1889.] 



NATURAL HISTORY OF DERO. 



441 



a complete list of literature referring to this genus, for the reason 

 that liousfield has gone into the matter somewhat fully. 



Concurrently with the clevelopment of the sexual organs, certain 

 of the lower aquatic Oligochaeta have been stated to show other 

 peculiarities, so that there is a kind of dimorphism among the 

 individuals. The sexual individuals of Bero, in common with other 

 Ohgochseta, show no traces of multiplication by budding ; connected 

 with this is a regularity in the arrangement of the seta-bundles at the 



Fig. 1. 



Ventral aspect of sexually mature Bero. 

 cp., spermatbecal pores; c?, male pores. 



distal extremity of the body. In the asexual form, on the contrary, 

 the continual growth of the posterior groups of setae appears to 

 produce an irregularity in the disposition of the seta-bundles ; the 

 termination of these, for example, in D. pet-rieri (Bousfield) (see 

 Proc. Zool, Soc— 1889, No. XXX. 30 



