26 



FRESH-WATER BIOLOGY 



shores of the Great Lakes down to a depth of eight feet or more are 

 usually almost entirely without bottom organisms. 



The character of terrigenous bottom is an important condition of 

 existence chiefly where current or wave action is strong and becomes 

 of little or no importance where there is no movement, as in the 



FIG. 3. 



The form of bottom and size of bottom materials in a cross section of the North Branch of the Chicago 

 River with distribution of animals, a to d natural size, a, burrowing may -fly nymph (Hexagenia sp.); 

 b, small bivalve (Spharium stamineum), two individuals, two views; c, viviparous snail (Campeloma in- 

 tegrum), seen from two sides; d, the long river snail, young and full grown (Pleurocera elevatum) ; e, cross 

 section of the stream with reference to a curve (/). (Original.) 



bottom of one of the Great Lakes. However, bottoms of soft muck 

 containing putrescible organic matter occur in the absence of current 

 and constitute a condition of existence sharply differentiated from 

 terrigenous bottoms because they can support only certain types of 

 organisms, mainly anaerobes, and but few of these. Many aquatic 

 animals use the bottom materials in the construction of their cases, 

 nests, etc. Thus the caddis worms (certain species of Mollana 

 and Geora) build cases of sand grains weighted at the sides by small 

 pebbles. The horned dace and several other fishes associated with 

 it use pebbles to build their nests. The pebbles must be of a cer- 

 tain average size. Many animals form associations (memory) with 



