448 



ZOOLOGY 



pigmented ; 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 some- 

 what 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. 

 f 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. 350 and 351, neph, Fig. 

 854) 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 ; 

 the inner end is continuous with 

 a narrow prolongation about 

 equal in length to the body, 

 which runs forwards and in- 

 wards to become attached to the 

 mesentery. The external open- 

 ing or nephridiopore {eoot. 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, ciliated except in its 

 most external part, which runs 

 through the anterior prolonga- 

 tion 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 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 



Fig. 354.— 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. 



