When you pick up a clam, it will draw in both foot and siphon and close its shell. If you return it to the water and watch for a few minutes, the foot will probably be pushed out so that you can see it. If it is near one of the glass sides of your aquarium, you can see how the clam digs and how it braces its foot while it pulls or pushes its body in some direction. Be sure to have the sand in the aquarium deep enough for the clam to burrow.

Young clams develop in the gills of female clams. In one interesting group of fresh-water clams, also called mussels, the larvae leave the female's shell before they are fully developed, and live for a time as parasites on the gills or skin of fishes. These larvae are called glochidia. When a glochidium touches a fish, its shell snaps shut into the fish tissues, and the skin of the fish gradually grows over it. For several weeks the glochidium grows and develops, living and feeding upon the body of the fish, but not killing its host, or even seeming to harm it very much. Finally it leaves the fish and settles to the bottom of the pond or lake for the remainder of its life. As the young clam continues to grow, it adds new layers to the outer edges of its shell.

A single adult clam of this group may produce many thousands of glochidia, but not all of them develop into adult clams in their turn. Many die because they do not find a fish to which to attach. Some of those which do develop, and leave the fish, are eaten by muskrats or other enemies.

Some fresh-water clams are three or four inches long. These are, of course, the best for aquarium study. You will usually find these in large bodies of fresh water. Other clams are less than an inch long and never grow any larger. You will find them among plant roots in ponds and streams, or in the sand and mud of the bottom.

The group of fresh-water clams shown at the beginning of this chapter were found in cold spring water near the roots and on the stems of water cress. Their average length was about one-quarter inch. The young clams of this group do not become parasites. They develop in the gills of the female until they are ready to begin life for themselves.