The velum is also provided with a circlet of twelve tantacles (in some species sixteen) which hang backwards into the pharynx; these are the velar tentacles.
M, Mouth; f, foot; v, velar area, the margin v corresponding with the ciliated band which demarcates the velar area or velum of the embryo Gastropod (see fig.
Farther up, within the velar area, the rudiments of the cerebral nerve-ganglion ng are seen separating from the ectoderm.
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.
The velar tenuis q, when labialized, became qu, without labialization became k; the velar media g became b or g.
Dv, Velar area or cephalic dome.
The ciliated band of the left side of the velar area is indicated by a line extending from v to v; the foot f is seen between the pharynx ph and the pedicle of invagination pi.
The outline of the velar area becomes strongly emarginated and can be traced through the more mature embr y os to the cephalic lobes or labial processes of the adult Limnaeus (fig.
The foot now protrudes below the mouth, and the post-oral hemisphere of the trochosphere grows more rapidly then the anterior or velar area.