364 Aquatic Societies 



lesser animals, of highly adapted form and habits, that 

 are able to dwell constantly in the rush of waters. 



These lesser animals may be roughly divided into two 

 categories according to the sources of their principal 

 food supply: 



1 . Plancton gathering forms, that are equipped with 

 an apparatus for straining minute organisms out of the 

 open current. 



2. Ordinary forms that gather home-grown food 

 about their dwelling places. 



I. Plancton Gatherers. — These are they that live 

 mainly on imported food, which by means of nets or 

 baskets or strainers they gather out of the passing 

 current. These are the most typical of lotic organisms, 

 for they must needs live on the exposed surfaces that 

 are washed by the current. They dwell on the bare 

 rock ledge, over which the water glides swiftty, or on the 

 top of the boulders in the stream bed, or on the exposed 

 side of the wave- washed pier. They are few in kinds, 

 and very diverse in form, and show many signs of 

 independent adaptation to life in such situations. 

 Among them are four that occur abundantly in the 

 Ithaca fauna. These four and their mode of attach- 

 ment and of plancton gathering are illustrated in the 

 accompanying diagram. The fly larva, Simulium, 

 adheres by a caudal sucker, gathers plancton by means 

 of a pair of fans placed beside its mouth, while its body 

 dangles head downward in the stream. The larva of 

 the caddis-fly, Hydropsyche, lives in a tube and con- 

 structs a net of silk that strains organisms out of the 

 water running through it. The caddis-worm, Brachy- 

 centrus, attaches the front end of its case firmly to the 

 top of a boulder in the stream bed, and then spreads its 

 bristle-fringed middle and hind feet widely to gather in 

 any organisms that may be adrift in the passing water. 



