NOTES ON SOME PROBLEMS OF ADAPTATION. 1 03 



illumination seems to cause a variable number of them to curl 

 and contract, but not sharply; they do not themselves appear 

 locally sensitive to light. Stimulation of a tentacle by weak 

 acetic acid in sea water caused the particular tentacle to retract 

 instantly, and the animal to immediately swim violently away. 



In swimming, Lima has open to it a variety of possibilities. 

 It may progress while lying on either side, after the manner of 

 Pecten, or it may swim with the medial plane in the vertical 

 position, the animal being out of contact with the substratum 

 save between "steps." The "steps" may in either case be 

 made in one of several ways either toward the gape of the shell, 

 as with Pecten, in which event the mechanics of the swimming 

 act is similar to that of Pecten (Vies, '06; Bauer, '12); or else 

 toward the hinge, by squirting water from between the valves. 

 The mantle fold is relatively wider than in most Pectinidae, and 

 is very mobile; at rest, it beats with a rhythmic pulsation, doubt- 

 less functionally homologous with that known in some other 

 lamellibranchs (Redfield, '17), but different in one regard: in 

 Anodonta and in other clams the opposite mantle-flaps beat 

 synchronously, whereas in Lima they unmistakably pulsate in 

 opposite phase, alternately. 



The Pecten-like mode of progression is the one usually adhered 

 to, whether the animal be swimming vertically or on one side 

 (and by actual count it was found that either side is used, in- 

 differently). This is almost the only type of swimming induced 

 by photic stimulation, the other mode of progression, in the 

 direction of the hinge, being slower and made up of very short 

 "steps." 



When a previously darkened Lima is illuminated by hori- 

 zontal sunlight, it turns the gape away from the light and swims, 

 usually with the valves vertical, in a straight line away from the 

 light. There is exhibited a definite and peculiar process of 

 orientation. The long, mobile foot is thrust out between the 

 valves, its free portion is bent sharply away from the light, and 

 its distal end becomes adherent to the substratum. The foot 

 then shortens longitudinally, so that, without moving its valves, 

 the body is turned somewhat away from the light; several 



