Page:Popular Science Monthly Volume 50.djvu/63

Rh they have a long foot lying the whole length of the body. Unlike the bivalves, they have a distinct head, in which the brain is situated. Often there are tentacles or feelers, as in the snail, on the ends of which the eyes are placed. Gasteropods have a wonderful eating apparatus called the odontophore or tooth ribbon. It is covered with hooked teeth, pointing backward, and is in the lower side of the mouth, situated about the same as our tongues. On the upper side of the mouth is a hard plate or jaw, and the food is ground up by the toothed ribbon against this plate. The



odontophore wears out rapidly, but as the front part is used up it grows from behind, and these animals are so fortunate as to have a new set of teeth every little while.

There is only one shell to take the place of two in the bivalves, so most of the univalves have an operculum. This is a little lid (either horny or calcareous) on the upper side of the foot which exactly fits the aperture in the shell. If a Gasteropod wishes for any reason to be alone and rest for a time, he only has to draw in his foot, pull to the door, and he is in complete seclusion from all the world.

The shells of the Gasteropods, like those of the bivalves, are often covered with a sort of horny membrane or epidermis which protects them from the eroding power of the water and other external injuries.

At the bottom of the Gasteropod group is a wonderful creature which we may call a multivalve, as its shell is made up of a number of plates (usually eight) which look like ancient armor. It is called the mail shell, or chiton, and is the only example in the world of a shell composed of more than two parts. It is common on the Atlantic coast, in some of the bays and inlets south of Boston, on the Pacific shores, in England, and other places. Chitons sometimes have as many as eight thousand eyes, their backs being covered with them.

The limpets range in shape from those which are almost flat to a perfect cone. Some of my prettiest from Sitka are