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.
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