436 



ZOOLOGY 



SECT. 



areas of integument occupied by plexuses of blood-vessels, subserve 

 the function of respiration. 



There is a well-developed nervous system (Fig. 358), which is 

 bilateral and metameric in its arrangement, like the other systems 

 of organs. Situated in the prostomium is a large bilobed mass 

 of nerve-matter containing numerous nerve-cells, the cerebral 

 ganglion or brain (c). This gives off tentacular nerves to the tentacles 

 and palpi, and two pairs of short thick optic nerves to the eyes. 



Behind, two thick 

 nerve-strands, the 

 cesophageal connec- 

 tives (d), curve 

 round the mouth 

 in the peristomiurn 

 to meet on the 

 ventral aspect be- 

 hind the mouth 

 and below the 

 pharynx. The 

 oesophageal con- 

 nectives with the 

 cerebral ganglion 

 thus form a ring 

 around the ante- 

 rior part of the 

 enteric canal. 

 From them are 

 given off nerves to 

 the two anterior 

 pairs o f peristo- 

 mial tentacles. 

 Running back- 

 wards from the 

 point of union of 

 the cesophageal 

 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 (h) . In each segment this cord presents a little dila- 

 tation 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 stoniatogastric or visceral 



FIG. 358. Nereis. Anterior portion of nervous system, com- 

 prising the brain (c), the oasophageal connectives (d), and the 

 anterior part of the ventral nerve-cord(A). (After Quatrefages.) 



