STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 215 



thrown open to the passage of food; and for this reason, if for 

 no other, one should find a mingling of digested with undigested 

 food in the rectum. In the Unionidse this mingling does not have 

 an adequate explanation in the style sac. For there is no loss of 

 crystalline style except with starvation. No normal mussel is 

 found without it. Under normal conditions the style has no 

 color, with a very slight core of green. The amount of food 

 which passes through the style or style sac under ordinary condi- 

 tions is very slight. Hence most of the digested diatoms whose 

 tests are found in the rectum have been through the stomach 

 but once. 



Resume. In the rectum and feces the simultaneous occur- 

 rence of green diatoms and empty tests shows that part, but not 

 all, of such material is digested. At any rate it is usable as food. 

 Ingestion is continuous, but digestion is discontinuous, and de- 

 pendent upon demand for nutrition on the part of the tissues. In 

 the Unionidse the return of material from the intestine to the 

 stomach through the style sac does not account for much of the 

 food actually digested, for such a transference of food is pos- 

 sible only when the style sac is empty. Since digestion is a chem- 

 ical reaction the contents of the stomach should be affected alike, 

 and not some wholly digested and some not at all, when equally 

 digestible. 



3. THE UTILIZATION OF NANNOPLANKTON AND 

 MEGALOPLANKTON. 



The materials recovered from the rectum grade down from 

 the largest phytoplankton to very small species. While stomach 

 and rectal contents vary, the larger particles are obviously oftener 

 found undigested than the smaller. This suggests that the 

 smaller are more easily digested ; that everything else being equal, 

 digestibility is inversely proportional to size. The question 

 arises : does the nannoplankton constitute an important, though 

 less conspicuous element of the food of a mussel? 



Juday (see Ward and Whipple, '18) has shown that the nanno- 

 plankton content of a lake may exist in vastly greater number 

 and in a much greater volume than does the net plankton. Even 

 though most of such organisms should escape through the gills 



