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,
could be defined in it. Unfortunately we possess only this single
complete series of observations, and its results cannot be regarded at
once as universally applicable; thus the young Hermit Crabs retain the
general aspect and mode of locomotion of Zoeae, whilst the rudiments of
the thoracic and abdominal feet are growing, and then, when these come
into action, appear at once in a perfectly new form, which differs from
that of the adult animal chiefly by the complete symmetry of the body
and by the presence of four pairs of well-developed natatory feet on the
abdomen.
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