Anatomy of the Food Mollusks 27 



formed automatically by cilia, over which the nervous 

 system has no control, are almost limited to opening and 

 closing the shell. It will be shown, however, that even in 

 this degenerated form, many wonderful responses of the 

 nervous system to various stimuli occur, by means of 

 which this automatic ciliary mechanism is rendered ef- 

 fective in inducing activities that result in the acceptance 

 of microscopic food particles from the water, and the 

 rejection of useless material. 



The Foot. This organ, which is characteristic of the 

 Mollusca, is simply a muscular thickening of the under 

 or ventral wall of the body. It varies greatly in size, 

 shape, and position in various bivalves, but the chief 

 features of its primitive form are probably represented 

 in Venus. Here the entire ventral wall is thickened, 

 forming an organ having some resemblance to a plow- 

 share, the point projecting forward. Though the adult 

 hard clam, and many others having a similar foot, seem 

 more or less completely to have abandoned the habit of 

 creeping, a few adult bivalves possessing a locomotor 

 organ of the same general character use it for that pur- 

 pose, sometimes covering considerable distances. The 

 very young of the long neck clam, mussel, and scallop 

 possess a foot much like that of the adult Venus, and all 

 are active creepers. Later in life the organ becomes 

 relatively much reduced in size and modified in form, and 

 in each the creeping habit is gradually abandoned. Be- 

 cause of the early form and function of the foot in these 

 species — and the same will probably be found to be true 

 of many other bivalves — it may be assumed that the 

 ancestral organ was large, covering the entire ventral side 

 of the body and projecting far forward, and that its 

 primary function was that of creeping. 



