THREADWORMS, HAIRWORMS, BRISTLEWORMS 



surface of the earth is being continually passed through the 

 bodies of earthworms, how they have literally overturned it 

 again and again. Just as important work is done by their 

 fresh water allies. The little red Tubifex (Fig. 114) dig their 

 heads an inch or more down into the bottom mud, and feed 

 there in a layer containing but little oxygen, and where but 

 for them decomposition would go on ver}^ slowly. But as 

 they feed, the waste matter from their bodies is cast out from 

 the ends projecting above the mud and thus they make a 

 constant overturn of the mud very like the other one con- 

 tinually being made on land. 



Collecting and aquarium study. — Hundreds of threadworms 

 and bristleworms can be found by simply rinsing freshly 

 gathered alg£e and water plants in a dish of clean water. 

 Those which live on the bottom can be collected in large 

 numbers by screening mud through a fine-meshed net or 

 sieve (p. 29). 



Despite their importance in the economy of nature, bristle- 

 worms are not interesting to the general collector. They 

 are too small to be seen easily and their features are mono- 

 tonous when visible; their chief interest lies in their works. 



Identification. — In most of these worms only the most prom- 

 inent structures can be seen with a hand-lens. A few of the 

 best known forms are here figured. 



Fig. III. — Nats, one of the commonest bristle- 

 worms. 



Nais. — There are several species of these transparent little 

 worms (Fig. iii). They are all similar, some of their setee 



143 



