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].
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.
Fig. 55. Ventral view of a larva of Acipenser of 11 millimetres.
m. mouth; st. suctorial (?) processes; op. eye; g. gills.
The openings of the nasal pits are at first single; but the opening of each becomes gradually divided into two by the growth of a flap on the outer side ([fig. 54], ol). It is probable that this flap is equivalent to the fold of the superior maxillary process of the Amniota, which by its growth roofs over the open groove which originally leads from the external to the internal nares; so that the two openings of each nasal sack, so established in these and in other fishes, correspond to the external and internal nares of higher Vertebrata.