PHYLUM ANNULATA 



447 



given off nerves to the two anterior jjairs of periatomial tentacles. 

 Running backwards from the point of nnion of the oesophageal 

 connectives along the entire length of the body of the worm, on 

 the ventral aspect, is a thick cord of nerve-matter, the ventral 

 nerve-cord (4). In each segment this cord presents a little dilata- 

 tion from which nerves are given off to the various parts of the 

 segment ; and each of these enlargements is really double, consist- 

 ing of a pair of closely-united ganglia. The intermediate parts of the 

 cord, between successive pairs of ganglia, are also double, consisting 

 of a pair of longitudinal connectives enclosed in a common sheath. 

 Given off behind from the cerebral ganglion is a system of fine 

 nerves with occasional small ganglia, the stomatogastric or visceral 

 sijstem, distributed to the anterior part of the alimentarj' canal. 



Fia. 3.53.— Nereis. 



-Section through one of the eyes. co. cornea ; cu. cuticle ; /. lens 

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



. layer 



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 oy 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 nuchid organs, all situated on the prostomium. 

 The eye (Fig. 353) consists of a darkly pigmented cup, the retina 

 {re.), with a small rounded aperture, the pujnl, and enclosing a 

 mass of gelatinous matter, the lens (/.) 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 forming part of the optic nerve ; near 

 this end is a nucleus ; the main body of the cell is densely 



