Within the velar area the eyes and the cephalic tentacles commence to rise up, and on the surface of the post-oral region is formed a cap-like shell and an encircling ridge, which gradually increases in prominence and becomes the freely depending mantle-skirt.
Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re surfaces of the nerve-end cells nearer the lens instead of with the opposite end.
A fact of special interest in regard to them is that the genus Poliochera, from the Coal Measures, appears to be a member of the same group. The name Cryptostemma, given to the first-known genus of the order, described by Guerin-Meneville, refers to the supposed concealment of the eyes by the movable cephalic sclerite.
Another form, Rhopalophorus, has two cephalic tentacles that are retractile and covered with hooks.
In the regions of greatest linguistic mixture is the greatest heterogeneity of cephalic index.
