x PHYLUM ANNULATA 449 



of narrow ciliated processes not represented in the figure. 

 Throughout its course the canal is excavated in a mass of nucleated 

 material of a granular character not distinguishable into cells. 



On the dorsal side of each segment, in close relation to the longi- 

 tudinal muscular bundle, is a specially developed ciliated tract of 

 the ccelomic epithelium of the nature of a short funnel without 

 external aperture, the dorsal ciliated organ. It is possible that at 

 the time of sexual maturity an aperture is formed through the 

 body-wall opposite this funnel, and that thus a genital duct of a 

 temporary character becomes formed : but no such opening has 

 ever been observed. 



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 (ccelomic epithelium) lining the ccelome 

 and the structures it contains. In Nereis dutnerilii there is in the 

 male only a single pair of these proliferating masses of cells (testes), 

 situated in one of the segments between the nineteenth and the 

 twenty-fifth. But in other species of Nereis they are much more 

 numerous. 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. 351, ov), formed by a similar process 

 of proliferation, take the form of rounded masses of cells, meta- 

 merically arranged, surrounding the principal vessels throughout 

 the length of the body. The young ova become detached from 

 the ovaries, and attain their full development 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 probably through apertures temporarily formed by 

 rupture of the body-wall (ef. above), and impregnation takes place 

 by contact between the two sets of elements while floating freely 

 in the sea- water. 



Nereis dumeriiii is an extremely variable species. If we 

 compare a number of specimens, we find numerous individual 

 differences between them. The most striking of these are 

 differences of colour and of the number of segments in the body ; 

 but a careful examination reveals many other points in which 

 individuals differ. Thus the precise form of the lobes of the 

 parapodia, together with the number of set* in the two bundles, 

 vary ; so also do the relative length of the tentacles, the 

 number of teeth on the jaws, and the number and arrangement 

 of the denticles in the bnccal cavity. Not only are such individual 



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