410 



ZOOLOGY 



SECT. 



nerves are given off to the various parts of the segment : and 

 each of these enlargements is really double, consisting 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 



Fi:;. 323. Nereis. Anterior portion of nervous system, comprising the brain, the oesophs 

 connectives, and the anterior part of the ventral nerve-cord. (After Quatrefages.) 



nerves with occasional small ganglia, the stomato gastric or visceral 

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



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, situated on the prostomium. The eye (Fig. 324) 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 (L). The wall of the cup is composed of numerous long 

 and narrow cells lying parallel with one another in a radial 



