It
cannot, therefore, surprise us that in transitions from the original
mode of metamorphosis to direct development, even when produced
independently, the larval life commences in the same way with this
Zoea-form in different families, in which the earlier stages of
development are effaced. But except what is common to all Zoeae, and
what may easily be explained as being transferred back from a later into
this stage, the Zoeae of the Crabs, for example, agree with those of
Pagurus and Palaemon in no single peculiarity of structure which leads
us to suppose a common inheritance. Consequently we may apparently
assume, without hesitation, that when the Brachyura and Macrura
separated, the primitive ancestors of each of these groups passed
through a more complete metamorphosis, and that the transition to the
present mode of development belongs to a later period. With regard to
the Brachyura, it may be added that in them this transition occurred
only a little later and indeed before the existing families separated.
The arrangement of the processes of the carapace, and, still more, the
similar number of the caudal setae in the most different Zoeae of Crabs
(Figures 19 to 23) prove this. Such an accordance in the number of
organs apparently so unimportant is only explicable by common
inheritance. We may predict with certainty that amongst the Brachyura no
species will occur which, like Peneus, still produces Nauplius-brood.
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