STUDIES ON BIOLOGY OF FRESHWATER MUSSELS. 237 



tion, we may feel safe in the reiterated conclusion that the 

 Unionids exercise choice in the ingestion of materials. As stated 

 by Zacharias ('07), Petersen ('n), the writer ('14), Baker 

 ('16), and others, considerable quantities of inorganic and organic 

 debris are carried into the stomach with food. Probably much 

 of the stuff which Evermann and Clark ('17) call "mud" is 

 organic. The fact that neither they nor other writers list sand in 

 the stomach contents is further evidence of a selection of food 

 material, and that river species are not an exception. Starved 

 mussels were placed in the lake in two localities (i) an open 

 leeward shore in clear water; (2) near the outlet of Pocahontas 

 creek, in muddy water following a rainstorm. The mussels of the 

 first situation reformed the crystalline styles within a few hours. 

 The others contained great quantities of muddy mucus, and did 

 not have well renewed styles until the following day. The slow 

 renewal of the style may be accounted for in part by the dilution 

 of the food. But the presence of mud must be held partially ac- 

 countable, for immediately after the subsidence and clearing of 

 the water it was always found to contain ample food material to 

 renew the style promptly. 



In most species the position of the siphons at some distance 

 above the substratum tends to keep out most of the grosser 

 particles, admitting little but plankton and other materials in 

 suspension. 



In the discussion of the crystalline style (p. 227) the feeding of 

 specific inert substances were recounted. When such materials 

 which were readily identifiable were admitted with the incurrent 

 water they were in no case found in the alimentary tract. As 

 far as size is concerned these particles could very readily have 

 entered the mouth. Since all were rigidly excluded we cannot 

 doubt that sense organs exist for their detection, and that the 

 assorting mechanism is a fairly effective one. Of the materials 

 mentioned only starch might be expected to have a food value, 

 though we cannot assume that it is in acceptable form. As a 

 matter of fact the rejecting reactions were more vigorous in re- 

 sponse to starch than to the other substances. 



In the above experiments on the crystalline style neutral sub- 



