Page:Popular Science Monthly Volume 13.djvu/418

402 The first, on the dorsal face (Fig. 3), is occupied in part by a pallial prolongation, a continuation of which is introduced by this opening into the interior of the shell, in part by the small adductor muscle.



The second opening is posterior (Fig. 4, a), and serves to open a passage to the internal organs contained in the cavity of the mantle.

Finally, the third, placed obliquely in front (Fig. 5, a a; Fig. 4, a), is the largest, and remains always gaping open to allow the foot to pass out (Fig. 5, b).

Each of these valves, which form the shell, is formed of three parts, viz.:



1. A posterior part (Fig. 3,f; Fig. 6, f; Fig. 7 f), which we can call the neck part; this posterior is the least arched, and thinner than the rest of the shell: its posterior edge is embraced by the folds of the mantle, which we have already mentioned, and thus the mantle is solidly attached to the shell.