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.
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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 dorsal surface? The points of attachment traveled 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.
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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
