24 JEAN DAWSON 



The inmarch of sedges into the water on the protected sides 

 of the lakes will probably always afford more or less of a harbor 

 for Physa. It cannot afford very favorable conditions however, 

 for there are two factors involved that cannot but affect snail 

 life, namely : decaying debris and enemies. It will be remembered 

 that it is under such conditions that peat beds are being formed 

 through the decay of sedges, and also that such regions are the 

 breeding grounds for fishes and frogs. 



It has been shown that Physa does not find favorable con- 

 ditions in the waters of large lakes, that spits. and bars form 

 beach pools and make life possible in the waters of medium 

 sized lakes and that it is only when the smaller lakes or ponds 

 are reached that Physa is found generally distributed in the 

 shallow waters of the open shores. In the series of lakes studied 

 the snail is found to have a very discontinuous distribution 

 in the larger lakes and this becomes more or less continuous 

 in shallow or marginal waters of the small lakes. Cases may 

 be found where small lakes may have the snails completely 

 girdling the shallow waters of its shores. 



We have seen that the various habitats in lakes are also sub- 

 ject to changes as are those of the river. The snail lives under 

 certain conditions and follows these conditions wherever they 

 may lead. This is strikingly illustrated in the distribution of snail 

 life behind the ice-rampart of Kavanaugh Lake and the spit 

 and bar of the east shore of Crooked Lake. In all probability 

 the snails followed the formation of this spit from year to year 

 always keeping a definite distance back from the open end and 

 aw^ay from the quieter debris laden w^ater at its base. When 

 such a spit reaches the opposite shore and the waters behind 

 it are cut ofT entirely from those of the lake, snail life will rapidly 

 die out in such a pool as is suggested by the absence of snails 

 in the waters behind the spit on the south side of the lake. 

 The trees and bushes bordering Crooked Lake in the region of 

 the spits and bars, have no doubt greatly lessened the number 

 of snails there. Few lakes offer more promising conditions, 

 as far as protection from waves is concerned, but the deciduous 

 leaves decaying in the quiet plant-grown waters -of the bays 

 will soon render snail life impossible. 



The bar that is forming to connect the island with the land 

 on the east side of Crooked Lake will form a new shore in whose 



