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 crayfish 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.) ] The shells were held together by two large _adductor muscles_. The anterior adductor (Fig. 193) is near the front end, above the foot. The posterior adductor is toward the rear end, but not so near the end as the anterior. Can you find both _muscle scars_ in the shells? Are they nearer the ventral or the dorsal surface? The points of attachment travelled downward and farther apart as the animal grew (see Fig. 191). Higher than the larger scars are small scars, or impressions, where the protractor and retractor muscles that extend and draw in the foot were attached. [Illustration: FIG. 193.—ANATOMY OF MUSSEL. (Beddard.) ] The muscular _foot_ extends downward in the middle, halfway between the shells (Fig. 193). On each side of the foot and behind it hang down the two pairs of =gills=, the outer pair and the inner pair (Fig. 192). They may be compared to four V-shaped troughs with their sides full of holes. The water enters the troughs through the holes and overflows above. Is there a marked difference in the size of the two pairs of gills? A kind of chamber for the gills is made by the joining of the mantle flaps below, along the ventral line. The mantle edges are separated at two places, leaving openings called _exhalent_ and _inhalent siphons_. [Illustration: FIG. 194.—MUSSEL. _A_, left shell and mantle flap removed. _B_, section through body. =Question=: Guided by other figures, identify the parts to which lines are drawn. ]