=Other Mollusca.=—The _oyster’s_ shells are not an exact pair, the shell which lies upon the bottom being hollowed out to contain the body, and the upper shell being flat. Can you tell by examining an oyster shell which was the lower valve? Does it show signs of having been attached to the bottom? The young oyster, like the young mussel, is free-swimming. Like the arthropoda, most molluscs undergo a metamorphosis to reach the adult stage (Fig. 199). [Illustration: FIG. 201.—CYPRÆA. (Univalve, with a long opening to shell.) ] Examine the shells of clams, snails, scallops, and cockles. Make drawings of their shells. The slug is very similar to the snail except that it has no shell. If the shell of the snail shown in Fig. 202 were removed, there would be left a very good representation of a slug. =Economic Importance of Mollusca.=—Several species of clams are eaten. One of them is the _hardshell clam_ (quahog) found on the Atlantic coast from Cape Cod to Texas. Its shell is white. It often burrows slightly beneath the surface. The _softshell clam_ is better liked as food. It lives along the shores of all northern seas. It burrows a foot beneath the surface and extends its siphons through the burrow to the surface when the tide is in, and draws into its shell the water containing animalcules and oxygen. _Oysters_ to the value of many millions of dollars are gathered and sold every year. The most valuable oyster fisheries of North America are in Chesapeake Bay. The young oysters, or “spat,” after they attach themselves to the bottom in shallow water, are transplanted. New oyster beds are formed in this way. The beds are sometimes strewn with pieces of rock, broken pottery, etc., to encourage the oysters to attach themselves. The dark spot in the fleshy body of the oyster is the digestive gland, or liver. The cut ends of the tough adductor muscles are noticeable in raw oysters. The starfish is very destructive in oyster beds. [Illustration: FIG. 202.—A SNAIL. _l_, mouth; _vf_, _hf_, feelers; _e_, opening of egg duct; _f_, foot; _ma_, mantle; _lu_, opening to lung; _a_, vent. ] _Pearls_ are deposited by bivalves around some irritating particle that gets between the shell and the mantle. The pearl oyster furnishes most of the pearls; sometimes pearls of great value are obtained from fresh-water mussels. Name articles that are made partly or wholly of mother-of-pearl.