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.
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