ix PHYLUM ANNULATA 195 



direction from before backwards. Connecting the dorsal 

 and ventral vessels, there are in each segment two pairs of 

 loop-like transverse vessels which give off branches to the 

 parapodia, the alimentary canal, and neighbouring parts. 



There is a well-developed nervous system (Fig. 112) 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 oesophageal connectives (d), curve round 

 the mouth in the peristomium to meet on the ventral aspect 

 behind the mouth and below the pharynx. The oesophageal 

 connectives, with the cerebral ganglion, thus form a ring 

 around the anterior part of the enteric canal. Running 

 backwards from the point of union of the oesophageal con- 

 nectives, 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 iji). In each segment this cord presents 

 a little dilatation from which nerves are given off to the 

 various parts of the segment : and each of these enlarge- 

 ments is really double, consisting of a pair of closely-united 

 ganglia. The intermediate parts of the cord, between suc- 

 cessive 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 j to ma togas trie 

 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 



