BIOLOGY OF PHYSA 39 



their isolation. The snail is quite generally dispersed in creeks 

 but only an occasional favorable habitat is found, as behind 

 stones and in quiet pools. Rivers contain but few snails, the 

 most favorable places being in the slack water above dams, 

 and in the quiet waters which have been cut off from the river 

 such as, back-waters, flood-plains and deserted channels or 

 ox-bows. 



a. Physa Habitats in Still Water. The pond offers, on the aver- 

 age, relatively a much greater area of water that is sufficiently 

 shallow for the snail, than do lakes and streams. The pond 

 may not only offer a more continuous habitat around its margin 

 but it may be shallow enough for the snail to live nearly through- 

 out its whole bed. On the other hand, a pond may be of the 

 sink-hole order with steep shores so that its waters, while rela- 

 tively shallow as compared with a lake, may not be sufficiently 

 shallow along its margin to enable the animal to live. The 

 amount of decaying debris in a pond habitat is usually more 

 than in rivers or lakes where currents and waves shift it about 

 or carry it away. The amount of this debris varies with the 

 growth of the vegetation which is necessarily less in ponds that 

 are subject to periods of drought or where the inwash from the 

 surrounding hills is sufficiently great in amount to bury the 

 debris. In general, the snail occurs abundantly in lakes only 

 in the littoral zone, as a rule the conditions are more favorable 

 than those of the river. The shallow wind-swept shores of 

 large lakes are barren and it is only in their beach pools that 

 the snail is found. It finds protection from severe waves and 

 currents behind spits and ice-ramparts that are formed in medium 

 sized lakes and upon their sedge-grown shores. In the littoral zone 

 of certain small lakes Physa is found even in unprotected places. 



The very nature of the pond brings Physa into new environ- 

 mental conditions of which nothing was known in larger bodies 

 of water. The snail's relation to pond weeds and drought brings 

 out possibilities of adaptation that would hardly have been 

 Suspected from a study of the snail in the waters of the river 

 system and lake proper. The double relationship which exists 

 between the snail and the water plant, namel}^ that of food 

 and aeration of water, would naturally lead one to suppose 

 that the optimum of the water plant would be consistent with 

 that of the snail. This is, however, not true even in water 



