MUSSELS OF W1NONA, PIKE, AND CENTER LAKES. 313 



shoreward edge by wave action, the muskrat, and human agency. 

 Wave action would keep the clumsy and slow-moving forms out 

 of the region from eighteen inches to the water's edge ; the 

 muskrat would keep mussels scarce about his house, as indeed 

 he has ; but neither nor both could keep them absent from favor- 

 able beds in from four feet to eighteen inches of water. This 

 peculiarity of distribution is noticeable only on sandy and 

 gravelly beaches, such as are frequented by bathers. We have 

 seen these throwing mussels on shore and out into deep water 

 and therefore believe this to be the cause of the dearth of mus- 

 sels in these regions. 



In Winona, Pike and Center lakes, mussels are most numerous 

 on the sandy and gravelly beaches ; they are scattered on gray 

 marl ; very rare on muck ; and are not present at all on the fine 

 black mud. They thrive best on gravelly and sandy beds, 

 poorly on gray marl, and not at all on muck and fine black mud. 

 The primary and essential factor in mussel distribution in these 

 three lakes is the character of the bottom, while wave action, 

 the muskrat, and human agency play varying and secondary parts. 



EXPERIMENTS. 



The work done in 1903 seemed to show that bottom condi- 

 tions are favorable or unfavorable as they do or do not allow the 

 animal to assume his natural position and to project his siphons 

 above the mud. When he cannot get his siphons above the 

 mud, his gills become choked and death follows. The animal, 

 therefore, which remains near enough the top of the mud to pro- 

 ject his siphons is bound to be the most successful in resisting it. 

 Any of the mussels can do this on sandy or gravelly bottoms, 

 but where the bottom is composed largely of the fine black mud, 

 only such forms as U. f aba/is, A. grand is and A. cdcntnla are able 

 to sustain themselves. These shells are all lig-ht and the last 



o 



two expose a large surface in proportion to weight. 



We placed eight baskets in water from nine and one-half feet 

 on sandy bottom to eighty-two feet on fine dark mud. Of these, 

 two were lost within a week, one in four weeks, and one in eight 

 weeks and three days. Four remained until the contents were 

 taken out, killed, and examined. We further carried on an ex- 



