STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 239 



circumstances ingestion may go on with less effort than an inter- 

 ruption of feeding. 



3. The return of undigested material from the intestine through 

 the style sac to the stomach is an unusual occurrence in the 

 Najades, which takes place only after periods of starvation, and 

 which is interrupted With the reformation of the style. It is a 

 function much less significant in the Najades than in the tidal 

 forms. 



4. Experiments in feeding relatively finer and coarser plankton 

 show that both are capable at least of stimulating the renewal of 

 the crystalline style. Both have food value. It is probable that 

 the nannoplankton furnishes a much greater part of a mussel's 

 food than has been suspected. The studies of intestinal contents 

 of the Unionids have not demonstrated what the actual food is, 

 but rather the undigested residue. Experiment here has shown, 

 however, that the organisms undigested in the feces are some- 

 times digested, under another set of conditions. 



5. Starved animals fed in Pocahontas creek below the outlet of 

 a sewer showed the following peculiarities in intestinal contents : 

 ( i ) the occurrence of many relatively large organic fragments ; 

 (2) abundance of minute flagellates; and (3) great quantities of 

 Oscillatoria filament. 



6. The regeneration of the crystalline style is in response to the 

 ingestion of food, and not due to the physico-chemical character 

 of the water. 



7. Creek-fed mussels show a variation in the color of the style 

 through hyaline, amber, and milky. The apparently rhythmic 

 character of this variation corresponds roughly to the variation of 

 sewage discharged from a septic tank. The milky color is ac- 

 counted for by the presence of bacteria. 



8. A repetition of experiments in feeding infusions indicates 

 that flagellates (and bacteria) present are responsible to a greater 

 extent for the renewal of the style than are the bulkier ciliates. 



9. The reformation of the crystalline style is a satisfactory 

 index of the renewal of the feeding activity. 



10. The function of the crystalline style varies. It may more 

 or less imperfectly perform several functions at once. The 



