Figs. 19 to 23. Tails of the Zoëæ of various Crabs.
Fig. 19. Pinnotheres. Fig. 20. Sesarma. Fig. 21. Xantho.
Figs. 22 and 23 of unknown origin.
It is further to be remarked that the Zoëæ of the Crabs, as also of the Porcellanæ, of the Tatuira and of the Shrimps and Prawns, are enveloped, on escaping from the egg, by a membrane veiling the spinous processes of the carapace, the setæ of the feet, and the antennæ, and that they cast this in a few hours. In Achæus I have observed that the tail of this earliest larval skin resembles that of the larvæ of Shrimps and Prawns, and the same appears to be the case in Maia (see Bell, ‘Brit. Stalk-eyed Crust.’ p. 44). Widely as they seem to differ from them at the first glance, the Zoëæ of the Porcellanæ (Fig. 24) approach those of the true Crabs very closely. The antennæ, organs of the mouth, and natatory feet, exhibit the same structure. But the tail bears five pairs of setæ, and the dorsal spine is wanting, whilst, on the contrary, the frontal process and the lateral spines are of extraordinary length, and directed straight forward and backward.
Fig. 24. Zoëa of Porcellana stellicola, F. Müll., magnified.
Fig. 25. Zoëa of the Tatuira (Hippa emerita), magnified.
Fig. 26. Zoëa of a small Hermit Crab, magnified.
The Zoëa of the Tatuira (Fig. 25) also appears to differ but little from those of the true Crabs, which it likewise resembles in its mode of locomotion. The carapace possesses only a short, broad frontal process; the posterior margin of the tail is edged with numerous short setæ.
The Zoëa of the Hermit Crabs (Fig. 26) possesses the simple inner antennæ of the Zoëa of the true Crabs; the outer antennæ bear upon the outside on a short stalk a lamella of considerable size analogous to the scale of the antennæ 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 setæ. 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 setæ. The tail bears five pairs of setæ. 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 Zoëæ of the Shrimps and Prawns (Fig. 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 antennæ 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 antennæ 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 setæ on its hinder margin.
Fig. 27. Zoëa of a Palæmon residing upon Rhizostoma cruciatum, Less., magnified.
The development of the Zoëa-brood to the sexually mature animal was traced by Spence Bate in Carcinus mænas. 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 Zoëæ, 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.[[4]]