238 WILLIAM RAY ALLEN. 



stances were introduced through the body wall into the stomach. 

 At later periods the intestine and rectum were opened. Carmine 

 and starch grains were recognized throughout the length of the 

 alimentary canal. Only a few carborundum flakes were found 

 in the intestine, and the rest were not carried out of the stomach. 

 It is thus shown that the ciliary streams of the intestine are 

 capable of manipulating only minute particles. The cilia are too 

 small or too sparse to take care of the I2o-gauge carborundum, 

 even in suspension in the liquid of the gut. Thus they must be 

 altogether inadequate to keep a stream of sand in motion, if sand 

 were ingested, unless it were of extremely fine grade. 



(c) The Marly Incrustation of the Shell. 



Since the dense, limy incrustation deposited on the exposed por- 

 tions of the shells of lake mussels is the site of the active prolifera- 

 tion of diatoms, I suggested (I.e.') that this might be a source of 

 food. In order to test its food value the following simple experi- 

 ment was made : 



A number of freshly collected mussels were placed in an aquar- 

 ium ; an equal number, having the incrustation scraped and 

 washed off, acted as controls. During intervals, covering several 

 days the animals were opened and the condition of the crystalline 

 styles noted. The experimental animals were found to contain a 

 trace of the style up to the fifth day, while the checks had virtually 

 lost it by the end of the second day and entirely lost it on the 

 third. Of course the crowding created rather special conditions, 

 unlike those of the lake. The conclusion is that the incrustation 

 contains considerable food. 



6. SUMMARY AND CONCLUSIONS. 



1. Feeding in the Najades is a nearly constant function under 

 normal conditions. The presence of much undigested and some- 

 times living matter in the rectum and feces shows that there is a 

 greater fluctuation in the degree of digestion than in the rate of 

 ingestion. 



2. The posture of a mussel has no effect upon the continuity 

 of the feeding process, a further indication that under normal 



