412 



ZOOLOGY 



SECT. 



wri 



into the cavity of the segment next in front of that in which the 

 body of the organ lies. Throughout its course the canal is exca- 

 vated in a mass of nucleated material of a granular character not 

 distinguishable into cells. 



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 proliferation of 

 the cells of the membrane (peritoneum) lining the ccelome and the 



structures it contains. In Nereis 

 dumerilii there is in the male 

 only a single pair of these 

 proliferating masses of cells 

 (testes), situated in one of the 

 segments between the nine- 

 teenth and the twenty-fifth. 

 These, during the season of their 

 active development, give off 

 groups of cells which become 

 disseminated throughout the 

 ccelomic fluid. The original 

 cells (mother-cells) undergo 

 division into smaller cells, each 

 of which develops into a sperm 

 with a minute rod-shaped head 

 and a long vibratile flagellum 

 or tail. In the female the 

 ovaries (Fig. 322, ov.), formed by 

 a similar process of prolifera- 

 tion, take the form of rounded 

 masses of cells, metamerically 

 arranged, surrounding the prin- 

 cipal vessels throughout the 

 length of the body. The young 

 ova become detached from the 

 ovaries, and attain their full de- 

 velopment while floating about 

 in the ccelomic fluid. Both 



ovaries and testes dwindle after they have given off the sexual 

 cells, and at the non-breeding season of the year are not to be 

 detected. 



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

 exterior, in the case of the sperms probably through the nephridia, 

 in the case of the ova, which are much too large to pass out in 

 this way, probably through apertures temporarily formed by 

 rupture of the body-wall ; and impregnation takes place by contact 

 between the two sets of elements while floating freely in the 

 sea-water. 



FKJ. 325. Nereis dumerilii. One of the 



nephridia'. e.rt. op. external opening or 

 nephridiopore ; fun. internal funnel or 

 nephrostome opening into the ccelome ; 

 i/.x. mesentery or septum. 



