scarcely half a line in length, whilst with the processes it is four
lines long, a mouth of eight times the width is necessary in order to
swallow the little animal when thus armed.* (* Persephone, a rare Crab,
belonging to the family Leucosiidae, is served in the same manner by its
long chelate feet. If we seize the animal, it extends them most
obstinately straight downwards, so that in all probability we should
more easily break than bend them.) Consequently these processes of the
carapace may be regarded as acquired by the Zoea itself in the struggle
for existence.

The formation of new limbs beneath the skin of the larvae is also to be
referred to an earlier occurrence of processes which originally took
place at a later period. The original course must have been that they
sprouted forth in a free form upon the ventral surface of the larva in
the next stage after the change of skin; whilst now they are developed
before the change of skin, and thus only come into action a stage
earlier. In larvae which, for other reasons, must be regarded as more
nearly approaching the primitive form, the original mode usually
prevails in this particular also. Thus the caudal feet (the "lateral
caudal lamellae") are formed freely on the ventral surface in Euphausia
and the Prawns with Nauplius-brood, and within the caudal lamellae in
the Prawns with Zoea-brood, in Pagurus and Porcellana.

A compression of several stages into one, and thereby an abridgement and
simplification of the course of development, is expressed in the
simultaneous appearance of several new pairs of limbs.

How earlier young states may gradually be completely lost, is shown by
Mysis and the Isopoda. In Mysis there is still a trace of the
Nauplius-stage; being transferred back to a period when it had not to