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
provide for itself, the Nauplius has become degraded into a mere skin;
in Ligia (Figures 36 and 37) this larva-skin has lost the last traces of
limbs, and in Philoscia (Figure 38) it is scarcely demonstrable.
Like the spinous processes of the Zoeae, the chelae on the penultimate
pair of feet of the young Brachyscelus are to be regarded as acquired by
the larva itself.
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