=Habits of the Mussel.=--Is it abundant in clear or muddy water; swift, still, or slightly moving water? Describe its track or furrow. What is its rate of travel? Can you distinguish the spots where the foot was attached to the ground? How long is one “step” compared to the length of the shell? The animal usually has the valves opened that it may breathe and eat. The hinge ligament acts like the case spring of a watch, and holds the valves open unless the adductor muscles draw them together (Fig. 189). When the mussel first hatches from the egg, it has a triangular shell. It soon attaches itself to some fish and thus travels about; after two months it drops to the bottom again. =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 mollusks undergo a metamorphosis to reach the adult stage (Fig. 199). [Illustration: FIG. 199.--OYSTER. _C_, mouth; _a_, vent; _g_, _g′_, ganglia; _mt_, mantle; _b_, gill.] [Illustration: FIG. 200.--TROCHUS.] [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 _hard-shell clam_ (quahog) found on the Atlantic coast from Cape Cod to Texas. Its shell is white. It often burrows slightly beneath the surface. The _soft-shell 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 the United States 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.