Page:Popular Science Monthly Volume 82.djvu/541

Rh anteriorly to the side of the head, there turns on itself and proceeds back to the stomach, where it again goes forward, passing through the ventricle of the heart, to terminate in the anus, which opens into the gill cavity. The shell is perforated, toward the left, by a series of openings lying above a slit in the mantle fold leading into the gill cavity, whence issues a stream bearing the excrement, respiratory and excretory wastes. Three tentacular processes from the edges of

 a, b, grasping kelp with anterior processes of foot; c, drawing kelp under foot; d, eating hole in kelp.

the mantle cleft project through these holes. As the animal grows the apertures in the shell behind the respiratory cavity are closed up and new ones are formed at the anterior edge.

The head terminates in a short snout on either side of which is a somewhat slender olfactory tentacle and slightly lateral to this a shorter and broader optic tentacle. Two elongated ganglia lying above the mouth cavity may be called the brain because they form the center for nerves from the eyes, olfactory tentacles, snout, lips and other parts of the head. The eye is a simple cup-shaped depression of the epithelium on the end of the tentacle. The cup is filled with a gelatinous lens and it has clear and pigmented retinal cells connected with fibrils from the