There are only a few points which call for special attention in the general form of the body. In the youngest larva figured the ventral part of the hyomandibular cleft is already closed: the dorsal part of the cleft is destined to form the spiracle (_sp_). The arch behind is the hyoid: on its posterior border is a membranous outgrowth, which will develop into the operculum. In older larvæ, a very rudimentary gill appears to be developed on the front walls of the spiracular cleft (Parker), but I have not succeeded in satisfying myself about its presence; and rows of gill papillæ appear on the hyoid and the true branchial arches (figs. 54 and 55, _g_). The biserially-arranged gill papillæ of the true branchial arches are of considerable length, and are not at first covered by the operculum; but they do not form elongated thread-like external gills similar to those of the Elasmobranchii. The oral cavity is placed on the ventral side of the head; it has at first a more or less rhomboidal form. It soon however (fig. 55) becomes narrowed to a slit with projecting lips, and eventually becomes converted into the suctorial mouth of the adult. The most remarkable feature connected with the mouth is the development of provisional teeth (fig. 55) on both jaws. These teeth were first discovered by Knock (No. 88). They do not appear to be calcified, and might be supposed to be of the same nature as the horny teeth of the Lamprey. They are however developed like true teeth, as a deposit between a papilla of subepidermic tissue and an epidermic cap. The substance of which they are formed corresponds morphologically to the enamel of ordinary teeth. As they grow they pierce the epidermis, and form hollow spine-like structures with a central axis filled with subepidermic (mesoblastic) cells. They disappear after the third month of larval life. In front of the mouth two pairs of papillæ grow out, which appear to be of the same nature as the papillæ on the suctorial disc in the embryo of Lepidosteus (_vide_ p. 115). They are very short in the embryo represented in fig. 53; soon however they grow in length (figs. 54 and 55, _st_); and it is probable that they become the barbels, since these occupy a precisely similar position[37]. [37] If these identifications are correct the barbels of fishes must be phylogenetically derived from the papillæ of a suctorial disc adjoining the mouth. [FIG. 54. SIDE VIEW OF A LARVA OF ACIPENSER OF 11 MILLIMETRES. _op._ eye; _ol._ olfactory pit; _st._ suctorial (?) processes; _m._ mouth; _sp._ spiracle; _g._ gills.]