PHYLUM ANNULATA 



437 



system, distributed to the anterior part of the alimentary canal. 

 The first ganglion of the ventral cord, which is situated in the 

 third segment, represents at least two double ganglia which have 

 coalesced, as is shown by the fact that it gives off nerves to the 

 two posterior pairs of peristomial tentacles and to the first pair of 

 parapodia. 



The tentacles and palpi, as well as the cirri, are probably organs 

 of the sense of touch. The only other sense-organs are the four 

 eyes and the two nuchal organs, all situated on the prostomium. 

 The eye (Fig. 359) consists of a darkly pigmented cup, the retina 

 (re.), with a small rounded aperture, the pupil, and enclosing a 

 mass of gelatinous matter, the lens (I.). The wall of the cup is 

 composed of numerous long and narrow cells lying parallel with 

 one another in a radial direction. The outer*end of each cell 

 narrows into a 

 nerve-fibre form- 

 ing part of the 

 optic nerve ; near 

 this end is a 

 nucleus ; the main 

 body of the cell is 

 densely pig- 

 mented ; the inner 

 part projects 

 towards the lens 

 as a clear hyaline 

 rod(r). The cuticle 

 of the general 

 surface passes 

 over the eye, and 



o 

 a 







Cll 

 CO 



f\( FIG. 359. Nereis. SectionCthrough one of the eyes. co. cornea 

 01 ni cuticle; 1. lens; r. layer o 1 rods; re. retina. (After Andrews.) 



the epidermis, 



with its cells somewhat flattened, constitutes the cornea (co). 

 The nuchal organs consist of a pair of pits lined by a special 

 ciliated epithelium with gland-cells, situated in close contact with 

 the posterior part of the brain near the posterior part of the 

 prostomium on the dorsal side. They are regarded as olfactory 

 in function. 



The organs which are supposed to perform the function of 

 excretion are a series of metamerically arranged pairs of tubes, 

 the segmental organs or nephridia (Figs. 356 and 357, neph, Fig. 

 360) occurring in all segments of the body with the exception 

 of several at the anterior and posterior ends. The nephridium 

 consists of two parts a body and a narrow anterior prolonga- 

 tion. The body is of an irregular oval shape directed nearly trans- 

 versely, but slanting somewhat ; the outer end, situated in the 

 base of the parapodium near its middle, is much the narrower ; 



