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. 

 Indeed, 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 

 For if, by chance, the larva of 



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

 tine. From Leunis. 



as a means of protection, 

 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 

 in which the burrowing habits have led to 

 a loss even of the segments in the adult. 

 This is the case in the group Gephyrea. 1 



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

 between holothurians and worms. 



. Phasco- 

 , Gephy- 

 rean. One-fourth 

 nat. size. From 

 Leunis. 



