PHYLUM ANNULATA 



411 



direction. The outer end of each cell narrows into a nerve-fibre 

 forming part of the optic nerve ; near this end is a nucleus ; the 

 main body of the cell is densely pigment ed ; the inner part projects 

 towards the lens as a clear hyaline rod (r.). The cuticle of the 

 general surface passes over the eye, and a continuation of the 

 epidermis, with its cells somewhat flattened, constitutes the 

 cornea (co.). 



The organs which are supposed to perform the function of 

 excretion are a series of metamerically arranged pairs of tubes, 

 the segmented organs or nephridia (Figs. 321 and 322, neph., Fig. 325 

 occurring in all the segments of the body. The nephridium 

 consists of two parts a body and a narrow anterior prolongation. 



FIG. 324. Nereis. Section through one of the eyes. co. cornea; cu. cuticle ; /. lens ; r. layer 

 of rods ; re. retina. (After Andrews.) 



The body is of an irregular oval shape directed nearly transversely, 

 but slanting somewhat. The outer end, situated in the base of 

 the parapodium near its middle, is much the narrower. 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 nephri- 

 diopore (cxt. 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. , This leads into a canal 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 coelome through a ciliated bell or 

 funnel (fun.), the nephrostome, projecting through the mesentery 



