636 FRESH-WATER BIOLOGY 



rapidity that they can extensively populate large masses of mori- 

 bund algae in a very short time, and their activity accounts in 

 part for the speedy disappearance of such masses in the autumn. 

 Although many of these worms will not thrive in polluted water, 

 others are adapted to foul conditions where fermentation is rife, 

 and, in fact, multiply most rapidly in such situations. Some 

 species of this sort feed extensively on the zoogloeic masses which 

 abound where fermentation is active. The food of most Oligo- 

 chaeta consists chiefly of decaying vegetable matter, but worms of 

 a common Naid species, Chaetogaster diaphanus, have a marked 

 preference for Chydorus sphaericus, a Cladoceran species which they 

 capture and devour in large numbers. Worms of the family Dis- 

 codrilidae are parasitic on crayfishes. 



Certain of the Naididae can swim effectively in open water, but 

 a great majority of the Oligochaeta are limited to crawling move- 

 ments for locomotion. 



Striking structural adaptations are not numerous in the group, 

 but the pecuHar modification of the posterior end in Aulophorus 

 and Dero for purposes of respiration, deserves mention. These 

 worms live chiefly in tubes of their own making or with their 

 bodies almost wholly buried in masses of vegetable material, and 

 respiration is aided by well-developed gill structures (Fig. 985). 

 The Discodrilidae in adaptation to their peculiar mode of life, have 

 become so leech-like in action and external appearance that for- 

 merly it was usual to treat them as belonging to the Hirudinea 

 rather than to the Oligochaeta. 



Collection and Preservation. The larger specimens may be ob- 

 tained by carefully screening mud from bottoms and shores and 

 from about the roots of coarse plants through fine-meshed nets or 

 sieves. Others may be obtained by carefully pulling to pieces 

 decaying rushes and masses of algae. Smaller specimens often 

 may be obtained from the sides of aquaria in which mud and vege- 

 table material have been allowed to stand for a few days. During 

 the fermentation of such masses large numbers of small worms 

 appear in the surface layers and about the margins. 



The most successful methods of preservation vary with different 

 species, and must be gained by experiment, but some general hints 



