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. 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