An aboral vessel embraces the sense-organ, but has no external opening.
The pedal pores (on the base of the aboral feet) are usually larger than the dental pores (on the base of the circoral teeth).
The mouth of the shell lies on the oral pole of the main axis, and is therefore opposed to the aboral hinge.
Besides, a more solid junction is often effected on the posterior or aboral part of the margins, which we shall call the hinge.
In the next stage a small disc—the ciliated disc—is formed in the middle of the aboral dome.
Ventral horn of the aboral hinge very large, pyramidal, one-third as long as the shell and three times as long as the dorsal.
The aboral end of the original body of the embryo gradually atrophies.
Make a drawing of the aboral surface showing all these parts.
Shell ovate, slightly compressed, with a single straight conical spine on the aboral pole, half as long as the radius.
Now begin at the tip of each ray and cut away the aboral wall of each, leaving, however, a single arm intact.