode of respiration,--all
indicate Zoea as one of those steps in development which persisted as a
permanent form throughout a long period of repose, perhaps through a
whole series of geological formations, and thus has also made a deeper
impression upon the development of its descendants, and formed a firmer
nucleus in the midst of other and more readily effaced young states. It
cannot, therefore, surprise us that in transitions from the original
mode of metamorphosis to direct development, even when produced
independently, the larval life commences in the same way with this
Zoea-form in different families, in which the earlier stages of
development are effaced. But except what is common to all Zoeae, and
what may easily be explained as being transferred back from a later into
this stage, the Zoeae of the Crabs, for example, agree with those of
Pagurus and Palaemon in no single peculiarity of structure which leads
us to suppose a common inheritance. Consequently we may apparently
assume, without hesitation, that when the Brachyura and Macrura
separated, the primitive ancestors of each of these groups passed
through a more complete metamorphosis, and that the transition to the
present mode of development belongs to a later period. With regard to
the Brachyura, it may be added that in them this transition occurred
only a little later and indeed before the existing families separated.
The arrangement of the processes of the carapace, and, still more, the
similar number of the caudal setae in the most different Zoeae of Crabs
(Figures 19 to 23) prove this. Such an accordance in the number of
organs apparently so unimportant is only explicable by common
inheritance. We may predict with certainty that amongst the Brachyura no
species will occur which, like Peneus, still produces Nauplius-brood.*
(* I must not omit remarking that what has been said as to the
development of the Crabs applies essentially only to the groups
Cyclometopa, Catometopa and Oxyrhyncha, placed together by Alph.
Milne-Edwards as "Eustomes." Among the Oxystomata, as also among the
"Anomura apterura," Edw., which approach so nearly to the Crabs, I am
unacquainted with the earliest young states of any of the species.)

As we have already seen, Mysis and the Isopoda depart from all other
Crustacea very remarkably by the fact that their embryos are curved
upwards, instead of, as elsewhere, downwards. Does not so isolated a
phenomenon as this, it might be asked, in the sense of Darwin's theory,
indicate a common inheritance? Does it not necessitate that we should
unite as the descendants of the same primitive ancestors, Mysis with the
Isopoda on the one hand, and on the other the rest of the Podophthalma
with the Amphipoda? I think not. Such a necessity exists only for those
who estimate a peculiarity at a higher value because it makes its
appearance at an earlier period of the egg-life. Whoever regards species
as not created independently and unchangeably, but as having gradually
become what they are, will say to himself that, when the ancestors of
our Mysides came (probably much later than those of the Amphipoda and
Isopoda) to develop numerous body-segments and limbs whilst still
embryos, as they could no longer find room in the egg when extended
straight out, and were therefore compelled to bend themselves, this
could only take place either upwards or downwards, and whatever
conditions may have decided the direction actually adopted, any near
relationship to either of the two orders of Edriophthalma could hardly
have taken part in it.

It may, however, be remarked, that the different curvature of the embryo
in the Amphipoda and Isopoda is so far instructive, as it proves that
their present mode of development was adopted only after the separation
of these orders, and that, in the primitive stock of the Edriophthalma,
the embryos were, if not Nauplii, at least short enough in the body to
find room in the egg in an extended position, like the larvae of
Achtheres enclosed by the Nauplius-skin. On the other hand the
uniformity of development that prevails in each of the two orders--which
is expressed in the Amphipoda for example in the formation of the
"micropylar apparatus," in the Isopoda in the want of the last pair of
ambulatory feet--testifies that the present mode of development has come
down from a very early period and extends back beyond the separation of
the present families. In these two orders also, as well as in the Crabs,
we can hardly hope to find traces of earlier young states, unless it be
in the family of the Tanaidae.* (* Whether the want of the abdominal
feet in the young of Tanais be an inheritance from the time of the
primitive Isopoda, or a subsequently acquired peculiarity, which appears
to me the more admissible view at present, may perhaps be decided with
some certainty, when we become acquainted with the development and mode
of life of its family allies, Apseudes and Rhoea. The latter, as is well
known, is the only Isopod which possesses a secondary flagellum on the
anterior antennae. I have recently obtained a new and unexpected proof
that the Tanaidae ("Asellotes heteropodes" M.-Edw.) of all known
Crustacea approach most closely to the primitive form of the
Edriophthalma. Mr. C. Spence Bate writes to me: "Apseudes, as far as I
know, is the ONLY Isopod in which the antennal scale so common in the
Macrura is present on the lower antenna.") If any one will furnish me
with an Amphipod or an Isopod with Nauplius-brood, the existence of
which would not be more remarkable in independently produced species
than that of a Prawn with Nauplius-brood, I will abandon the whole
Darwinian theory.

With regard to the Crabs, and also to the Isopoda and Amphipoda, we were
led to the assumption that, about the period when these groups started
from the common stem, a simplification of their process of development
took place. This also seems to be intelligible from Darwin's theory.
When any circumstances favourable to a group of animals caused its wider
diffusion and divergence into forms adapting themselves to new and
various conditions of existence, this greater variability, which betrays
itself in the production of new forms, will also favour the
simplification of the development which is almost always advantageous,
and moreover, exactly at this period, during adaptation to new
circumstances, as has already been indicated with regard to fresh-water
animals, this simplification will be doubly beneficial, and therefore,
in connexion with this, a doubly strict selection will take place.

So much for the development of the higher Crustacea.

A closer examination of the developmental history of the lower Crustacea
is unnecessary after what has been said in general upon the historical
significance of the young states, and the application of this which has
just been made to the Malacostraca. We may see, without further
discussion, how the representation given by Claus of the development of
the Copepoda may pass almost word for word as the primitive history of
those animals; we may find in the Nauplius-skin of the larvae of
Achtheres and in the egg-like larva of Cryptophialus, precisely similar
traces of a transition towards direct development, as were presented by
the Nauplius-envelope of the embryos of Mysis and the maggot-like larva
of Ligia, etc.

It will be sufficient to indicate an essential difference in the process
of development in the higher and lower Crustacea. In the latter all new
body-segments and limbs which insert themselves between the two terminal
regions of the Nauplius, are formed in uninterrupted sequence from
before backwards; in the former there is further a new formation in the
middle of the body (the middle-body), which pushes itself in between the
fore-body and the abdomen in the same way, as these have done on their
part between the head and tail of the Nauplius. Thus, that which appears
probable even from the comparison of the limbs of the adult animal,
finds fresh support in the developmental history, namely, that the lower
Crustacea, like the Insects, are entirely destitute of the region of the
body corresponding to the middle-body of the Malacostraca. It seems
probable that the swimming feet of the Copepoda, as also of the pupae of
Cirripedia and Rhizocephala, represent the abdominal feet of the
Malacostraca, that is to say, are derived by inheritance from the same
source with them.

It would be easy to weave together the separate threads furnished by the
young forms of the various Crustacea, into a general picture of the
primitive history of this class. Such a picture, drawn with a little
skill, and finished in lively colours, would certainly be more
attractive than the dry discussions which I have tacked on to the
developmental history of these animals. But the mode of weaving in the
loose threads would still in many cases be arbitrary, and to be effected
with equal justice in various ways; and many gaps would still have to be
filled up by means of more or less bold assumptions. Those who have not
wandered much in this region of research would then readily believe that
they were standing upon firm ground, where mere fancy had thrown an airy
bridge; those acquainted with the subject, on the other hand, would soon
find out these weak points in the structure, but would then be easily
led to regard even what was founded upon well considered facts, as
merely floating in the air. To obviate these misconceptions of its true
contents from either side, it would be necessary to accompany such a
picture throughout with lengthy, dry explanations. This has deterred me
from further filling in the outline which I had already sketched.

I will only give, as an example, the probable history of the production
of a single group of Crustacea, and indeed of the most abnormal of all,
the RHIZOCEPHALA, which in the sexually mature state differ so
enormously even from their nearest allies, the Cirripedia, and from
their peculiar mode of nourishment stand quite alone in the entire
animal kingdom.

I must preface this with a few words upon the homology of the roots of
the Rhizocephala, i.e. the tubules which penetrate from its point of
adhesion into the body of the host, ramify amongst the viscera of the
latter, and terminate in caecal branchlets. In the pupae of the
Rhizocephala (Figure 58) the foremost limbs ("prehensile antennae")
bear, on each of the two terminal joints, a tongue-like, thin-skinned
appendage, in which we may generally observe a few small strongly
refractive granules, like those seen in the roots of the adult animal. I
have therefore supposed these appendages to be the rudiments of the
future roots. A perfectly similar appendage, "a most delicate tube or
ribbon," was found by Darwin in free-swimming pupae of Lepas australis
on the last joints of the "prehensile antennae." From the perfect
accordance in their entire structure shown by the pupae of the
Rhizocephala and Cirripedia, there can be no doubt that the appendages
of Sacculina and Lepas, which are so like each other and spring from the
same spot, are homologous structures.

Now in three species of Lepas, in Dichelaspis Warwickii and in
Scalpellum Peronii, Darwin saw, on tearing recently-affixed animals from
their point or support, that a long narrow band issued from the same
point of the antennae; its end was torn away, and in Dichelaspis,
judging from its ragged appearance, it had attached itself firmly to the
support. From this it follows that this appendage in Lepas australis can
hardly be anything but a young cement-duct. If, therefore, the
supposition that the appendages on the antennae of the pupae of
Rhizocephala are young roots be correct, the roots of the Rhizocephala
are homologous with the cement-ducts of the Cirripedia. And this,
strange as it may appear at the first glance, seems to me scarcely
doubtful. It is true that the act of adhesion of the Rhizocephala has
never yet been observed, but it is more than probable that they attach
themselves, just like the Cirripedia, by means of the antennae, and that
therefore the points of attachment in the two groups indicate homologous
parts of the body. From the point of attachment in the Rhizocephala the
roots penetrate into the body of the host, whilst in the Cirripedia, the
cement-ducts issue from the same point. The roots are blind tubes,
ramified in different ways in different species. The cement-ducts in the
basis of the Balanidae likewise constitute a generally remarkably
complicated system of ramified tubes, with regard to the mode of
termination of which nothing certain has yet been made out. Individual
caecal branches are not unfrequently seen even in the vicinity of the
carina; and, at least in some species, in which the cement-ducts divide
into extremely numerous and fine branchlets, forming a network which
gradually becomes denser towards the circumference of the basis, these
seem nowhere to possess an orifice.

Now as to the question: How were Cirripedia converted by natural
selection into Rhizocephala?

A considerable number of existing Cirripedia settle exclusively or
chiefly upon living animals;--on Sponges, Corals, Mollusks, Cetaceans,
Turtles, Sea-Snakes, Sharks, Crustaceans, Sea Urchins, and even on
Acalephs. Dichelaspis Darwinii was found by Filippi in the branchial
cavity of Palinurus vulgaris, and I have met with another species of the
same genus in the branchial cavity of Lupea diacantha.

The same thing may have taken place in primitive times. The supposition
that certain Cirripedes might once upon a time have selected the soft
ventral surface of a Crab, Porcellana or Pagurus, for its
dwelling-place, has certainly nothing improbable about it. If then the
cement-ducts of such a Cirripede instead of merely spreading on the
surface, pierced or pushed before them the soft ventral skin and
penetrated into the interior of the host, this must have been beneficial
to the animal, because it would be thereby more securely attached and
protected from being thrown off during the moulting of its host.
Variations in this direction were preserved as advantageous.

But as soon as the cement-ducts penetrated into the body-cavity of the
host and were bathed by its fluids, an endosmotic interchange must
necessarily have been set up between the materials dissolved in these
fluids and in the contents of the cement-ducts, and this interchange
could not be without influence upon the nourishment of the parasite. The
new source of nourishment opened up in this manner was, as constantly
flowing, more certain than that offered by the nourishment accidentally
whirled into the mouth of the sedentary animal. The individuals favoured
in the development of the cement-ducts now converted into nutriferous
roots, had more than others the prospect of abundant food, of vigorous
growth, and of producing a numerous progeny. With the further
development, assisted by natural selection, of the roots embracing the
intestine of the host and spreading amongst its hepatic tubes, the
introduction of nourishment through the mouth and all the parts
implicated in it, such as the whirling cirri, the buccal organs, and the
intestine, gradually lost their importance, became aborted by disuse,
and finally disappeared without leaving a trace of their existence.
Protected by the abdomen of the Crab, or by the shell inhabited by the
Pagurus, the parasite also no longer required the calcareous test, in
which, no doubt, the first Cirripedes settling upon these Decapods
rejoiced. This protective covering, having become superfluous, also
disappeared, and there remained at last only a soft sack filled with
eggs, without limbs, without mouth or alimentary canal, and nourished,
like a plant, by means of roots, which it pushed into the body of its
host. The Cirripede had become a Rhizocephalon.

If it be desired to form a notion of what our parasite may have looked
like when half way in its progress from the one form to the other, we
may consult the figures given by Darwin, (Lepadidae Plate 4 figures 1 to
7) of Anelasma squalicola. This Lepadide, which lives upon Sharks in the
North Sea, seems, in fact, to be in the best way to lose its cirri and
buccal organs in the same manner. The widely-cleft, shell-less test is
supported upon a thick peduncle, which is immersed in the skin of the
Shark. The surface of the peduncle is beset with much-ramified, hollow
filaments, which "penetrate the Shark's flesh like roots" (Darwin).
Darwin looked in vain for cement-glands and cement. It seems to me
hardly doubtful, that the ramified hollow filaments are themselves
nothing but the cement-ducts converted into nutritive roots, and that it
is just in consequence of the development of this new source of
nourishment, that the cirri and buccal organs are in the highest degree
aborted. All the parts of the mouth are extremely minute; the palpi and
exterior maxillae have almost disappeared; the cirri are thick,
inarticulate, and destitute of bristles; and the muscles both of the
mouth and cirri are without transverse striation. Darwin found the
stomach perfectly empty in the animal examined by him.

...

Having reached the Nauplius, the extreme outpost of the class, retiring
furthest into the gray mist of primitive time, we naturally look round
us to see whether ways may not be descried thence towards other
bordering regions. By the structure of the abdomen in Nauplius we might
be reminded, like Oscar Schmidt, of the moveable caudal fork of the
Rotatoria, which many regard as near allies of the Crustacea, or at any
rate of the Arthropoda; in the six feet surrounding the mouth we might
imagine an originally radiate structure, and so forth. But I can see
nothing certain. Even towards the nearer provinces of the Myriopoda and
Arachnida I can find no bridge. For the Insecta alone, the development
of the Malacostraca may perhaps present a point of union. Like many
Zoeae, the Insecta possess three pairs of limbs serving for the
reception of nourishment, and three pairs serving for locomotion; like
the Zoeae they have an abdomen without appendages; as in all Zoeae the
mandibles in Insects are destitute of palpi. Certainly but little in
common, compared with the much which distinguishes these two
animal-forms. Nevertheless the supposition that the Insecta had for
their common ancestor a Zoea which raised itself into a life on land,
may be recommended for further examination.

Much in what has been adduced above may be erroneous, many an
interpretation may have failed, and many a fact may not have been placed
in its proper light. But in one thing, I hope, I have succeeded,--in
convincing UNPREJUDICED readers, that Darwin's theory furnishes the key
of intelligibility for the developmental history of the Crustacea, as
for so many other facts inexplicable without it. The deficiencies of
this attempt, therefore, must not be laid to the charge of the plan
drawn out by the sure hand of the master, but solely to the clumsiness
of the workman, who did not know how to find the proper place for every
portion of his material.


INDEX.

Acanthonotus Owenii.

Acanthosoma.

Achaeus.

Achtheres.
-- percarum.

Allorchestes.

Alpheus.

Amphilochus.

Amphipoda.

Amphithoe.

Anceus.

Anelasma squalicola.

Anilocra.

Aratus.
-- Pisonii.

Artemia.

Asellus.

Atylus.
-- carinatus.

Batea.

Bodotria.

Bopyridae.

Bopyrus.

Brachyscelus.
-- crusculum.

Brachyura.

Branchiopoda.

Calanidae.

Caligus.

Caprella.
-- attenuata.
-- linearis.

Carcinus maenas.

Caridina.

Cassidina.

Cerapus.

Chalimus.

Chondracanthus.

Chthamalus.

Cirripedia.

Cladocera.

Copepoda.

Corophium.
-- dentatum.

Corycaeidae.

Crangon.

Crayfish.

Cryptoniscus planarioides.

Cryptophialus.
-- minutus.

Cuma.

Cumacea.

Cyclograpsus.

Cyclopidae.

Cyclops.

Cyclopsine.

Cymothoa.

Cymothoadiens.

Cypridina.

Cypris.

Cyrtophium.

Cythere.

Daphnia pulex.

Dercothoe.

Diastylidae.

Dichelaspis Warwickii.

Dulichia.

Edriophthalma.

Entomostraca.

Entoniscus.
-- cancrorum.
-- porcellanae.

Erichthus.

Eriphia gonagra.

Euphausia.

Eurynome.

Evadne.

Filograna.

Gammarus.
-- ambulans.
-- Dugesii.
-- puteanus.

Gecarcinus.

Gelasimus.
-- vocans.

Glaucothoe Peronii.

Grapsus.

Hermit Crabs.

Hippa emerita.

Hippolyte.

Hyperia galba.
-- Latreillei.
-- Martinezii.

"Hyperines anormales et ordinaires".

Idothea.

Insecta.

Isopoda.

Kepone.

Laemodipoda.

Lepas.
-- anatifera.
-- australis.

Lernaeodiscus porcellanae.

Lernanthropus.

Lestrigonus.

Leucifer.

Leucothoe.

Ligia.

Lobster.

Lupea diacantha.

Macrura.

Maia.

Megalops.

Melita.
-- anisochir.
-- exilii.
-- Fresnelii.
-- insatiabilis.
-- Messalina.
-- palmata.
-- setipes.
-- valida.

Microdeutopus.

Montagua.

Mysis.

"Nauplius-larvae".

Nebalia.

Niphargus.

Ocypoda.
-- rhombea.

Orchestia.
-- Darwinii.
-- gryphus.
-- sylvicola.
-- tahitensis.
-- telluris.
-- Tucurauna.
-- Tucuratinga.

Orchestoidea.

Pagurus.

Palaemon.

Palinurus.

Peltogaster.
-- socialis.

Peneus.
-- setiferus.

Persephone.

Philoscia.

Phronima.
-- sedentaria.

Phryxus.

Phyllopoda.

Phyllosoma.

Pinnotheres.

Podophthalma.

Polyphemus.

Pontellidae.

Porcellana.
-- stellicola.

Porcellionides.

Praniza.

Prawns.

Protella.

Protula.

Pycnogonidae.

Ranina.

Rhizocephala.

Sacculina purpurea.

Scalpellum Peronii.

Sergestes.

Serpulae.

Sesarma.

Shrimps.

Sphaeroma.

Squilla.

Talitrus.

Tanais.
-- dubius.
-- Dulongii.

Tatuira.

Tetraclita porosa.

Trilobites.

Xantho.

Xiphosura.

Zoeae.





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