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).