176 



PRACTICAL ZOOLOGY 



Fresh-water Segmented Worms. — Smaller and less con- 

 spicuous segmented worms are abundant in fresh-water ponds 

 and streams. The tube worm, Tubifex, is a reddish colored 

 creature that makes tubes in the mud on the 

 bottom of slow-running brooks. The duckweed 

 worm, Dero, frequents the surface of ponds, where 

 it constructs a shelter for itself by fastening to- 

 gether leaves of the duckweed or other plants. 



Some of the fresh-water worms have the inter- 

 esting method of reproducing by fission. This is 

 true of Nais (Fig. 100, A), a little worm whose 

 body sometimes becomes pinched in two, each 

 part then growing into an entire animal. This is 

 one sort of asexual reproduction, or reproduction 

 without the use of germ (sex) cells. Sexual repro- 

 duction, the opposite form, is brought about 

 by the sex cells, the eggs and spermatozoa. 



Marine Segmented Worms. — Most of us have 

 no opportunities to see the worms that live in the 



sea, but many of 

 these are very inter- 

 esting animals. The 

 sandworm or clam- 

 worm, Nereis (Fig. 

 100, B), is a common 

 marine annelid that 

 swims freely about in 

 the water by means 

 of pairs of oarlike 

 appendages, one pair 

 on each segment. It 

 also possesses four 

 eyes and a number of tentacles on the head, which bear sense 

 organs of touch and smell. Because of the large number of 

 bristles on its appendages Nereis and its relatives are called 



w 



m 



Fig. 100. -- A, a fresh-water worm, Nais. 

 B, a marine worm, Nereis. (After Oersted.) 



