In the Hermit Crabs a similar, usually
moveable, spiniform process occurs as the remains of the scale; their
Zoeae have a well-developed but inarticulate scale. A precisely similar
scale is possessed by the adult Prawns, in the Zoeae of which it exists
still in a jointed form, like the outer branch of the second pair of
feet of the Nauplius or Peneus-Zoea.
The long, spiniform processes on the carapace of the Zoeae of the Crabs
and Porcellanae are not to be explained in this way, but their advantage
to the larvae is evident. Thus, for example, if the body of the Zoea of
Porcellana stellicola (Figure 24), without the processes of the carapace
and without the abdomen, which however is not rigidly extensible, is
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.
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