Ciliary Mechanisms 53 



warm waters like those of the Gulf of Mexico. They 

 are more numerous in the shallow waters of coast lines 

 than in the open sea, probably because the salts in solu- 

 tion, forming a necessary part of the material required 

 for their nourishment, exist in sufficient quantities only 

 near the mouths of rivers or small streams which, in 

 turn, have derived them from the soil. The tempera- 

 ture of the water on extensive flats and in shallow estu- 

 aries, also, is higher than in the open ocean. As a re- 

 sult of this distribution of diatoms, bivalves are most 

 numerous in comparatively shallow waters near the land. 



The normal process of feeding, when the water is free 

 from mud, is much the same in all bivalves. Imagine 

 an oyster, for example, lying with valves open in water 

 containing only diatoms. The cilia on both surfaces of 

 each gill continually drive water from the branchial 

 chamber through the spaces between filaments and into 

 the gill interior. Here it passes upward to the base of 

 the gill, falls into the epibranchial space, and is forced 

 backward and discharged from the body above the gills. 



Now and then the stream bears a diatom to the gill 

 surface. On touching a filament, it instantly adheres to 

 the sticky mucus produced by the gland cells. This 

 mucus, with its entangled diatoms, is then moved 

 by the gill cilia down to the free edge of the gill, 

 as indicated by the arrows on the gill surface in Figure 

 3. It now proceeds forward on the gill margin until 

 the palps are reached. The material is transferred from 

 the gill to the ciliated inner surfaces of the palps, and 

 proceeds directly across their ridges toward, and finally 

 into the funnel-like mouth. So much of the function of 

 the gill and palp cilia has been known to biologists for 

 many years, but practically nothing more has been ob- 



