NERVOUS SYSTEM OF NEREIS 



255 



pair to the first parapodium. In the remainder of the body 

 there is a ganglion in each segment, whence nerves pass out- 

 wards to the parapodium and muscles of the segment (Fig. 124). 



In Nereis the apparently single ganglion in each segment 

 really consists of two halves, and the apparently single cord which 

 traverses the whole length of the body consists of two closely 

 apposed cords. In some worms, such as Serpulidae, the two 

 cords are more or less widely separated, and the two ganglia of 

 each segment are thus distinct, and connected by a transverse 

 commissure. In Nereis, as well as in many other Polychaeta, the 

 nerve-cords lie within the body-wall, but in other cases they 

 lie in the epidermis, as they do in Archiannelida. 



The visceral nervous system, supplying the muscles of the 

 pharynx, is frequently highly developed. In Nereis it arises on 

 each side by two roots, one from the brain, the second from the 

 circum-buccal commissure. 



The organs of sense in Nereis are eyes, tentacles, palps, and 

 cirri. The four eyes, which rest upon the brain, have the struc- 

 ture represented in Tig. 130. The retina consists of a single 

 layer of cells containing pigment ; each cell is drawn out peri- 

 pherally into a nerve-fibre, whilst centrally it forms a cuticular 

 product the " rod " (h). The edges of the retina are continuous 

 with the surrounding epidermis, and the cup thus formed remains 

 widely opened to the cuticle in a few Polychaetes, e.g. Autolytus, 

 and in the young of Nereis, but more usually it has the relations 

 represented in the 

 figure. The lens is 

 produced by the re- 

 tinal cells (accord- 

 ing to Andrews 1 ), 

 and is in some 

 cases (Eunice, Am- 

 phinome) continu- 

 ous with the cuticle. 

 It appears to be 

 composed in other 

 cases (Lepidonotus) 

 of continuations of 

 the retinal rods. The structure of the other sense organs indi- 



Fig. 130. Eye of Nereis. (After Andrew.?.) x 150. A, 

 Section through the entire eye ; c, cuticle ; e, epidermis ; /, 

 lens ; h, rods ; r, retina ; n, optic nerve : B, isolated re- 

 tinal element ; c, cell ; p, pigment ; h, rod : /, nerve-fibre. 



Andrews, "The Eyes of Polychaetes," J. Morj)h. vii. 1892, p. 169. 



