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).
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Fig. 199.—Oyster.
C, mouth; a, vent; g, g', ganglia; mt, mantle; b, gill.
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Fig. 200.—Trochus.
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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
