But let us stick to the Crustacea. In Polyphemus Leydig finds the first
traces of the intestinal tube even during segmentation. In Mysis a
provisional tail is first formed, and in Ligia a maggot-like larva-skin.
The simple median eye appears earlier, and would therefore be more
important than the compound paired eyes; the scale of the antennae in
the Prawns would be more important than the flagellum; the maxillipedes
of the Decapoda would be more important than the chelae and ambulatory
feet, and the anterior six pairs of feet in the Isopoda, than the
precisely similarly formed seventh pair; in the Amphipoda the most
important of all organs would be the "micropylar apparatus," which
disappears without leaving a trace soon after hatching; in Cyclops the
setae of the tail would be more important than all the natatory feet; in
the Cirripedia the posterior antennae, as to which we do not know what
becomes of them, would be more important than the cirri, and so forth.
The most unimportant of all organs would be the sexual organs, and the
most essential peculiarity would consist in colour, which is to be
referred back to the ovarian egg.
"The embryos, or young states of different animals, resemble each other
the more, the younger they are," or, as Johannes Muller expresses it,
"they approach the more closely to the common type." Different as may be
the ideas connected with the word "type," no one will dispute that the
typical form of the penultimate pair of feet in the Amphipoda is that of
a simple ambulatory foot, and not that of a chela, for the latter occurs
in no single adult Amphipod; we know it only in the young of the genus
Brachyscelus, which therefore in this respect undoubtedly depart more
widely than the adults from the type of their order.
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