naupliiform larva brings with it out of the egg; it is afterwards
divided, by the younger sections which become developed in its middle,
into the head and tail. To this primitive body belong the two pairs of
antennae, the mandibles and the caudal feet ("posterior pair of
pleopoda," Sp. B.). Even in the mature animal the fact that these
terminal sections belong to one another is sometimes betrayed by the
resemblance of their appendages, especially that of the outer branch of
the caudal feet, with the outer branch (the 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.