The Zoea of the Hermit Crabs (Figure 26) possesses the simple inner
antennae of the Zoea of the true Crabs; the outer antennae bear upon the
outside on a short stalk a lamella of considerable size analogous to the
scale of the antennae of the Prawns; on the inside, a short, spine-like
process; and between the two the flagellum, still short, but already
furnished with two apical setae. As in the Crabs, there are only two
pairs of well-developed natatory feet (maxillipedes), but the third pair
is also present in the form of a two-jointed stump of considerable size,
although still destitute of setae. The tail bears five pairs of setae.
The little animal usually holds itself extended straight in the water,
with the head directed downwards.

This is also the position in which we usually see the Zoeae of the
Shrimps and Prawns (Figure 27), which agree in their general appearance
with those of the Hermit Crabs. Between the large compound eyes there is
in them a small median eye. The inner antennae bear, at the end of a
basal joint sometimes of considerable length, on the inside a plumose
seta, which also occurs in the Hermit Crabs, and on the outside a short
terminal joint with one or more olfactory filaments. The outer antennae
exhibit a well-developed and sometimes distinctly articulated scale, and
within this usually a spiniform process; the flagellum appears generally
to be still wanting. The third pair of maxillipedes seems to be always
present, at least in the form of considerable rudiments. The spatuliform
caudal lamina bears from five to six pairs of setae on its hinder
margin.

The development of the Zoea-brood to the sexually mature animal was
traced by Spence Bate in Carcinus maenas. He proved that the
metamorphosis is a perfectly gradual one, and that no sharply separated
stages of development, like the caterpillar and pupa of the Lepidoptera,