438 



ZO 



OLOGY 



SECT. 



the inner end is continuous with a narrow prolongation about 

 equal in length to the body, which runs forwards and inwards 

 to become attached to the mesentery. The external opening 

 or nephridiopore (ext. op) is a fine circular pore capable of being 

 widened or contracted, situated on the ventral surface not far 

 from the base of the ventral cirrus. It leads into a canal, ciliated 

 except in its most external part, which runs through the anterior 

 prolongation to its extremity, where it bends sharply back again and 

 runs to the body, through which it pursues an extremely tortuous 

 course to the outer end, and then bends back again and runs in 



the anterior prolongation to the 

 extremity of the latter, where it 

 opens into the ccelome through a 

 ciliated bell or funnel (fun), the 

 nephrostome, projecting through 

 the septum into the cavity of 

 the segment next in front of that 

 in which the body of the organ 

 lies. The edge of the nephrostome 

 is produced into a number of 

 narrow ciliated processes not 

 represented in the figure. 

 Throughout its course the canal 

 is excavated in a mass of nu- 

 cleated material of a granular 

 character not distinguishable into 

 cells. 



On the dorsal side of each seg- 

 ment, in close relation to the 

 longitudinal 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 pos- 

 sible 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 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 nineteenth and the 



FIG. 360. Nereis dumerilii. One of the 

 nephridia. ext. op. external opening or 

 nephridiopore ; fun. internal funnel or 

 nephrostome [opening into the ccelome ; 

 mes. transverse mesentery or septum. 



