Facts and Arguments for Darwin | Page 6

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

The comprehension of the feet of the hind-body and tail in a single
group (as "fausses pattes abdominales," or as "pleopoda") seems not to
be justifiable. When there is a metamorphosis, they are probably
always produced at different periods, and they are almost always quite
different in structure and function. Even in the Amphipoda, in which
the caudal feet usually resemble in appearance the last two pairs of
abdominal feet, they are in general distinguished by some sort of
peculiarity, and whilst the abdominal feet are reproduced in wearisome
uniformity throughout the entire order, the caudal feet are, as is
well-known, amongst the most variable parts of the Amphipoda.)
And if at the present day the majority of the Crabs and Macrura, and
indeed the Stalk-eyed Crustacea in general, pass through Zoea-like
developmental states, and the same mode of transformation was to be
ascribed to their ancestors, the same thing must also apply, if not to the
immediate ancestors of the Amphipoda and Isopoda, at least to the
common progenitors of these and the Stalk-eyed Crustacea. Any such
assumption as this was, however, very hazardous, so long as not a
single fact properly relating to the Edriophthalma could be adduced in
its support, as the structure of this very coherent group seemed to be
almost irreconcilable with many peculiarities of the Zoea. Thus, in my
eyes, this point long constituted one of the chief difficulties in the
application of the Darwinian views to the Crustacea, and I could
scarcely venture to hope that I might yet find traces of this passage
through the Zoea-form among the Amphipoda or Isopoda, and thus
obtain a positive proof of the correctness of this conclusion. At this
point Van Beneden's statement that a cheliferous Isopod (Tanais
Dulongii), belonging, according to Milne-Edwards, to the same family
as the common Asellus aquaticus, possesses a carapace like the
Decapoda, directed my attention to these animals, and a careful
examination proved that these Isopods have preserved, more truly than
any other adult Crustacea, many of the most essential peculiarities of
the Zoeae, especially their mode of respiration. Whilst in all other
Oniscoida the abdominal feet serve for respiration, these in our
cheliferous Isopod (Figure 2) are solely motory organs, into which no
blood-corpuscle ever enters, and the chief seat of respiration is, as in
the Zoeae, in the lateral parts of the carapace, which are abundantly

traversed by currents of blood, and beneath which a constant stream of
water passes, maintained, as in Zoeae and the adult Decapoda, by an
appendage of the second pair of maxillae, which is wanting in all other
Edriophthalma.
For both these discoveries, it may be remarked in passing, science is
indebted less to a happy chance than immediately to Darwin's theory.
Species of Peneus live in the European seas, as well as here, and their
Nauplius-brood has no doubt repeatedly passed unnoticed through the
hands of the numerous naturalists who have investigated those seas, as
well as through my own,* for it has nothing which could attract
particular attention amongst the multifarious and often wonderful
Nauplius-forms. (* Mecznikow has recently found Naupliiform
shrimp-larvae in the sea near Naples.) When I, fancying from the
similarity of its movements that it was a young Peneus-Zoea, had for
the first time captured such a larva, and on bringing it under the
microscope found a Nauplius differing toto coelo from this Zoea, I
might