PHYLUM ANNULATA 



197 



Nereis is unisexual. The sexual elements, ova or sperms, 

 are formed from temporary masses of cells {ovaries or testes), 

 which are developed towards the breeding season by a pro- 



Fig. 112. — Nereis. Anterior portion of nervous system, comprising the brain, the 

 oesophageal connectives, and the anterior part of the ventral nerve-cord. (After 

 Quatrefages.) 



liferation of the cells of the membrane {peritoneum) lining 

 the ccelom and the structures it contains. 



Ova and sperms, when fully ripe, are discharged, reaching 



