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.