Bristle-Bearing Worms 



173 



Oligochetes — ^Associated with the nematodes in the 

 trash and ooze, there is a group of minute bristle-bear- 

 ingworms, thenaiads (Family Naidae), similar in slender- 

 ness and transparency of body, but very different on 

 close examination; for the body in Nais is segmented, 

 and each segment is armed with tufts of bristles of 

 variable length and form. There are many common 

 members of this family. Besides the graceful Nais 

 shown in our figure there is Chcetogaster , which creeps 

 on its dense bristle-clusters as on feet. There is 

 Stylaria with a long tongue-like 

 proboscis. There is Dero that lives 

 at the surface in a tube of some 

 floating plant stuffs, such as seeds 

 (fig. 82) or Lemna leaves, slipping 

 in and out or changing ends in 

 the tube with wonderful celerity; 

 and there are many others. 



Dero bears usually two pairs of 

 short gill-lobes at the posterior end 

 of the body. 



All these naiads reproduce habitually by automatic ' 

 division of the body, which when in process of develop- 

 ment, forms chains of incompletely formed individuals, 

 as in certain of the fiatworms before described. 



Another group of Oligochetes is represented by 

 Tubifex and its allies. These dwell in the bottom mud, 

 living in stationary tubes, which are in part burrows, 

 and in part chimneys extended above the surface. ' 

 The worms remain anchored in these and extend their 

 lithe bodies forth into the water. On disturbance they 

 vanish instantly, retreating into their tubes. They are 

 often red in color, and when thickly associated, as on 

 sludge in the bed of some polluted pool, they often 

 cover the bottom as with a carpet of a\pale mottled 

 reddish color. 



Fig. 81. Nais. (after 

 Leunis). 



