352 ZOOLOGY. 



burrowing to swimming in the open water. Accordingly 

 when the sexual products are ripe the worms of both sexes 

 swim about actively while the eggs and sperms are dis- 

 charged, and thus fertilisation takes place. A parapodium 

 in this condition is shown in Fig. 181. Originally speci- 

 mens of Nereis with such parapodia were supposed to 

 belong to a distinct genus, which was named Heteronereis, 



PERISTOMIAL 

 TENTAQLES 



Fig. 183.-ANTERIOR END OP NKREIS WITH PHARYNX EVKRTED, PROM DORSAL 



SIDE, ENLARGED. 



and this term may be used to signify the sexual modifica- 

 tion ; the two stages have been distinguished also as the 

 atokous and epitokous conditions. 



5. Head and Pharynx. Fig. 183 represents the an- 

 terior end of a Nereis from the dorsal side, with the 

 pharynx everted. The pharynx is usually seen in this 

 condition in preserved specimens, owing to the contraction 

 of the muscles which occurs when the worms are killed. 

 The extremity of the body is formed by the prostomium, 

 which is of triangular outline and projects dorsally in 

 front of the oral aperture. The prostomium bears two 



