BIOLOGY OF PHYSA 9 



whose physical condition greath^ resembles the optimum, but 

 upon closer examination it was found that there were additional 

 factors, which in all probability caused their barrenness. The 

 following illustrations will serve to show^ these influences: On 

 the north side of Detroit avenue,. Ann Arbor, Mich., where the 

 creek crosses the street, the stream presents conditions almost 

 exactly like those of the optimum; there is the shallow water 

 with stones and Cladophora showing the same relation to each 

 other and the current, but there are no snails to take advantage 

 of their shelter and food. The presence of a flock of ducks 

 which frequent this section of the creek may perhaps explain 

 the absence of the snails at this place. The problem of snail 

 enemies is one to which no special study was given, but it w^as 

 observed that where fish, ducks, and crayfish abound in the 

 creek, snails are either absent or very few in number. 



In Fig. 4 is represented another place in the creek bottom 

 where one would naturally look for snails, but here it is seen 

 upon closer inspection that a layer of silt covers the stones 

 and bed. There are no pond weeds growing here except a few 

 opposite the basket, shown on the right bank in Fig. 4. These are 

 in a little pool of quiet water, whefe the only Physa in this 

 region are found. The stones are covered with a thick, soft 

 mass of silt, which at a glance resembles Cladophora, but upon 

 examination proves to be a fine, muddy deposit. None of this 

 deposit is found beyond a deep pool in the creek which lies in the 

 stream between Fig. 4 and Fig. 3. The location of the pool may be 

 seen in Fig. 3 near the fence. The bottom of this pool is covered 

 with a deposit of silt and it is concluded that the mud which the 

 stream w^as carrying above this point had settled when the flowing 

 water entered the deeper quiet pool. A small amount of algae grows 

 about its edges and upon it are living a few adult Physa. It was 

 not determined why Physa is not found in the region shown 

 in Fig. 4, unless it is on account of the absence of food, or perhaps 

 it is more probable that the snail has been unable to gain a 

 foothold in the current of w^ater; however, it might also be 

 due to the mode of respiration of the snail. A pulmonate snail 

 like' Physa whose lung is full of air has a specific gravity much 

 less than the water so the only way it can descend into the 

 water is by adhering to the substratum by means of its foot. 

 In consequence of its lightness the snail exerts a pull upon 



