The many salt water and fresh water clams are bivalves, or two¬ shelled animals. The shells, which protect their soft bodies and make their lazy lives possible, start to form almost before the youngsters can be called clams at all. And they continue to grow, ridge by ridge, throughout their lives.

There are slight differences in the life stories of the various clams which enjoy life in salt or fresh water. But the growth of the all important shells is the same in all cases. A clam is a bivalve because his soft body is safely inside his pair of valves, or sturdy shells. The leathery hinge which joins the two valves together is the head end of the clam's soft body and the shells cover his right hand left sides.

Under the shells, the body is enfolded in two loose flaps of soft tissue called the mantle. Special cells in the mantle extract chemicals from the water and process them into sticky secretions. These secretions ooze out and harden, adding new layers and extensions on the growing shells. The outer shell is ridged with fine grooves which show where the extensions were added to make room for the growing clam inside,

The concentric ridges follow the curved edge of the shell and those inside show the various sizes of the clam in his youth. "'he outer layer of shell is rough and tough enough to protect the inner layers from being dissolved by the water. The inside layer of snell is hard and satin smooth to protect the clam's soft body from scratches. A middle layer is sandwiched between the rough outer and smooth inner layer of clam shell.

The outer and middle layers are secreted by the outer edge of the mantle, which follows the curve of the clams body like the hem of a cloak. The glossy layer which lines the walls of the clams home is secreted by cells throughout the mantle surface which touches the shells. The middle and inside layers are made from fine crystals of calcium carbonate. In the middle, or prismatic layer, the crystals are stacked in piles. In the lower, or nacerous layer, they are arranged flat, like overlapping tiles

The clam stages life as an egg which hatches to a larva, measuring no more than one 300th part of an inch. But even this tiny larva has the beginnings of his shells. Hinged together, they open like a miniature pair of jaws and some larvae clams use Crem to hitch a ride on the gills or fins of some passing fish. A young clam burrows into the mud or sand to start his stodgy life when his valves are about a quarter inch wide.