FRESH-WATER MUSSELS. 89 



or lesser fragments of various thread algse, such as Spirogyra, increased in numbers 

 until they outranked all others in importance. The list of diatoms showed these forms 

 as important: Coscinodiscus, Synedra, Asterionella, and Navicula. In far smaller 

 quantities but generally present were Gyrosigma, Tabellaria, Gomphonema, Epithemia, 

 and a few others that are negligible for practical purposes. 



In both quantitative and qualitative constitution no appreciable difference was 

 noticed between samples from Lake Pepin and those from the Mississippi River at 

 Homer, Minn. 



FOOD DISCRIMINATION UNDER NORMAL CONDITIONS. 



Having ascertained the materials available in the water as a possible source of 

 food, the next step was to determine whether the different mussels showed a preference 

 or dislike for any of these constituents. This necessitated an examination of the 

 stomach and intestinal contents of naturally feeding mussels. In every case the con- 

 stituents of the material obtained from the stomachs corresponded to those found in 

 a free state in the water. This refers not only to the kind of material found but also to 

 the percentages, which were discovered always to correspond, at least roughly, to those 

 obtaining in the water at that period. Lastly, no difference was observed — ^in stomach 

 or intestinal contents — among any of the four forms of mussels concerned. Thus, 

 under normal conditions no discernible degree of discrimination is evinced. 



UTILIZATION OF FOOD MATERIALS. 



The question of the utilization of these materials was best solved by an examina- 

 tion of the feces. It was astonishing to note that only about one-half of the green algae 

 and diatoms were attacked to any degree by the digestive processes. In fact the 

 green algse, with their often delicate cell walls, on many occasions did not even lose 

 color. It was the detritus that underwent the greatest changes. The vegetable origin 

 of this material was easily discerned under the microscope before digestion had taken 

 place, but in the feces, after digestion, the substance was found almost always to have 

 undergone a radical change in appearance and in structure. It was evidently attacked 

 by the digestive processes to a much greater degree than the plankton. 



These observations point to a comparatively unimportant role as played by algae 

 and diatoms in the food of mussels. Not only are these forms present in very much 

 smaller amounts than the dead-food materials but also they are not digested as well. 



EXPERIMENTS IN FEEDING VEGETABLE MATTER. 



Although under normal natural conditions no discrimination of food was observed, 

 conditions might easily arise which would bring about a radical change in the constit- 

 uents of this normal food supply. Feeding with different materials was tried, therefore, 

 to determine any preference that the mussels might have. 



The method used was to starve the mussels for four to five days and then feed 

 them with the material under investigation. In this way the intestine was first cleared 

 and the state of digestion of the fed material determined without any disturbing con- 

 tamination from substances previously present in the intestine. As starved mussels 

 may lose their sense of discrimination to a certain degree, an equal number of control 

 mussels feeding and living in a tank with a flow of river water were always experimented 

 on at the same time. The food was administered from a long pipette into the intak- 

 ing siphon. It is unnecessary to go into the details here, but it may be mentioned that 



