ix PHYLUM ANNULATA 195 



behind forwards, so as to drive a current of blood in that 

 direction. 



Along the middle of the ventral surface below the 

 alimentary canal runs another large longitudinal vessel, the 

 ventral vessel (vent, ves.), in which the current of blood takes 

 a 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. 105) 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 ossophageal connectives (d.\ curve round 

 the mouth in the peristomium to meet on the ventral aspect 

 behind the mouth and below the pharynx. The oeso- 

 phageal 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 cesophageal 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 (/i.). 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 

 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 



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