The Scallops 341 



tional activity. Whenever the creature is disturbed, it, 

 like the veligers of other bivalves, immediately with- 

 draws the velum within the shell, which closes, and sinks. 

 Sometimes it resumes its course before settling far, but 

 even if it reaches the bottom it may soon rise. 



During the last three days of the short swimming or 

 veliger period, a foot begins to develop on the under side 

 of the body. It becomes relatively very large, and its 

 first function is a curious one. Being extended from be- 

 tween the valves of the shell to a distance equal to that 

 of two-thirds of the body, its end is seen to be covered 

 with cilia, the movements of which begin to aid in swim- 

 ming, and thus supplement the woFk~~per formed by the 

 velum. The latter organ soon atrophies and disappears, 

 and the creature ceases to spend the greater part of the 

 time afloat. But it is still able to swim, and frequently 

 rises from the bottom not yet, however, by the flapping 

 movement of the shell valves that characterizes the swim- 

 ming of the adult, nor by the action of the foot cilia 

 alone, for the body soon becomes too heavy to be moved 

 by them unaided but by a paddling motion of the foot. 

 Swimming by a paddle-like foot action is sometimes prac- 

 tised by adults of other species of bivalves (Mactra, 

 Ensis, Solenomia, Yoldia), and the habit is retained by 

 the young Pecten for some time. Gradually it begins 

 to swim by the shell, being aided for a time by the foot, 

 but as the animal grows, the latter organ becomes rela- 

 tively small and ceases to have a part in the performance 

 of this function. 



Thus in early life the foot aids in swimming in two 

 ways by the action of its cilia and by paddling: but in 

 addition to this, it performs two functions that of spin- 

 ning the byssus for attachment, and of creeping. Prob- 



