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? [Illustration: FIG. 188.--ANODON, or fresh-water mussel.] [Illustration: FIG. 189.--DIAGRAM OF SHELL open and closed, showing muscle, _m_, and ligament, _b_.] [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. 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_. [Illustration: 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 the pallial line, or the line to which the mantle extended on each shell when the animal was alive? A free portion of the mantle extended like a fringe below the pallial line. [Illustration: FIG. 192.--CROSS SECTION OF MUSSEL. (Diagram, after Parker.)]