Facts and Arguments for Darwin | Page 6

from those of the following group. The segments of the middle-body seem always to put forth limbs immediately after their own appearance, whilst the segments of the hind-body often remain destitute of feet through long portions of the larval life or even throughout life (as in many female Diastylidae), a reason, among many others, for not, as is usual, regarding the middle-body of the Crustacea as equivalent to the constantly footless abdomen of Insects. The appendages of the middle-body (pereiopoda) seem never, even in their youngest form, to possess two equal branches, a peculiarity which usually characterises the appendages of the hind-body. This is a circumstance which renders very doubtful the equivalence of the middle-body of the Malacostraca with the section of the body which in the Copepoda bears the swimming feet and in the Cirripedia the cirri.
The comprehension of the feet of the hind-body and tail in a single group (as "fausses pattes abdominales," or as "pleopoda") seems not to be justifiable. When there is a metamorphosis, they are probably always produced at different periods, and they are almost always quite different in structure and function. Even in the Amphipoda, in which the caudal feet usually resemble in appearance the last two pairs of abdominal feet, they are in general distinguished by some sort of peculiarity, and whilst the abdominal feet are reproduced in wearisome uniformity throughout the entire order, the caudal feet are, as is well-known, amongst the most variable parts of the Amphipoda.)
And if at the present day the majority of the Crabs and Macrura, and indeed the Stalk-eyed Crustacea in general, pass through Zoea-like developmental states, and the same mode of transformation was to be ascribed to their ancestors, the same thing must also apply, if not to the immediate ancestors of the Amphipoda and Isopoda, at least to the common progenitors of these and the Stalk-eyed Crustacea. Any such assumption as this was, however, very hazardous, so long as not a single fact properly relating to the Edriophthalma could be adduced in its support, as the structure of this very coherent group seemed to be almost irreconcilable with many peculiarities of the Zoea. Thus, in my eyes, this point long constituted one of the chief difficulties in the application of the Darwinian views to the Crustacea, and I could scarcely venture to hope that I might yet find traces of this passage through the Zoea-form among the Amphipoda or Isopoda, and thus obtain a positive proof of the correctness of this conclusion. At this point Van Beneden's statement that a cheliferous Isopod (Tanais Dulongii), belonging, according to Milne-Edwards, to the same family as the common Asellus aquaticus, possesses a carapace like the Decapoda, directed my attention to these animals, and a careful examination proved that these Isopods have preserved, more truly than any other adult Crustacea, many of the most essential peculiarities of the Zoeae, especially their mode of respiration. Whilst in all other Oniscoida the abdominal feet serve for respiration, these in our cheliferous Isopod (Figure 2) are solely motory organs, into which no blood-corpuscle ever enters, and the chief seat of respiration is, as in the Zoeae, in the lateral parts of the carapace, which are abundantly traversed by currents of blood, and beneath which a constant stream of water passes, maintained, as in Zoeae and the adult Decapoda, by an appendage of the second pair of maxillae, which is wanting in all other Edriophthalma.
For both these discoveries, it may be remarked in passing, science is indebted less to a happy chance than immediately to Darwin's theory.
Species of Peneus live in the European seas, as well as here, and their Nauplius-brood has no doubt repeatedly passed unnoticed through the hands of the numerous naturalists who have investigated those seas, as well as through my own,* for it has nothing which could attract particular attention amongst the multifarious and often wonderful Nauplius-forms. (* Mecznikow has recently found Naupliiform shrimp-larvae in the sea near Naples.) When I, fancying from the similarity of its movements that it was a young Peneus-Zoea, had for the first time captured such a larva, and on bringing it under the microscope found a Nauplius differing toto coelo from this Zoea, I might have thrown it aside as being completely foreign to the developmental series which I was tracing, if the idea of early Naupliiform stages of the higher Crustacea, which indeed I did not believe to be still extant, had not at the moment vividly occupied my attention.
And if I had not long been seeking among the Edriophthalma for traces of the supposititious Zoea-state, and seized with avidity upon everything that promised to made this refractory Order serviceable to me, Van Beneden's short statement could hardly have affected me so much in the manner of an electric shock, and impelled