FRESH-WATER MUSSELS. 91 



greatest care had to be taken in handling it, while just the opposite was true of itg near 

 relative, the Lake Pepin mucket, which fed readily on most of the experimental material 

 and was not so fastidious regarding the physical state of the food; that is, the size of 

 food particles and the amount given at one time. This may explain to a certain degree 

 the success attending the culture of the latter species in ponds, but the question then 

 arises why the river mucket is not crowded out everywhere by this mussel, since one 

 species, judging from the shell structure, is as well adapted to live in moving water as 

 the other. As was observed above, however, Lampsilis lutcola is typically an inhab- 

 itant of water with little or no current, while Lampsilis ligamenlina is a true river mussel. 

 The available data of shell structure and feeding habit evidently offer no explanation. 



The blue-point and the spike take a midway position as regards their feeding 

 habits, although the former is perhaps less exacting than the latter. 



Detritus undoubtedl}' forms the main bulk of the food of fresh-water mussels. 

 Dissolved substances may also play a part (Churchill, 1915 and 1916), but their role is 

 probably a comparatively unimportant one when compared with the solid food matter. 

 This must be especially true of streams with relatively pure water, in which mussels 

 have been found to thrive just as well or better than those carrjang large quantities of 

 dissolved matter. 



In view of the universal presence of plants in or near waters productive of mussels 

 there is little likelihood of a shortage of food, for detritus will always be forthcoming. 

 There can be only a very little competition among mussels as far as food is concerned, 

 and the noncommercial species are not objectionable from this standpoint. 



OBSERVATIONS OF H. WALTON CLARK ON FOOD OF MUSSELS." 



In general it may be said that the food of fresh-water mussels, as indicated by their 

 stomach contents, includes about everything obtainable and not positively harmful, 

 organic or inorganic substances, living or dead matter, if not too large or too active for 

 the mussels to take in. As the mussel has no means of mastication it can not use long 

 objects such as filaments of alga; and the like. 



In the course of general biological investigations and of mussel surveys opportunity 

 was had to study the stomach contents of mussels from widely separated areas and 

 under widely different conditions. One of the striking features of the case is that the 

 size and apparent health of mussels bear no direct relation to the apparent nutritive- 

 ness of the material in the stomach. Thickness of shell is partly a matter of heredity; 

 thick-shelled species of Lampsilis are found in fairly good currents where nutritious food 

 material is scarce; thin-shelled Anodontas are usually found in quiet places where the 

 food supply is rich. Moreover, generally speaking, Lampsilis of any species in a quiet 

 lake where food in the form of plankton is abundant, are thinner shelled and smaller 

 than those of rivers. 



Although, generally speaking, thickness of shell seems to be almost always in 

 inverse ratio to richness of food, that relation itself may be partly accidental. In mus- 

 sels the secretion of shell is in relation to current or to mineral content of the water. 



The stomach contents of some large heavy pocketbooks, Lampsilis venlricosa, 

 from the mussel beds in Yellow River, Ind., where this species reaches maximum size, 



o For additional data see Clark and Wilson (1913) and Evermann and Clark (1918). 



