It is to be supposed that, not only originally but even still, in
the larvae of the first Malacostraca, the new body-segments and pairs of
limbs are formed singly,--first of all the segments of the fore-body,
then those of the abdomen, and finally those of the middle-body,--and,
moreover, that in each region of the body the anterior segments were
formed earlier than the posterior ones, and therefore last of all the
hindermost segment of the middle-body. Of this original mode more or
less distinct traces still remain, even in species in which, in other
respects, the course of development of their ancestors is already nearly
effaced. Thus the abdominal feet of the Prawn-larva represented in
Figure 33, are formed singly from before backwards, and after these the
last feet of the middle-body; thus, in Palinurus, the last two pairs of
feet of the middle-body are formed later than the rest; thus in the
young larvae of the Stomapoda the last three abdominal segments are
destitute of limbs, which are still wanting on the last of them in older
larvae; and thus, in the Isopoda, the historically newest pair of feet
is produced later than all the rest. In the Copepoda this formation of
new segments and limbs, gradually advancing from before backwards, is
more perfectly preserved than in any of the higher Crustacea.* (* It is
well known that, in many cases, even in adult animals the last segment
of the middle-body, or some of its last segments, either want their
limbs or are themselves deficient (Entoniscus Porcellanae male,
Leucifer, etc.
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