=External Features.=—The shell is an elongated oval, broader and blunter at one end (Fig. 188). Why does the animal close its shell? Does it open the shell? Why? Does it thrust the foot forward and pull up to it, or thrust the foot back and push? (Mussels and clams have no bones.) Does it go with the blunt end or the more tapering end of the shell forward? (Fig. 188.) Can a mussel swim? Why, or why not? [Illustration: FIG. 188.—ANODON, or fresh-water mussel. ] 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 at 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? [Illustration: FIG. 189.—DIAGRAM OF SHELL open and closed, showing muscle, _m_, and ligament, _b_. ] 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 ploughs 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 to the 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? [Illustration: FIG. 190.—MUSSEL crawling in sand. ] Notice the _lines_ on the outside of the shell (Figs. 188 and 190). What point do they surround? They are _lines of growth_. Was each line once the margin of the shell? If the shell should increase in size, what would the present margin become? (Fig. 191.) Does growth take place on the margin only? Did the shell grow thicker as it grew larger? Where is it thinnest? =Draw= the outside of the shell from the side. Draw a dorsal view. Near the drawings write the names of the margins of the shell (p. 98) and of other parts learned, using lines to indicate the location of the parts. Study the surface of the shell inside and out. The inside is called _mother-of-pearl_. Is it of lime? Is the deeper layer of the shell of lime? (When weak hydrochloric acid or strong vinegar is dropped on limy substances, a gas, carbon dioxide, bubbles up.) Compare the thickness of the _epidermal layer_, the middle _chalky layer_, and the inner, _pearly layer_. [Illustration: FIG. 191.—DIAGRAM.