PHYLUM ANNULATA 



435 



case of the dorsal vessel than in that of any of the others, and 

 run with great regularity from behind forwards, so as to drive a 

 current of blood in that direction. The contractions are brought 

 about partly by a series of muscular fibres which are arranged in 

 rings round the wall of the vessel at short intervals ; but the wall 

 of the vessel is itself contractile. 



Along the middle of the ventral surface below the alimentary 

 canal runs another large longitudinal vessel, the ventral vessel (vent, 

 vess), in which the current of blood takes a direction from before 

 backwards. Connecting the dorsal and ventral vessels, there are in 



circ.fnus 



r. Cress 



>'.> ^Otrl. /////,-. 



rwt act 



rtcur. &Ct 



C/J/t, 



fang. ?fu<fi 



FIG. 357. Nereis dumerilii. Semi -diagrammatic transverse section of the middle region of 

 the body. circ. mus. (external), circular layer of muscle of body-wall ; circ. mus. (internal) 

 circular layer of muscle of wall of enteric canal ; ccel. ccelome ; cut. cuticle ; dors. long. mus. 

 dorsal longitudinal muscles of body-wall ; dors. vess. dorsal vessel ; ent. ep. enteric epithe- 

 lium ; ep. epidermis ; long. mus. longitudinal muscle of wall of enteric canal ; ne. co. nerve- 

 cord ; neph. nephridium ; neur. set. neuropodial setae and aciculum with their muscles ; 

 not. set. notopodial setae and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri. 

 parietal layer of coalomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent, 

 vess. ventral vessel ; vise. peri, visceral layer of crelornic epithelium. (The entire extent of 

 the ccelomic epithelium is not represented.) 



each segment two pairs of loop-like transverse vessels which give 

 off branches to the parapodia, the alimentary canal, and neighbour- 

 ing parts. Some of these branches communicate with plexuses 

 of fine vessels in the interior of the lobes of the parapodia and in 

 the integument of the dorsal surface, and with dilatations or sinuses 

 situated in the bases of the parapodia. A delicate longitudinal 

 neural vessel accompanies the nerve-cord. 



Nereis is devoid of any branchice ; but there can be little doubt 

 that the lobes of the parapods with their rich blood-supply, and the 



p F 2 



