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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. By 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 dioxid, bubbles up.) Compare the thickness of the epidermal layer, the middle chalky layer, and the inner, pearly layer.
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Fig. 191.—Diagram. Change of points of attachment of muscles as mussel enlarges. (Morgan.)
Anatomy of the Mussel.—What parts protrude at any time beyond the edge of the shell? (Fig. 190.) The shell is secreted by two folds of the outer layer of the soft body of the mussel. These large, flaplike folds hang down on each side, and are called the mantle. The two great flaps of the mantle hang down lower than the rest of the body and line the shell which it secretes (Fig. 192). The epidermis of the mantle secretes the shell just as the epidermis of the crawfish secretes its crust. Can you find
