Almost all the peculiarities by which
they depart from the primitive form of the Zoea of Peneus (Figures 29,
30 and 32), may in fact be conceived as transferred back from a later
period into this early period of life. This is the case with the large
compound eyes,--with the structure of the heart,--with the raptorial
feet in Squilla,--and with the powerful, muscular, straightly-extended
abdomen in Palaemon, Alpheus, Hippolyte, and the Hermit Crabs. (In the
latter, indeed, the abdomen of the adult animal is a shapeless sac
filled with the liver and generative organs, but it is still tolerably
powerful in the Glaucothoe-stage, and was certainly still more powerful
when this stage was still the permanent form of the animal.) It is also
the case with the abdomen of the Zoeae of the Crabs, the Porcellanae,
and the Tatuira, which is still powerful, although usually bent under
the breast; the two last swim tolerably by means of the abdomen, even
when adult, as do the true Crabs in the young state known as Megalops.
It is the case, lastly, with the conversion of the two anterior pairs of
limbs into antennae. The second pair of antennae, which, in the various
Zoeae always remains a step behind that of the adult animal, is
particularly remarkable. In the Crabs the "scale" is entirely wanting;
their Zoeae have it indicated in the form of a moveable appendage, which
is often exceedingly minute.
Pages:
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135