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The highly developed swimming ability of the scallops accounts for the migratory powers of the great schools of these active bivalves. One would normally expect the direction of swimming taken by a scallop snapping its valves together to be “backward” in the direction of the hinges. Although that type of movement is on rare occasions used as an escape measure, the typical swimming movement is in the opposite direction with the free edge of the shell going in front, so that the animal appears to be taking a series of bites out of the water. This odd action is made possible by the vertical, curtain-like edges of the muscular mantle. When the valves are snapped shut by the powerful adductor muscle, water is driven out, not past the mantle curtains but through the regions around the hinge or ears of the shell. By manipulating these curtains, which can be extended or withdrawn locally, the scallop is able to vary the amount and position of exodus of water and hence can direct its course. If accidentally turned over onto the wrong valve, the scallop can execute a neat flip and regain its normal position.

While the scallop always swims with its valves in a horizontal plane, the Lima File Clams most frequently progress edgewise, that is, with the breadth of the valves vertical or slightly oblique. The long, colorful tentacles of the Lima keep the animal momentarily suspended in water while the valves are being opened in preparation for another “bite” forward. The Lima is a poor swimmer and, because of its habit of building nests under rocks, apparently has no incentive to undertake migrations as is done by

Figure 17. The mode of locomotion in the bivalve, Yoldia limatula. a, the foot is thrust forward; b, the muscular flaps are spread apart to form an anchor; c, the foot is withdrawn, thus pulling the animal forward; d, by closing the flaps together, the foot is made ready for another thrust forward. (After Drew 1900.)