At the front border of the disc is placed the unjointed but elongated first pair of antennæ (at1). Externally to and behind these there spring the short posterior antennæ (at2), at the base of which the green gland is already formed. Surrounding the mouth are the mandibles (md) and anterior pair of maxillæ (mx1), and some distance behind the second pair of maxillæ (mx2), consisting of a cylindrical basal joint and short terminal joint armed with bristles. The first pair of maxillipeds is absent.
The thoracic region is formed of an oval segmented disc attached to the under surface of the cephalic disc. From its front segment arises the second pair of maxillipeds (mxp2) as single five-jointed appendages, and from the next segment springs the five-jointed elongated but uniramous third pair of maxillipeds (mxp3), and behind this there arise three pairs of six-jointed ambulatory appendages (p1, p2, p3, of which only the basal joint is represented in the figure) with an exopodite springing from their second joint. The two posterior thoracic rings and their appendages cannot be made out.
The abdomen is reduced to a short imperfectly segmented stump, ending in a fork, between the prongs of which the anus opens. Even the youngest larval Phyllosoma, such as has just been described, cannot be compared with a Zoæa, but belongs rather, in the possession of biramous thoracic feet, to a Mysis stage. In the forked tail and Nauplius eye there appear however to be certain very primitive characters carried on to this stage.
The passage of this young larva to the fully formed Phyllosoma ([fig. 222] C) is very simple. It consists essentially in the fresh development of the first pair of maxillipeds and the two last ambulatory appendages, the growth and segmentation of the abdomen, and the sprouting on it of biramous swimming feet. In the course of these changes the larva becomes a true Decapod in the arrangement and number of its appendages; and indeed it was united with this group before its larval character was made out. In addition to the appearance of new appendages certain changes take place in those already present. The two posterior maxillipeds, in the Palinurus Phyllosoma at any rate, acquire again an exopodite, and together with the biramous ambulatory feet develop epipodites in the form of gill pouches.
The mode of passage of the Phyllosoma to the adult is not known, but it can easily be seen from the oldest Phyllosoma forms that the dorsal cephalic plate grows over the thorax, and gives rise to the cephalo-thoracic shield of the adult.
There are slight structural differences, especially in the antennæ, between the Phyllosoma of Scyllarus and that of Palinurus, but the chief difference in development is that the first pair of maxillipeds of the Palinurus embryo, though reduced in the embryonic state, does not completely vanish, at any rate till after the free larval state has commenced; and it is doubtful if it does so even then. The freshly hatched Palinurus Phyllosoma is very considerably more developed than that of Scyllarus.
Fig. 223. The Appendages of a Crab Zoæa.
At. I. first antenna; At. II. second antenna; md. mandible (without a palp); mx. 1. first maxilla; mx. 2. second maxilla; mx. 3. third maxilla; mxp. 1. first maxilliped; mxp. 2. second maxilliped.