Ciliary Mechanisms 61 



It is practically the same in other lamellibranchs, though 

 in some cases the entire mechanism is much more com- 

 plex than in the little neck or oyster. 



Though diatoms and useless particles are sometimes 

 brought to the gills at the same time, an examination 

 of the stomach contents of any bivalve will show that 

 there has been a selection of the food organisms and an 

 exclusion of mud or fine sand. The latter are some- 

 times found in the stomach, but in small amounts. When 

 this mixed mass of food and mud is brought, it is not 

 possible to separate them, and the whole is rejected and 

 sent out of the body. It thus happens that the bivalve 

 ceases to feed when the water is muddy to a certain de- 

 gree, though it still allows water to enter the mantle 

 chamber, presumably for the sake of the oxygen it 

 bears. 



It appears that the animal is not able to distinguish 

 food from mud particles either by the gills or palps, 

 but the selection of food results directly from the dif- 

 ferences in the response of the palps to varying quan- 

 tities of matter transported to them by the gills. Under 

 favorable conditions, diatoms are collected a few at a 

 time, and are sent across the palp ridges to the mouth. 

 In muddy water the particles increase in number until 

 the palps respond by diverting everything received to 

 the outgoing tracts. 



This may be proved experimentally, though the 

 operator must exercise care and skill. Very fine grains 

 of carmine, dropped on gills or palps of any bivalve, in 

 very small quantities, may be observed, with the aid of a 

 glass, to pass into the mouth. This simulates conditions 

 favorable for feeding, when diatoms are steadily 

 strained, a few at a time, from clear water. When 



