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 with- 

 drawn 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foot is withdrawn, thus pulling the animal forward; d, by closing the flaps together, 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.) 



