10 JEAN DAWSON 



the substratum, which, if it is not very coherent, yields and the 

 snail is buoyed to the surface of the water. If this happens 

 in a creek with a swift current, the animal is carried down the 

 stream on the surface of the water, or if it expels the air from 

 its lung, as it often does when unexpectedly pushed upward, 

 it sinks to the bottom of the creek bed and is rolled along by 

 the current. An examination of the diagram of Kavanaugh 

 swamp, Fig. i, will show the lower part of Creek F and the 

 large ditches leading into, and the one leading out of the lake 

 are practically devoid of Physa gyrina. In all three the water 

 is flowing swiftly (faster than in the other streams of the swamp) 

 over coarse peat which is so unstable that it does not offer a 

 sufficiently firm substratum for the snails to maintain a foot- 

 hold. 



b. Shallow Eddies as a Habitat. In the background of Fig. 5, 

 the stream may be seen to spread out ; where the banks on both 

 sides are low, into a shallow eddy-pool, near the middle of which 

 the stream divides into two unequal parts. The larger, swifter 

 part flows to the right in the picture, while the gently flowing 

 water of the left division passes around through the pool to 

 meet and be carried on by the swifter stream as it flows again 

 into the deeper channel. In the relatively quiet waters a 

 quantity of algae grows and furnishes food for the abundant 

 snail population of the pool. On the sides of the creek is 

 found an occasional small quiet pool containing Physa here and 

 there, except where the fish are abundant or where a great 

 many pond weeds grow. A large shallow pool, well grown 

 over with Chara and Ceratophyllum, was found in the creek 

 east of Lima Center, Mich., and contained a moderate number 

 of Physa anatina Lea. 



Physa is found in creeks in five other habitats which deserve 

 mention for reasons other than the number of snails found 

 in them. They are as follows: 



1. Where a stream is not building up its convex bank too 

 rapidly a small number of Physa are found in company with 

 some pond weeds. The creek on the west side of Lima Center, 

 Mich., is an example of this sort. 



2. In deep pools (where fish and other enemies are not too 

 abundant), the long grasses bend over into the moving water 

 and to them cling small Physa. The noticeable fact about 



