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44 American Seashells
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 snap-
ping 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 By manipulating these curtains, which can be extended or withdrawn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 Lw/^ File Clams most frequently progress edgewise, that is, with the
breadth of the valves vertical or slightly oblique. The long, colorful tenta-
cles of the Lifna keep the animal momentarily suspended in water while
the valves are being opened in preparation for another "bite" forward. The
Li7na 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
--FOOT
.6HELL
Figure 17. The mode of locomotion in tlie bivalve, Yoldia Intiatnla. 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foot is withdrawn, thus pulling the animal forward; d, by closing the flaps together,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.)