STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 213 



another and to gravity, was obviously of no consequence. Food 

 could be passed from the gills to the palps as readily upward as 

 downward or horizontally. Due to the buoyancy of the mucus in 

 which food material is gathered and passed to the stomach, so 

 near the specific gravity of the surrounding water, cilia need 

 exert only a very slight traction upon the food masses to move 

 them in any direction. 



No Unionids have acquired an asymmetry like that of the 

 oyster, but some species, particularly Quadrulse, are always found 

 lying upon one side or the other. They feed as readily upon 

 one side as upon the other. Neither the right side nor the left is 

 favored. There is no exceptional arrangement of ciliated parts 

 within the mantle chamber for countering the unsymmetrical pull 

 of gravity, unless the unusual size of the labial palps of some 

 species may be interpreted as a means of preventing the loss of 

 food between gills and palps. 



No matter, then, in what position the gills and palps may be. 

 food is readily passed mouthward, and the process of food collect- 

 ing goes on constantly in the absence of adverse stimuli. Only 

 with the closure of the siphons is the streaming of fresh material 

 interrupted. Only in case of powerful stimulus are the palps 

 caused to move out of line, away from the inner gill, thus refus- 

 ing food masses entirely. It has been difficult to demonstrate 

 the constancy of the food stream upon the contiguous faces of 

 the labial palps. The ciliated furrows (p. 234) ha've the func- 

 tions of accepting or rejecting material, but to what extent the 

 latter function is exercised under normal circumstances it is 

 difficult to determine directly. When fed in high concentrations 

 certain specific substances were rejected entirely, others taken 

 readily, e.g., starch grains were never recovered in the alimentary 

 tract, while Glococapsa passed through in great numbers (p. 227 ). 

 It is permissible, therefore, to postulate that the labial palps 

 behave in harmony with the rest of the food-gathering apparatus. 



(b) The Degree of Digestion a Response to Tissue Demands. 



The cilia of the alimentary tract are virtually in constant move- 

 ment. (Nelson, '18, has reported the partial suspension of the 



