FIELD BOOK OF PONDS AND STREAMS 



the refuse in the water and in turn becoming the food of larger 

 insects and young fishes. Except for the aquatic earthworm 

 they are all small animals; few are more than an inch or two 

 long, most of them less than half an inch; many are 



Fig. 109. — I, nematode worm; 2, oligochaete 

 worm; 3, midge (chironomid) larv^a. (i, after Jagers- 

 kiold.) 



microscopic. Although their size is insignificant their num- 

 bers are so enormous that they play an important and con- 

 structive part in water life. 



Threadworms — Nematoda 



Form and habits of nematodes. — The threadworms are slen- 

 der, strikingly smooth, unsegmented worms, seldom more 

 than a few millimeters long, very often microscopic. They 

 are almost inconceivably abundant in pond and stream and 

 ocean as well as in the soil. Great numbers of them live in 

 the bodies of aquatic mollusks, crustaceans, and worms, 



140 



