Page:Popular Science Monthly Volume 50.djvu/59

Rh and looking like a row of little frills. This edge secretes carbonate of lime from the water and adds it to the shell all along the outer edge, forming a new line of growth. Thus, as the clam increases in size his house grows proportionately, so that it always exactly fits him. The two halves of the shell are joined by a curious hinge. In some kinds of shells the hinge is external and in some internal. It consists of teeth (two or more) with spaces between on either half. These lock together, and are held by a strong, elastic muscle. On the inside of the clam shell are two slight depressions, where the powerful adductor muscles of the body were fastened.

The clam's body is completely enshrouded in the mantle, except for two openings, through one of which the foot can be



pushed out. The other is for the siphon, or what is commonly known as the "neck" of the clam. In some respects the clam may be better off than we are, for he has a little brain in his foot and also a gland for secreting strong fibers. With this he spins a byssus by which he can attach himself to whatever he likes. He does not even have to search for his food, but waits for it to come to him. He makes a burrow in the mud or sand, attaching himself to the bottom by the byssus. Then he thrusts his siphon up through the mud and water until it reaches the surface. The siphon is made up of two tubes, the water flowing in through one and out through the other.

When the inflowing current, laden with minute plants and animals, reaches the gill chamber, some of these are sifted out and retained for food, while the water and waste matter flow out through the other tube.

The clam's eggs are carried by the mother on her gills. When there are fish in the water with them the mother clams discharge the eggs, which soon hatch, but if there are no fish they carry the eggs until they decay. The reason of this strange behavior is