184 College of Forestry 



bottom. Mud and saiid bottoms have the greatest number 

 of species (64 and 62 respectively), gravel the smallest number 

 (25 species), and boulder, sandy clay, and clay bottoms almost 

 an equal number (46, 47, 48 species respectively). 



Several groups of animals are characteristic o£ a particular 

 kind of bottom; thus, Camharus, Hcptagenia, Argia, Heli- 

 copsyche, and Pscphcnus are usually found on a boulder or 

 gravel bottom. The great majority of associated animals 

 inhabit sand, clay, or mud bottoms in Lower South Bay. Of 

 the 23 higher groups represented in the collections, 18 fre- 

 quented a boulder bottom, 11 gravel bottom, 19 sand, 18 

 sandy clay bottom, and 15 a mud bottom. A few animals 

 were associated with vegetation, usually the water-lilies, such 

 as the lepidopterous larvae, and the larvae and some adults 

 of Coleoptera. As in the case of the Mollusca the study of 

 the higher vegetation was unexpectedly scanty owing, probably, 

 to the heavy growth of filamentous algae. 



Relation to Depth of Water. In Table No. 50 the asso- 

 ciated animals are shown in relation to depth, from the 

 surface to a depth of 18 feet. As with the Mollusca there is 

 first an increase followed by a decrease in number of species 

 as the depth of water increases. Dividing the depth into 

 seven parts the following result is obtained: 



Table No. 50. Showing Decrease of Associated Animals with 



Depth 



Surface 3 species 



I- 3 feet 63 species 



3- 6 feet 78 species 



6- 9 feet Z7 species 



9-12 feet 29 species 



12-15 feet 16 species 



15-18 feet 2 species 



The depth at which the maximum number of species is 

 found is thus seen to be between three and six feet. The 

 sudden drop in depth (9-18 feet) from 29 species to 16, and 

 then to two species in the deeper water is striking. The table 

 also shows that a large number of species are confined to cer- 

 tain depths, mostly in comparatively shallow water. Only 



