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 Lay the shells, fitted together, in your hand with the hinge side away from you and the blunt end to the left (Fig. 188). Is the right or the left shell uppermost? Which is the top, or dorsal, side? Which is the front, or anterior, end? Is the straight edge at the top or the bottom? Our word "valve" is derived from a word meaning shell, because the Romans used shells for valves in pumps. Is the mussel a univalve or a bivalve? Which kind is the oyster? The snail?

Does the mussel have bilateral symmetry? Can you find a horny covering, or epidermis, over the limy shell of a fresh specimen? Why is it necessary? Does water dissolve lime? Horn? Find a bare spot. Does any of the shell appear to be missing there?

The bare projection on each shell is called the umbo. Is the umbo near the ventral or the dorsal line? The posterior or anterior end? Is the surface of the umbones worn? Do the umbones rub against the sand as the mussel plows its way along? How are the shells held together? Where is the ligament attached? (Fig. 189.) Is it opposite the umbones or more to the front or rear? (Fig. 189.) Is the ligament of the same material as the shell? Is the ligament in a compressed condition when the shell is open or when it is closed? (Fig. 189.) When is the muscle relaxed?

, or fresh-water mussel.

open and closed, showing muscle, m, and ligament, b.