BIOLOGY OF PHYSA 



43 



h. The General Relation of Physa to Its Environment. The con- 

 ditions under which Physa Hves in the different habitats appar- 

 ently varies greatly and there is a corresponding variation in 

 the number of snails found in different situations. However, an 

 analysis of the five habitats having the largest number of snails 

 shows a striking similarity in the conditions, as may be seen 

 from the following table: 



Creek-Bed 

 Habitat 



Shallow water, 

 from 1 inch to a 

 foot. 



Shallow water, 

 from 5-8 

 inches deep. 



Running water 



Growth of Clad- 

 ophora upon 

 stones. 



Little or no de- 

 bris. 



Gravelly substra- 

 tum. 



Full sunlight. 



No enemies ob- 

 served. 



Beach 

 Pool 



Shallow water, 

 from 2-10 

 inches deep. 



Water supplied 

 from the lake 

 without severe 

 wave action. 



Small amount 

 of Algae. 



No debris. 



Clean, sandy 

 substratum. 



Full sunlight. 



Behind Ice- 

 Rampart 



Exchange of 

 water from the 

 lake. 



A medium 

 number of 

 plants. 



Little or no 

 vegetable de- 

 bris. 



Clear, gravelly 



substratum. 



Little or no 

 shade. 



No enemies ob- 

 served. 



No enemies ob- 

 served. 



Behind Spits 

 and Bars 



Large area of 

 shallow water 

 from 1-6 

 inches deep. 



Free exchange 

 of water from 

 the lake. 



Comparatively 

 few plants. 



Very little de- 

 bris. 



Clear loam sub- 

 stratum. 



Practically 

 shade. 



No enemies ob- 

 served. 



Pond near Uni- 

 versity Campus 



Large zone of 

 shallow water 

 from 1-12 

 inches deep. 



Still, shallow 

 water. 



Medium amount 

 of Spirogyra 

 and Alisma 

 plantagu. 



Medium amount 

 of debris. 



Clay substra- 

 tum. 



Shade from 

 Alisma plan- 

 tago, when ma- 

 ture. 



No enemies ob- 

 served. 



In all the different habitats studied, Physa gyrina Say, Physa 

 gyrina hildrethiana Lea, Physa anatina Lea, Physa ancillaria 

 Say, Physa heterostropha Say and Physa elliptica Lea were found. 

 Physa gyrina was the most common. Four of the above species, 

 namely, Physa gyrina, Physa ancillaria, Physa heterostropha 

 and Physa hildrethiana Lea have an optimum with the follow- 

 ing conditions : Shallow water ; minimum amount of shade ; few 



