The Zoëæ of the Crabs (Fig. 17) are usually distinguished by long, spiniform processes of the carapace. One of these projects upwards from the middle of the back, a second downwards from the forehead, and frequently there is a shorter one on each side near the posterior inferior angles of the carapace. All these processes are, however, wanting in Maia according to Couch, and in _Eurynome_ according to Kinahan; and in a third species of the same group of the _Oxyrhynchi_ (belonging or nearly allied to the genus _Achæus_) I also find only an inconsiderable dorsal spine, whilst the forehead and sides are unarmed. This is another example warning us to be cautious in deductions from analogy. Nothing seemed more probable than to refer back the beak-like formation of the forehead in the Oxyrhynchi to the frontal process of the Zoëa, and now it appears that the young of the Oxyrhynchi are really quite destitute of any such process. The following are more important peculiarities of the Zoëæ of the Crabs, although less striking than these processes of the carapace which, in combination with the large eyes, often give them so singular an appearance:—the anterior (inner) antennæ are simple, not jointed, and furnished at the extremity with from two to three olfactory filaments; the posterior (outer) antennæ frequently run out into a remarkably long spine-like process (“styliform process,” Spence Bate), and bear, on the outside, an appendage, which is sometimes very minute (“squamiform process” of Spence Bate), corresponding with the antennal scale of the Prawns,[3] and the first rudiment of the future flagellum is often already recognisable. Of natatory feet (afterwards maxillipeds) only two pairs are present; the third (not, as Spence Bate thinks, the first) is entirely wanting, or, like the five following pairs of feet, present only as a minute bud. The tail, of very variable form, always bears _three_ pairs of setæ at its hinder margin. The Zoëæ of the Crabs usually maintain themselves in the water in such a manner that the dorsal spine stands upwards, the abdomen is bent forwards, the inner branch of the natatory feet is directed forwards, and the outer one outwards and upwards. (Figs. 19 to 23. Tails of the Zoëæ of various Crabs. Fig. 19. Pinnotheres. Fig. 20. Sesarma. Fig. 21. Xantho. Fig. 22 and 23 of unknown origin. 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.