True molluscan shells come in two main varieties: BIVALVES and UNIVALVES. Bivalves have two valves, fitting together along a toothed hinge on one side, and kept closed by means of ADDUCTOR MUSCLES. Univalves have only one shell, usually coiled, but sometimes shaped like a cap or miniature volcano. Some marine univalves can seal themselves inside with an operculum, which covers the open end of the shell like a trap door. Although shells take on many different shapes, they are much alike inside. Each has a foot, a breathing siphon, a tiny brain and heart, and a fleshy mantle which secretes lime for shell-building. Most true mollusks have eyes, but a few are blind. Many have teeth, called RADULAE. Like any other animal, the mollusk generally moves about. It pushes along on the ocean floor on its foot, or it might swim a little. It lays millions of eggs and hatches countless baby mollusks. It lives its life in its shell, lugging it around, snuggling into it when alarmed, burrowing into mud, fastening itself to a rock and creating ingenious camouflage. It builds its calcareous house with a great instinctive talent for color and sculpture. . .and the closer it lives to the tropical zones, the more beautifully spectacular is its art. The two parts of a bivalve shell are like thin saucers, concave inside, convex outside. The inside is smooth, polished. The outside is rougher, sometimes with graceful ribs or concentric ridges or combinations of both. Univalves are conical and spiraling, with a series of whorls coming down like widening steps from the tiny nucleus on top. Univalves may have spines on their shoulders. The opening, called the aperture, has a delicate right-hand rim called the lip and a heavy, left-hand edge called the columella. [figure captions] BIVALVE'S anatomy: a) foot, b) adductor muscles, c) gills, d) hinge, e) adductor muscles, f) siphon, g) stomach, h) mantle. Oysters, clams, mussels all have them. UNIVALVE'S anatomy: As before, a) foot, b) siphon, c) mantle, but also d) operculum. Univalves include whelks, winkles, conchs. Chambered nautilus is brother to the octopus, but he wears his castle permanently--and on the outside. THE SHELL AS AN ARCHITECT. . .HOW DOES HE DO IT? Picture a vast undersea factory with billions of shells in constant production. Each is made slowly and entirely of lime which the little animal inside extracts from its food, almost from the first day of its life. Each shell builder flawlessly follows the shape and design of the species to which it belongs. All these sea animals come from eggs, all different according to species, but all laid in measureless abundance--sometimes released into the open sea, sometimes protected in homemade nests, sometimes encased in capsules strung like beads. Hatched, most baby mollusks swim freely for a while, their tiny, transparent bodies almost invisible to the naked eye. Then they start building a heavier shell and sink to the bottom.