138 



ZOOLOGY 



filament at the hinder end of the body (Fig. 124). It also 



has eyes ; while Dero has none. Both Dero and Nais have 



the interesting habit 

 of reproducing by 

 dividing the body 

 transversely. In the 

 middle of the body 

 tentacles begin to 

 arise, a new mouth is 

 formed, and the worm 

 constricts into two. 

 I n d e e d, sometimes 

 several new heads may 

 be forming in the 

 midst of a single 

 worm. This habit is 

 of advantage not only 

 in multiplying the 

 number of individuals 

 of the species, but also 



as a means of protection. For if, by chance, the larva of 



the water-beetle Dytiscus seizes a Nais 



and bites it in two, the part which escapes 



can go on developing new^ individuals. 

 The slow-moving, burrowing habit of 



the earthworm has led to a nearly complete 



absence of such appendages as Nereis 



possesses. There are other ringed worms 



i T , i i . i -i , FIG. 125. Phasci)- 



in which the burrowing habits have led to i os oma, a Gephy- 



a loss even of the segments in the adult. rean - One-fourth 



r , i ,-, ,. nat. size. From 



11 us is the case in the group Gephyrea. 1 Leunis. 



1 ytfivpa, bridge ; because they were once considered to bridge the gap 

 between holothurians and worms. 



FIG. 124. Nais: a, mouth; b, anus; c, intes- 

 tine. From Leunis. 



