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.). This might be due to the animals having separated from
the common stem before these limbs were formed at all. But in those
cases with which I am best acquainted, it seems to me more probable that
the limbs have been subsequently lost again. That these particular limbs
and segments are more easily lost than others is explained by the
circumstance that, as the youngest, they have been less firmly fixed by
long-continued inheritance. ("Mr. Dana believes, that in ordinary
Crustaceans, the abortion of the segments with their appendages almost
always takes place at the posterior end of the cephalothorax."--Darwin,
Balanidae, page 111.))

The original development of the Malacostraca starting from the Nauplius,
or the lowest free-living grade with which we are acquainted in the
class of Crustacea, is now-a-days nearly effaced in the majority of
them. That this extinction has actually taken place in the way already
deduced as a direct consequence from Darwin's theory, will be the more
easily demonstrated, the more this process is still included in the
course of life, and the less completely it is already worn out. We may
hope to obtain the most striking examples in the still unknown
developmental history of the various Schizopoda, Peneidae, and, indeed,
of the Macrura in general. At present the multifarious Zoea-forms appear
to be particularly instructive. Almost all the peculiarities by which
they depart from the primitive form of the Zoea of Peneus (Figures 29,