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Title: Facts and Arguments for Darwin

Author: Fritz Müller

Translator: W. S. Dallas

Release date: September 1, 2004 [eBook #6475]
                Most recently updated: June 21, 2020

Language: English

Credits: Produced by Sue Asscher


*** START OF THE PROJECT GUTENBERG EBOOK FACTS AND ARGUMENTS FOR DARWIN ***




Produced by Sue Asscher




Facts and Arguments for Darwin

by Fritz Müller

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.




Contents

 TRANSLATOR’S PREFACE
 AUTHOR’S PREFACE
 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.
 Index 




TRANSLATOR’S PREFACE


My principal reason for undertaking the translation of Dr. Fritz
Müller’s admirable work on the Crustacea, entitled ‘Für 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 Müller 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. Müller’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, 15_th 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, 7_th September_, 1863.



HISTORY OF CRUSTACEA




CHAPTER I.
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 to 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, _primâ 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
Diastylidæ, the Amphipoda and Isopoda, the Ostracoda and Daphnidæ, 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 Acalephæ, 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 _Penëus_, 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 II.
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 Raninidæ (_Ranina_), Eriphinæ (_Eriphia
gonagra_), Grapsoidæ (_Aratus, Sesarma,_ etc.), Ocypodidæ (_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.

Fig. 1. Melita exilii n. sp., male, enlarged. The large branchial
lamellæ are seen projecting between the legs. Fig. 1. _Melita exilii_
n. sp., male, enlarged. The large branchial lamellæ 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 (Fig. 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 _Melitæ_ 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—

From the structure of the clasp-forceps:	From the presence or
absence of the
secondary flagellum.
_M. palmata, etc., M. exilii, etc., M. Fresnelli._	_M. palmata,
etc., M. exilii, etc., 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 _ Leucothoë, 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 (_coxæ_ Sp. B.) of the caudal feet (_uropoda_
Westw.), etc., to be a true _Amphithoë_[1] possesses it—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 branchiæ—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.

 [1] 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’).




CHAPTER III.
MORPHOLOGY OF CRUSTACEA—NAUPLIUS-LARVÆ.


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 larvæ of Shrimps (‘Archiv für Naturgeschichte’ 1860, i, p.
8), and I must admit that this discovery gave me the first decided turn
in Darwin’s favour.

The similar number of segments[1] 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. And if at the present day the majority of the Crabs and
Macrura, and indeed the Stalk-eyed Crustacea in general, pass through
Zoëa-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 _Zoëa._ 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 Zoëa-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 _Zoëae,_
especially their mode of respiration. Whilst in all other Oniscoida the
abdominal feet serve for respiration, these in our cheliferous Isopod
(Fig. 2) are solely motory organs, into which no blood-corpuscle ever
enters, and the chief seat of respiration is, as in the _Zoëae,_ 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 _Zoëae_ and the adult Decapoda, by an appendage of
the second pair of maxillæ, which is wanting in all other
Edriophthalma.

Tanais dubius Fig. 2. _Tanais dubius_ (?) Kr. hermaphrodite, magnified,
showing the orifice of entrance (_x_) into the cavity overarched by the
carapace, in which an appendage of the second pair of maxillæ (_f_)
plays. On four feet (_i, k, l, m_) are the rudiments of the lamellæ
which subsequently form the brood-cavity.


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 _Penëus_ 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,[2] for it has nothing which could attract
particular attention amongst the multifarious and often wonderful
_Nauplius_-forms. When I, fancying from the similarity of its movements
that it was a young _Penëus-Zoëa,_ had for the first time captured such
a larva, and on bringing it under the microscope found a _Nauplius_
differing _toto cœlo_ from this _Zoëa,_ 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 _Zoëa_-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.

 [1] 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’ p. xx), states that he observed limbs of the last segment
 in _Palæmon 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
    antennæ, 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 antennæ. Like the antennæ, 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
    Diastylidæ), 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.


 [2] Mecznikow has recently found Naupliiform shrimp-larvæ in the sea
 near Naples.




CHAPTER IV.
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 chelæ 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 Cambriú, 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 antennæ 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 Diastylidæ, 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 (Fig. 3) acquire
powerful, long-fingered, and very mobile chelæ, 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 (Fig. 5) retain the short thick form of the chelæ
of the females; but, on the other hand, their antennæ (Fig. 6) are
equipped with a far greater number of olfactory filaments, which stand
in groups of from five to seven together.


Fig. 3. Head of the ordinary form of the male of Tanais dubius (?) Kr.
magnified. The terminal setæ of the second pair of antennæ project
between the cheliferous feet.
Fig. 4. Buccal region of the same from below; lambda, labrum.
Fig. 5. Head of the rarer form of the male, magnified.
Fig. 6. Flagellum of the same, with olfactory filaments, magnified.


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 Confervæ, 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 chelæ, 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 chelæ 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.

Fig. 7. Orchestia Darwinii, n. sp. male. Fig. 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 (Fig. 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 chelæ 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 chelæ—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 (Figs. 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 chelæ of the males occur in this species is so far
worthy of notice, because the formation of the chelæ, 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.

Figs. 8 and 9. The two forms of the chelæ of the male of Orchestia
Darwinii, magnified. Figs. 8 and 9. The two forms of the chelæ of the
male of _Orchestia Darwinii,_ magnified.


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.

Fig. 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. Fig. 11. Coxal
lamella of the same pair of feet of the female of M. insatiabilis. Fig.
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. Fig. 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 lamellæ 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 lamellæ 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 lamellæ.

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 V.
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 branchiæ, 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 Grapsoidæ, 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 (“régions ptérygostomiennes”), 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 branchiæ from behind.

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

Fig. 12. Posterior entrance to the branchial cavity of Ocypoda rhombea,
Fab. The carapace and the fourth foot of the right side are removed.
Fig. 13. Points of some of the hairs of the basal joints of the foot,
magnified. Fig. 12. Posterior entrance to the branchial cavity of
_Ocypoda rhombea,_ Fab. The carapace and the fourth foot of the right
side are removed. Fig. 13. Points of some of the hairs of the basal
joints of the foot, magnified.


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.[1] In these a peculiar arrangement on the feet of the
third and fourth pairs (Fig. 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 (Fig. 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 branchiæ 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._[2] 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 Ocypodidæ and
Grapsoidæ, 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 branchiæ,—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 Grapsoidæ and yet does
not agree with them, but with the arenicolous _Ocypoda._

 [1] 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.


 [2] 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.




CHAPTER VI.
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 Caprellidæ and Cyamidæ (Latreille’s _Læmodipoda_), 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.[1]

The sole unimportant exception which I have hitherto met with is
presented by the genus _Brachyscelus,_[2] 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.[3]

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.

Fig. 14. Heart of a young Cassidina. Fig. 15. Heart of a young
Anilocra. Fig. 16. Abdomen of the male of Entoniscus Cancrorum. h.
Heart. l. Liver. Fig. 14. Heart of a young _Cassidina._ Fig. 15. Heart
of a young _Anilocra._ Fig. 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 _Porcellanæ_ (_Entoniscus
porcellanæ_), 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 (Fig. 16) is situated in the third abdominal
segment. In the _Cassidinæ,_ the heart (Fig. 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 (Fig. 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
suprà_) 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 infrà_)
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 larvæ 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
suprà_). Now, where, as in the majority of the Isopoda, branchiæ 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 Zoëa, as in the _Tanaides,_ or by the development of
branchiæ 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 setæ
directed obliquely upwards, and at some distance from this, upon the
ventral surface, a group of thicker setæ 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
Amphinomidæ; 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
Amphinomidæ, by the formation and position of the inferior setæ, etc.
But that a worm belonging to this particular family Amphinomidæ 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.”

 [1] 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 Kröyer in his ‘Grönlands Amfipoder’ was
 acquainted with only 28 species, including 2 Læmodipoda, from the
 Arctic Seas, although these had been investigated by a far greater
 number of Naturalists.


 [2] According to Milne-Edwards’ arrangement the females of this genus
 would belong to the “Hypérines ordinaires” and the previously unknown
 males to the “Hypérines anormales,” the distinguishing character of
 which, namely the curiously zigzagged inferior antennæ, 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.


 [3] I find, in Milne-Edwards’ ‘Leçons 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 Caprellæ (probably the young
 of Caprella attenuata, Dana, with which they occurred), but can only
 find the usual three pairs.




CHAPTER VII.
STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA.


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[1] in the European fresh-water Crayfish, and
according to Westwood in a West Indian Land Crab (_Gecarcinus_). 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 _Planariæ_ 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 _Zoëa._ 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 Zoëa-brood,[2] which is also met with in other
terrestrial Crabs (_Ocypoda, Gelasimus,_ etc.). All the Anomura seem
likewise to commence their lives as Zoëæ: witness the _ Porcellanæ,_
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, Palæmon, 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 Zoëa-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. Branchiæ
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
_Daphniæ,_ 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 Zoëa. 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 maxillæ (Fig. 18). The addition of fine coloured particles to
the water allows this current of water to be easily detected even in
small Zoëæ.

Fig. 17. Zoëa of a Marsh Crab (Cyclograpsus ?), magnified. Fig. 18.
Maxilla of the second pair in the same species, magnified. Fig. 17.
Zoëa of a Marsh Crab (_Cyclograpsus ?_), magnified. Fig. 18. Maxilla of
the second pair in the same species, magnified.


The Zoëæ of the Crabs (Fig. 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 _Achæus_) 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 Zoëa, 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 Zoëæ 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) antennæ are simple, not jointed, and furnished at the
extremity with from two to three olfactory filaments; the posterior
(outer) antennæ 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,[3]
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 setæ at its hinder margin. The Zoëæ 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.

(Figs. 19 to 23. Tails of the Zoëæ of various Crabs. Fig. 19.
Pinnotheres. Fig. 20. Sesarma. Fig. 21. Xantho. Fig. 22 and 23 of
unknown origin. Figs. 19 to 23. Tails of the Zoëæ of various Crabs.
Fig. 19. Pinnotheres.  Fig. 20. Sesarma.  Fig. 21. Xantho. Figs. 22 and
23 of unknown origin.

It is further to be remarked that the Zoëæ of the Crabs, as also of the
_Porcellanæ,_ 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 setæ of the feet, and the antennæ, and
that they cast this in a few hours. In _Achæus_ I have observed that
the tail of this earliest larval skin resembles that of the larvæ of
Shrimps and Prawns, and the same appears to be the case in _Maia_ (see
Bell, ‘Brit. Stalk-eyed Crust.’ p. 44). Widely as they seem to differ
from them at the first glance, the Zoëæ of the _ Porcellanæ_ (Fig. 24)
approach those of the true Crabs very closely. The antennæ, organs of
the mouth, and natatory feet, exhibit the same structure. But the tail
bears _five_ pairs of setæ, 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.

Fig. 24. Zoëa of Porcellana stellicola, F. Müll. Magnified. Fig. 25.
Zoëa of the Tatuira (Hippa emerita), magnified. Fig. 26. Zoëa of a
small Hermit Crab, magnified . Fig. 24. Zoëa of _Porcellana
stellicola,_ F. Müll., magnified.
Fig. 25. Zoëa of the Tatuira (_Hippa emerita_), magnified.
Fig. 26. Zoëa of a small Hermit Crab, magnified.

The Zoëa of the Tatuira (Fig. 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
setæ.

The Zoëa of the Hermit Crabs (Fig. 26) possesses the simple inner
antennæ of the Zoëa of the true Crabs; the outer antennæ bear upon the
outside on a short stalk a lamella of considerable size analogous to
the scale of the antennæ 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 setæ. 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 setæ. The tail bears five pairs of
setæ. 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 Zoëæ of the
Shrimps and Prawns (Fig. 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 antennæ 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 antennæ
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 setæ on its
hinder margin.

Fig. 27. Zoëa of a Palæmon residing upon Rhizostoma cruciatum, Less.,
magnified. Fig. 27. Zoëa of a _Palæmon_ residing upon _Rhizostoma
cruciatum,_ Less., magnified.


The development of the Zoëa-brood to the sexually mature animal was
traced by Spence Bate in _Carcinus mænas._ 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 Zoëæ, 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.[4]

The development of the Palinuridiæ 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 antennæ 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 larvæ 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 _Penëus_ 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. Müller 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!

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 (Fig. 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.

Fig. 28. Nauplius of a Prawn, magnified. Fig. 29. Young Zoëa of the
same Prawn, magnified. Fig. 28. Nauplius of a Prawn, magnified.
Fig. 29. Young Zoëa of the same Prawn, magnified.

In a subsequent moult the new limbs (maxillæ, and anterior and
intermediate maxillipedes) come into action, and in this way the
Nauplius becomes a Zoëa (Fig. 29), agreeing perfectly with the Zoëa 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 setæ;
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 Zoëæ 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 Zoëæ of _Alpheus,
Palæmon, Hippolyte,_ and other Prawns, of the Hermit Crabs, the Tatuira
and the _ Porcellanæ._ The heart possesses only one pair of fissures,
and has no muscles traversing its interior like trabeculæ, whilst in
other Zoëæ two pairs of fissures and an interior apparatus of trabeculæ
are always distinctly recognisable.

Fig. 30. Older Zoëa of the same Prawn, magnified. Fig. 31. Mysis-form
of the same Prawn, magnified. Fig. 30. Older Zoëa of the same Prawn,
magnified.
Fig. 31. _Mysis_-form of the same Prawn, magnified.

During this Zoëal 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 (Fig. 30). The caudal appendages sprout forth like other
limbs freely on the ventral surface, whilst in other Prawns, the
_Porcellanæ,_ 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 Zoëa passes into the _ Mysis-_ or
Schizopod-form (Fig. 31). The antennæ cease to serve for locomotion,
their place is taken by the thoracic feet, furnished with long setæ,
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
antennæ have lost their long setæ, 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 antennæ 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 setæ, and kept, as in _Mysis,_ in constant
whirling motion. The heart acquires new fissures, and interior muscular
trabeculæ.

During the _Mysis_-period, the auditory organs in the basal joint of
the anterior antennæ are formed; the inner branches of the first three
pairs of feet are developed into chelæ and the two hinder pairs into
ambulatory feet; palpi sprout from the mandibles, branchiæ 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 setæ, and the branchiæ 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 Zoëa (Fig. 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.

Fig. 32. Youngest (observed) Zoëa 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.
Fig. 33. Older larva produced from the Zoëa represented in Fig. 32. The
last segment and the last two pairs of feet of the middle-body are
wanting. Magnified. Fig. 32. Youngest (observed) Zoëa 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.
Fig. 33. Older larva produced from the Zoëa represented in Fig. 32. The
last segment and the last two pairs of feet of the middle-body are
wanting. Magnified.


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 chelæ
are developed; the outer branches are lost before an inner branch has
made its appearance on the fourth segment (Fig. 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 chelæ, will certainly be very nearly allied to the preceding
species.[5]

The youngest larva of the Schizopod genus Euphausia observed by Claus,
stands very near the youngest Zoëa of our Prawns; but whilst its
anterior antennæ 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 Zoëa?

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-Zoëa 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 antennæ
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- (maxillæ, 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.[6]

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.

Fig. 34. Embryo of a Squilla, magnified. a. heart. Fig. 35. Older larva
(Zoëa) of a Stomapod, magnified. Fig. 34. Embryo of a Squilla,
magnified. _a._ heart.
Fig. 35. Older larva (Zoëa) of a Stomapod, magnified.


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[7] 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 (Fig. 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 antennæ 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 branchiæ; 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 Zoëa.

 [1] Authorities are cited only for facts which I have had no
 opportunity of confirming.


 [2] Bell (‘Brit. Stalk-eyed Crust.’ p. xlv) 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
 Zoëa 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?


 [3] In a memoir on the metamorphoses of the Porcellanæ I have
 erroneously described this appendage as the “flagellum.”


 [4] _Glaucothoë Peronii,_ M.-Edw., may be a young and still
 symmetrical _Pagurus_ of this kind.


 [5] The oldest observed larvæ (see Fig. 33) are characterised by the
 extraordinary length of the flagella of the outer antennæ, and in this
 respect resemble the larva of _Sergestes_ found by Claus near Messina
 (Zeitschr. für Wiss. Zool. Bd. xiii. Taf. 27, Fig. 14). This unusual
 length of the antennæ leads to the supposition that they belong to our
 commonest Prawn, which is very frequently eaten, and is most nearly
 allied to _Penëus setiferus_ of Florida. Claus’s _Acanthosoma_ (_l.
 c._ Fig. 13) is like the younger _ Mysis_-form of the larva figured by
 me in the ‘Archiv für Naturgeschichte,’ 1836, Taf. 2, Fig. 18, and
 which I am inclined to refer to _Sicyonia carinata._


 [6] Van Beneden, who regards the eye-peduncles as limbs, cannot
 however avoid remarking upon _Mysis_: “Ce pédicule n’apparaît
 aucunement comme les autres appendices, et paraît avoir une autre
 valeur morphologique.”


 [7] ‘Archiv für Naturgeschichte’ 1863. Taf. 1.




CHAPTER VIII.
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.

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 (Fig. 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.

Fig. 36. Embryo of Ligia in the egg, magnified. D. yelk; L. liver. Fig.
37. Maggot-like larva of Ligia, magnified. R remains of the
egg-membrane. We see on the lower surface, from before backwards:--the
anterior and posterior antennæ, the mandibles, the anterior and
posterior maxillæ, maxillipedes, six ambulatory feet, the last segment
of the middle-body destitute of appendages, five abdominal feet, and
the caudal feet. Fig. 36. Embryo of _Ligia_ in the egg, magnified. _D._
yelk; _L._ liver. Fig. 37. Maggot-like larva of _Ligia,_ magnified. _R_
remains of the egg-membrane. We see on the lower surface, from before
backwards:—the anterior and posterior antennæ, the mandibles, the
anterior and posterior maxillæ, 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.

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 Bopyridæ,—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 (Fig. 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_).[1] 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 (_Sphæroma_) and Shield-Slaters (_Cassidina_),
to the Bopyridæ (_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.

Fig. 38. Embryo of a Philoscia in the egg, magnified. Fig. 39. Embryo
of Cryptoniscus planarioides, magnified. Fig. 40. Last foot of the
middle-body of the larva of Entoniscus Porcellanæ, magnified. Fig. 38.
Embryo of a _Philoscia_ in the egg, magnified.
Fig. 39. Embryo of _Cryptoniscus planarioides,_ magnified.
Fig. 40. Last foot of the middle-body of the larva of _Entoniscus
Porcellanæ,_ magnified.

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_ (Fig. 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
Porcellanæ_ very short, imperfectly jointed, and with a large ovate
terminal joint (Fig. 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 _Pranizæ,_ 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.

Fig. 41. Entoniscus Cancrorum, female, magnified. Fig. 42. Cryptoniscus
planarioides, female, magnified. Fig. 43. Embryo of a Corophium,
magnified. Fig. 41. _Entoniscus Cancrorum,_ female, magnified.
Fig. 42. _Cryptoniscus planarioides,_ female, magnified.
Fig. 43. Embryo of a _Corophium,_ magnified.

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 Bopyridæ (_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 antennæ 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 setæ into foliaceous or tongue-shaped and
sometimes ramified branchiæ. In the dwarfish males the eyes, antennæ,
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 _
Porcellanæ,_ the eyes, antennæ, and buccal organs, the segmentation of
the vermiform body, and in one species (Fig. 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 Bopyridæ, that of _Entoniscus
Porcellanæ_ 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,[2] 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.[3]
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,_[4] or where one or more segments are deficient,
as in _Dulichia_ and the _Caprellæ,_ we find the same fusion and the
same deficiencies in young animals taken out of the brood-pouch of
their mother. 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 setæ 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
Hyperinæ which usually live upon Acalephæ. 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.[5]

Figs. 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.) Figs.
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.
Figs. 44–46. 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.)
Figs. 47–49. 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.

Thus, in order to give a few examples, the powerful chelæ of the
antepenultimate pair of feet, of _Phromina sedentaria,_ are produced,
according to Pagenstecher, from simple feet of ordinary structure; and
_vice versà,_ the chelæ 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 Hyperinæ, attain an enormous size, and almost entirely
occupy the head, which then appears spherical, etc.

The difference of the sexes which, in the Gammarinæ is usually
expressed chiefly in the structure of the anterior feet (gnathopoda,
Sp. Bate) and in the Hyperinæ in the structure of the antennæ, 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 _Dercothoë, Lestrigonus_ and _Hyperia_) or
even families (_Hypérines anormales_ and _Hypérines 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,[6]—the females of _Brachyscelus_ are destitute of the posterior
(or inferior) antennæ; the male possesses them like other Amphipodæ; 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.

For example, the younger sexually mature males of _Orchestia
Tucurauna,_ n. sp., have slender inferior antennæ, 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 antennæ 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 Fig.
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.

Fig. 50. Foot of the second pair ('second pair of gnathopoda') of the
male of Orchestia Tucurauna, magnified. Fig. 51. Foot of the second
pair ('second pair of gnathopoda') of the female of Orchestia
Tucurauna, magnified. Fig. 52. Male of a Bodotria, magnified. Note the
long inferior antennæ, which are closely applied to the body, and of
which the apex is visible beneath the caudal appendages. Fig. 50. Foot
of the second pair (“second pair of gnathopoda”) of the male of
_Orchestia Tucurauna,_ magnified.
Fig. 51. Foot of the second pair (“second pair of gnathopoda”) of the
female of _Orchestia Tucurauna,_ magnified.
Fig. 52. Male of a _Bodotria,_ magnified. Note the long inferior
antennæ, 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 _ Diastylidæ_ or _Cumacea._ The
young, which Kröyer 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.[7] The caudal portion of the
embryo in the _Diastylidæ,_ 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.”

 [1] 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 antennæ, 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).


 [2] nIn the genera _Orchestoidea, Orchestia, Allorchestes, Montagua,
 Batea_ n.g., _Amphilochus, Atylus, Microdeutopus, Leucothoë, Melita,
 Gammarus_ (according to Meissner and La Valette), _Amphithoë, Cerapus,
 Cyrtophium, Corophium, Dulichia, Protella_ and _Caprella._ote


 [3] 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._


 [4] 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.


 [5] 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 _Beroë
 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 (Fig. 44) the two following pairs
 (Fig. 45) and finally the three last pairs (Fig. 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 (Fig. 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 antennæ, of which the
 anterior bear an unusual abundance of olfactory filaments.
    Their youngest larvæ of course cannot swim; they are helpless
    little animals which firmly cling especially to the swimming laminæ
    of their host; the adult _ Hyperiæ,_ 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 rapidité extrême,” says
    Van Beneden of _H. Latreillii_ M.-Edw.)
    The transformation of the _Hyperiæ_ 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. für
    Naturgeschichte’ 1852, I. p. 91).


 [6] “I know of no case in which the inferior (antennæ) are obsolete,
 when the superior are developed,” Dana. (Darwin, ‘Monograph on the
 Subclass Cirripedia, Lepadidæ’ p. 15.)


 [7] A trustworthy English Naturalist, Goodsir, described the
 brood-pouch and eggs of _Cuma_ as early as 1843. Kröyer, 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 Diastylidæ are full-grown
 animals or larvæ, 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 larvæ (see Van Beneden, ‘Rech. sur la Fauna
 littor. de Belgique’ Crustacées, pp. 73, 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’édriophthalmes que nous avons observées sur nos côtes, nous n’en
 avons pas vu une seule qui eût même la moindre resemblance avec un
 _Cuma_ quelconque.” The _only thing_ that suits the larvæ of
 _Hippolyte, Palæmon_ and _Alpheus,_ in the family character of the
 Cumacea as given by Kröyer which occupies three pages (Kröyer,
 ‘Naturh. Tidsskrift, Ny Raekke,’ Bd. ii. pp. 203–206) 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 _Palæmon_ (Fig. 27) and the Cumacean (Fig. 52)
 in order to be convinced of their extraordinary similarity!




CHAPTER IX.
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,[1] have to undergo a
metamorphosis. Mecznikow has recently observed the development of
_Nebalia,_ and concludes from his observations “that _Nebalia,_ during
its embryonal life, passes through the Nauplius- and Zoëa-stages, which
in the Decapoda occur partly (in _Penëus_) in the free state.”
“Therefore,” says he, “I regard Nebalia as a Phyllopodiform Decapod.”
The youngest larvæ [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 maxillæ of the higher Crustacea.[2] We might regard the
Phyllopoda as Zoëæ 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 larvæ 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.[3]

All the larvæ of the free Copepoda investigated by Claus, have, at the
earliest period, three pairs of limbs (the future antennæ and
mandibles), the anterior with a single, and the two following ones with
a double series of joints, or branchiæ. 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 setæ,
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 maxillæ.” 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 antennæ 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 setæ, and the body consists of the
oval cephalothorax, the second, third, and fourth thoracic segments,
and an elongated terminal joint. In the Cyclopidæ the posterior antennæ
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.”

Figs. 53 and 54. Nauplii of Copepoda, the former magnified, the latter
magnified 2x. Fig. 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. Fig. 53 and 54. Nauplii of
Copepoda, the former magnified, the latter magnified 2x.
Fig. 55. Nauplius of _Tetraclita porosa_ after the first moult,
magnified. 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.[4]

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 (antennæ), here occur separated from them.

The anterior antennæ of the Copepoda, Cladocera, Phyllopoda (Leydig,
Claus), Ostracoda (at least the Cypridinæ), Diastylidæ, 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 larvæ of the Cirripedia and Rhizocephala, directly from the
brain.

(Fig. 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 setæ. Fig. 57. Pupa of a Balanide (Chthamalus ?), magnified.
The adherent feet are retracted within the rather opaque anterior part
of the shell. Fig. 58. Pupa of Sacculina purpurea, magnified. The
filaments on the adherent feet may be the commencements of the future
roots. Fig. 56. Nauplius of _Sacculina purpurea,_ shortly before the
second moult, magnified. We may recognise in the first pair of feet the
future adherent feet, and in the abdomen six pairs of natatory feet
with long setæ.
Fig. 57. Pupa of a Balanide (_Chthamalus ?_), magnified. The adherent
feet are retracted within the rather opaque anterior part of the shell.
Fig. 58. Pupa of _Sacculina purpurea,_ magnified. The filaments on the
adherent feet may be the commencements of the future roots.


At the base of the inferior antennæ 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 antennæ, 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
antennæ). 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
antennæ (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 larvæ of _Lepas,_ and
indeed the resemblance of the frontal horns with the conical processes
on the inferior antennæ of the Amphipoda, is complete throughout.[5]

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, setæ, 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 “pupæ,” 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 antennæ,” 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 setæ
have been formed beneath the Nauplius-skin, and behind these are two
short, setigerous caudal appendages (Fig. 58).

The pupæ of the Cirripedia (Fig. 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;[6] they are especially distinguished from them by the
possession of a pair of composite eyes. Sometimes also traces of the
frontal horns seem to persist.[7]

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 pupæ 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,
_Porcellanæ,_ 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 (Figs. 59, 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.[8]

Fig. 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. Fig. 60. Young
Sacculina purpurea with its roots; the animal purple-red, the roots
dark grass-green. Magnified. Figs. 61 to 63. Eggs of Tetraclita porosa
in segmentation, magnified. The larger of the two first-formed spheres
of segmentation is always turned towards the pointed end of the egg.
Fig. 64. Egg of Lernæodiscus Porcellanæ, in segmentation, magnified 90
Fig. 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.
Fig. 60. Young _Sacculina purpurea_ with its roots; the animal
purple-red, the roots dark grass-green. Magnified.
Figs. 61–63. Eggs of _Tetraclita porosa_ in segmentation, magnified.
The larger of the two first-formed spheres of segmentation is always
turned towards the pointed end of the egg.
Fig. 64. Egg of _Lernæodiscus Porcellanæ,_ in segmentation, magnified.


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 larvæ to leave the
egg in the pupa-form, in the case of a Rhizocephalon (_Peltogaster ?_)
found by Dr. Powell in the Mauritius.

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 (Fig. 64), and, as I can add,
in the Cirripedia (Figs. 61–63) 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.[9]

 [1] 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 _Artemiæ_ 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.


 [2] “The maxilla of the Decapod-larva (Krebslarve) is a sort of
 Phyllopodal foot” (Claus).


 [3] I am still unacquainted with Claus’ latest and larger work, but no
 doubt the same may be said of it.


 [4] 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 (_Eleuthéronotes_). 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
 larvæ,” or Cirriped-pupæ 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_ (Fig. 59) with the family of the sausages.


 [5] In connexion with this it may be mentioned that, in the females of
 Brachyscelus, in which the posterior antennæ are deficient, the
 conical processes with the canal permeating them are nevertheless
 retained.


 [6] Compare the figure given by Darwin (Balanidæ Pl. xxx fig. 5) of
 the first natatory foot of the pupa of _Lepas australis,_ with that of
 _Lernæodiscus Porcellanæ_ published in the ‘Archiv für
 Naturgeschichte’ (1863, Taf. iii, fig. 5). The sole distinction, that
 in the latter there are only 3 setæ 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.


 [7] Darwin describes as “acoustic orifices” small apertures in the
 shell of the pupæ 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.


 [8] The roots of _Sacculina purpurea_ (Fig. 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_
 (Fig. 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.


 [9] I have not mentioned the Pycnogonidæ, 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 X.
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,”[1] who may have written here, as a
Portuguese proverb says “straight in crooked lines.”[2] 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; _Palæmon,_ like the Crabs, as a Zoëa; and
_Penéus,_ 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 Müller, 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 Müller 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,[3] 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” (§ 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”
(§ 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” (§ 319). “_Hence the embryos of different animals resemble
each other the more, the younger they are_” (§ 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” (§ 321). “_A system, in order to be true and natural,
must agree with the sequence of the organs in the development of the
embryo_” (§ 322).

I do not know whether any one at the present day will be inclined to
subscribe to this proposition in its whole extent.[4] 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 (§ 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 _à 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 antennæ in the Prawns would be more important than the
flagellum; the maxillipedes of the Decapoda would be more important
than the chelæ 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 setæ of the tail would be more important
than all the natatory feet; in the Cirripedia the posterior antennæ, 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 Müller
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 Figs. 61–64) proceed different forms of Nauplius,
these become converted into exceedingly similar pupæ, and from the pupæ
again proceed sexually mature animals, differing from each other _toto
cœlo._

“_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 larvæ. 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
vertebræ, 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 Corycæidæ, Claus found all the ventral ganglia fused
together into a single broad mass, and in the Calanidæ 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
Corycæidæ and the Crabs, or the Calanidæ and the Lobsters.—Or to the
organs of circulation? We have among the Copepoda, the Cyclopidæ and
Corycæidæ without a heart, side by side with the Calanidæ and
Pontellidæ with a heart. And in the same way among the Ostracoda, the
_Cypridinæ,_ 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 branchiæ 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
_Lernææ_ 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 _Penëus_ 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, _Penëus._)


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. _Zoëogona._—Full number of limbs not produced in the egg.
Zoëa-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.”

 [1] “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’).


 [2] “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.


 [3] ‘Principles of Zoology’ Part I. Comparative Physiology. By Louis
 Agassiz and A.A. Gould. Revised Edition. Boston, 1856.


 [4] 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.




CHAPTER XI.
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 (_Polynoë_) 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 larvæ 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.

(Figs. 65 to 67. Young Tubicolar worms, magnified with the simple lens:
65. Without operculum, Protula-stage. 66. With a barbate opercular
peduncle, Filograna-stage; With a naked opercular peduncle,
Serpula-stage. Figs. 65–67. Young Tubicolar worms, magnified with the
simple lens: 65.[1] Without operculum, _Protula_-stage; 66. With a
barbate opercular peduncle, _Filograna_-stage; 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 (Fig. 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 (Fig. 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 (Fig. 67), and the worms had become
_Serpulæ._ 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 _Protulæ,_ 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
_Serpulæ_ 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 _Serpulæ_ 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 larvæ 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 larvæ 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 larvæ 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 larvæ, 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 larvæ which have to provide for
themselves, as upon adult animals. The changes of the larvæ,
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 Blattidæ, 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 larvæ and
pupæ; but, on the contrary, Nauplii and Zoëæ 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.[2]

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.

 [1] Fig. 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.


 [2] 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
 pupæ 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 maxillæ” (Gerstäcker).
    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” (Gerstäcker). The
    Orthoptera with eleven segments in the abdomen, agree perfectly in
    the number of their body-segments with the Prawn-larva represented
    in Fig. 33, or indeed, with the higher Crustacea (Podophthalma and
    Edriophthalma) in general, in which the historically youngest last
    thoracic segment (see Chapter 12), 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” (Gerstäcker).
    4. Their palæontological 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” (Gerstäcker).
    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, Schizæa_ and _Lygodium,_ belonging to the group
    _Schizæaceæ_ which is very poor in species, differ much more from
    each other than any two forms of the group _Polypodiaceæ_ 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 (_Penéus_)
    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 larvæ 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 larvæ of the existing Orthoptera. The circumstance that
    there are still numerous wingless species among the Orthoptera, and
    that some of these (_Blattidæ_) 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 larvæ of
    the Coleoptera, or the caterpillars of the Lepidoptera, still less
    any bearing even a distant resemblance to the quiescent pupæ of
    these animals. The pupæ, 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 larvæ, 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 palæontological 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.




CHAPTER XII.
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 Zoëa
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, _Porcellanæ,_
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 Zoëa of
this species particularly from other Zoëæ (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 Zoëæ 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 Zoëa of five pairs of
limbs. It is to be supposed that, not only originally but even still,
in the larvæ 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 Fig. 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 larvæ of the Stomapoda the last three abdominal segments
are destitute of limbs, which are still wanting on the last of them in
older larvæ; 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.[1]

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, Peneïdæ, and, indeed,
of the Macrura in general. At present the multifarious Zoëa-forms
appear to be particularly instructive. Almost all the peculiarities by
which they depart from the primitive form of the Zoëa of _Penëus_
(Figs. 29, 30, 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 _Palæmon, 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 _Glaucothoë_-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 Zoëæ
of the Crabs, the _Porcellanæ,_ 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 antennæ.
The second pair of antennæ, which, in the various Zoëæ always remains a
step behind that of the adult animal, is particularly remarkable. In
the Crabs the “scale” is entirely wanting; their Zoëæ 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 Zoëæ have a well-developed
but inarticulate scale. A precisely similar scale is possessed by the
adult Prawns, in the Zoëæ of which it exists still in a jointed form,
like the outer branch of the second pair of feet of the Nauplius or
_Penëus_-Zoëa.

The long, spiniform processes on the carapace of the Zoëæ of the Crabs
and _Porcellanæ_ are not to be explained in this way, but their
advantage to the larvæ is evident. Thus, for example, if the body of
the Zoëa of _Porcellana stellicola_ (Fig. 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.[2]
Consequently these processes of the carapace may be regarded as
acquired by the Zoëa itself in the struggle for existence.

The formation of new limbs beneath the skin of the larvæ 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 larvæ 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 lamellæ”) are formed freely on the ventral surface in
_Euphausia_ and the Prawns with Nauplius-brood, and within the caudal
lamellæ in the Prawns with Zoëa-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_ (Figs. 36, 37) this larva-skin has lost the last traces of
limbs, and in _Philoscia_ (Fig. 38) it is scarcely demonstrable.

Like the spinous processes of the Zoëæ, the chelæ 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 Zoëæ of the Crabs, the _Porcellanæ,_
the Tatuira, the Hermit Crabs, and the Prawns with Zoëa-brood agree,
and by which they are in common distinguished from the larvæ of
_Penëus_ 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 Zoëa-form. But then neither _Penëus_ with its
Nauplius-brood, nor even apparently the _Palinuri_ could be referred
back to this ancestor. The mode of development of _Penëus_ and
_Palinurus,_ as also several peculiar larvæ 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 Zoëa-form in all the above-mentioned Decapoda, its existence in
_Penëus_ 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 Zoëa
of _Penëus_ in the Schizopod genus _Euphausia,_) and the reminiscence
of the structure of Zoëa, which even the adult _Tanais_ has preserved
in its mode of respiration,—all indicate Zoëa 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 Zoëa-form in
different families, in which the earlier stages of development are
effaced. But except what is common to all Zoëæ, and what may easily be
explained as being transferred back from a later into this stage, the
Zoëæ of the Crabs, for example, agree with those of _Pagurus_ and
_Palæmon_ 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 setæ in the most different Zoëæ of Crabs (Figs. 19–23) 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 _Penëus,_ still produces Nauplius-brood.[3]

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
larvæ 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 Tanaidæ.[4] 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 larvæ 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 pupæ 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 cæcal branchlets. In the pupæ of the
Rhizocephala (Fig. 58) the foremost limbs (“prehensile antennæ”) 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 pupæ of _Lepas australis_
on the last joints of the “prehensile antennæ.” From the perfect
accordance in their entire structure shown by the pupæ 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 antennæ; 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 antennæ of the
pupæ 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
antennæ, 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 Balanidæ 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 cæcal 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, (Lepadidæ Pl. iv, figs. 1–7)
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 maxillæ 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
Zoëæ, the Insecta possess three pairs of limbs serving for the
reception of nourishment, and three pairs serving for locomotion; like
the Zoëæ they have an abdomen without appendages; as in all Zoëæ 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 Zoëa 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.

 [1] 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 Porcellanæ_
 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, Balanidæ, page 111.)


 [2] _Persephone,_ a rare Crab, belonging to the family Leucosiidæ, 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.


 [3] 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 “Eustomés.” 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.


 [4] 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 _Rhœa._ The latter, as is
 well known, is the only Isopod which possesses a secondary flagellum
 on the anterior antennæ. I have recently obtained a new and unexpected
 proof that the _Tanaidæ_ (“Asellotes hétéropodes” 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.”




INDEX.


(Numbers represent chapter number. Click on number and use 'Find' in
the 'Edit' menu of your browser. Numbers in parentheses denote number
of references in the chapter.)

Acanthonotus Owenii, 2.
Acanthosoma, 7.
Achæus, 7 (2).
Achtheres, 12.
—— percarum, 9 (2).
Allorchestes, 4, 8.
Alpheus, 6, 7, 12.
Amphilochus, 2, 8.
Amphipoda, 3, 6, 8 (2), 12.
Amphithoë, 2, 8.
Anceus, 8.
Anelasma squalicola, 12.
Anilocra, 6.
Aratus, 2.
—— Pisonii, 5.
Artemia, 9.
Asellus, 8.
Atylus, 8.
—— carinatus, 2.

Batea, 8.
Bodotria, 4, 8.
Bopyridæ, 8 (2).
Bopyrus, 8 (2).
Brachyscelus, 6, 8 (2), 9, 10 (2), 12.
—— crusculum, 8.
Brachyura, 12.
Branchiopoda, 9.

Calanidæ, 10.
Caligus, 8.
Caprella, 8 (3).
—— attenuata, 6.
—— linearis, 6.
Carcinus mænas, 7.
Caridina, 7.
Cassidina, 6, 8 (3).
Cerapus, 2, 8 (3).
Chalimus, 8.
Chondracanthus, 9.
Chthamalus, 9.
Cirripedia, 9 (2), 10.
Cladocera, 9.
Copepoda, 4, 9 (2), 10, 12.
Corophium, 8 (2).
—— dentatum, 8.
Corycæidæ, 10.
Crangon, 7.
_Crayfish_, 10.
Cryptoniscus planarioïdes, 8 (2).
Cryptophialus, 12.
—— minutus, 9.
Cuma, 8.
Cumacea, 8.
Cyclograpsus, 4, 5, 7.
Cyclopidæ, 10.
Cyclops, 9 (2), 10.
Cyclopsine, 9.
Cymothoa, 8.
_Cymothoadiens_, 8.
Cypridina, 10.
Cypris, 8, 10.
Cyrtophium, 2, 8, 10.
Cythere, 10.

Daphnia pulex, 8.
Dercothoë, 8.
Diastylidæ, 4, 8.
Dichelaspis Warwickii, 12.
Dulichia, 8 (2), 10.

Edriophthalma, 3, 6, 8.
Entomostraca, 9.
Entoniscus, 8.
—— cancrorum, 6, 8 (2).
—— porcellanæ, 6, 8 (2), 12.
Erichthus, 7.
Eriphia gonagra, 2, 5.
Euphausia, 7 (2), 10, 12 (2).
Eurynome, 7.
Evadne, 8.

Filograna, 11.

Gammarus, 8.
—— ambulans, 8.
—— Dugesii, 4.
—— puteanus, 6.
Gecarcinus, 7 (2).
Gelasimus, 2, 4 (2), 5, 7.
—— vocans, 5.
Glaucothoë Peronii, 7.
Grapsus, 5 (2).

_Hermit Crabs_, 7 (3), 12.
Hippa emerita, 7 (2).
Hippolyte, 7 (2), 12.
Hyperia galba, 8.
—— Latreillei, 8.
—— Martinezii, 8.
“_Hypérines anormales et ordinaires,_” 6, 8.

Idothea, 8 (2).
Insecta, 11.
Isopoda, 3, 6 (2), 8, 12.

Kepone, 8.

Læmodipoda, 6.
Lepas, 9 (2).
—— anatifera, 6.
—— australis, 9, 12.
Lernæodiscus porcellanæ, 9.
Lernanthropus, 9.
Lestrigonus, 8 (2).
Leucifer, 7, 8, 10, 12.
Leucothoë, 2, 8.
Ligia, 8 (2), 10, 12.
_Lobster_, 7, 10.
Lupea diacantha, 5.

Macrura, 7, 12.
Maia, 7 (2).
Megalops, 12.
Melita, 8.
—— anisochir, 2.
—— exilii, 2 (2), 4.
—— Fresnelii, 2 (3).
—— insatiabilis, 4 (2).
—— Messalina, 4 (2).
—— palmata, 2, 4.
—— setipes, 2.
—— valida, 2.
Microdeutopus, 3, 8.
Montagua, 8.
Mysis, 7, 8, 10 (3), 12 (2).

“Nauplius-larvæ”, 3 (3), 7, 8, 9 (3), 12 (2).
Nebalia, 9.
Niphargus, 6.

Ocypoda, 2, 5 (2), 7.
—— rhombea, 5.
Orchestia, 8 (2), 10.
—— Darwinii, 4 (2).
—— gryphus, 4.
—— sylvicola, 4 (2).
—— tahitensis, 4.
—— telluris, 4 (2).
—— Tucurauna, 8.
—— Tucuratinga, 8.
Orchestoidea, 8.

Pagurus, 12 (2).
Palæmon, 7 (3), 10, 12 (2).
Palinurus, 7, 10, 12 (2).
Peltogaster, 9.
—— socialis, 9.
Penëus, 3, 10, 12 (2).
—— setiferus, 7.
Persephone, 12.
Philoscia, 8 (2), 12.
Phronima, 6, 10.
—— sedentaria, 8.
Phryxus, 8.
Phyllopoda, 9.
Phyllosoma, 7 (2).
Pinnotheres, 7.
Podophthalma, 7.
Polyphemus, 10.
Pontellidæ, 10.
Porcellana, 7 (3), 10, 12 (2).
—— stellicola, 7, 12.
Porcellionides, 8.
Praniza, 8.
_Prawns_, 7, 12.
Protella, 8.
Protula, 11.
Pycnogonidæ, 9.

Ranina, 2, 5.
Rhizocephala, 9 (2), 10, 12.

Sacculina purpurea, 9 (5).
Scalpellum Peronii, 12.
Sergestes, 7.
Serpulæ, 11.
Sesarma, 2, 4, 5, 7.
_Shrimps_, 7.
Sphæroma, 8.
Squilla, 6, 7 (2), 12.

Talitrus, 8.
Tanais, 6 (2), 8 (2), 10, 12.
—— dubius, 4.
—— Dulongii, 3, 4.
_Tatuira_, 7 (3), 12.
Tetraclita porosa, 9 (2).
_Trilobites_, 9.

Xantho, 7.
Xiphosura, 9.

Zoëæ, 3 (2), 6, 7 (2), 12 (3).






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