232 WILLIAM RAY ALLEN. 



tidal forms, where the breaking down and renewal of the style 

 occur rhythmically. 



Of passing interest is the observation that small, newly formed 

 styles sometimes may be seen coiled up in the stomach, where they 

 have pushed forth more rapidly than they could be broken up 

 and dissolved against the gastric shield. 



Resume. It is here contended that the crystalline style accom- 

 plishes a number of purposes, for none of which it is entirely 

 indispensable, nor entirely a perfect adaptation ; that it is no longer 

 performing an identical, single, primitive function traceable to a 

 primitive Lamellibranch ancestor. The response of this organ 

 to similar conditions is much the same in various bivalves ; but 

 the tranquil life of the lake has stabilized the feeding activity and 

 the style formation in the Unionids, while the styles of some 

 species inhabiting the marine littoral are profoundly affected by 

 the tidal phenomena. 



The formation and dissolution of the crystalline style goes 

 on in the same way that a paper, candle-lighter might, if extended 

 to an indefinite length by rolling up a sheet of paper of indefinite 

 length, and burning off the free end as rapidly as new paper is 

 added at the other. 



There is no evidence that digestion is furthered during the 

 passage of food through the style or style sac. 



The feeding of inert substances, both normally and through 

 the stomach wall, indicates that the mechanical stimulation of the 

 wall of the enteron is not alone the cause of the secretion of a 

 new style. The rate of formation of the style is shown to depend 

 in part upon temperature. Little food and little time are required 

 to set the process going. 



5. THE MECHANISM OF INGESTION. 



It has long been known that the gills with their great multiplica- 

 tion of surface are responsible for the movement of respiratory 

 water, and for the concentration of food material from the 







water. It is surprising to encounter in the work of so eminent a 

 student as Simpson, written only a score of years ago ('99), a 

 statement that the siphoning is due to the waving of the palps. 



