PHYLUM ANNULATA 



445 



the majority of the vessels, undergoes contractions which are of 

 a peristaltic character waves of contraction passing along the 

 wall of the vessel so as to cause the movement of the contained 

 blood. These peristaltic contractions are more powerful in the 

 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 



CIST-, fnus 



dcrs 



ru>t. set 



ev 



FIG. 351. Nereis dumerilii. Semi-diagrammatic transverse section of the middle region of 

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

 circular layer of muscle of wall of enteric canal ; caul, coelome ; cut. cuticle ; dors. long. mus. 

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

 ep. epidermis ; long. mus. 'longitudinal muscle of wall of enteric canal ; tie. co. nerve-cord ; 

 nepli. nephridium ; neur. set. neuropodial setae and aciculum with their muscles ; not. set. 

 notopodial sette and aciculum ; obi. mus. oblique muscle ; ov. ovary ; par. peri, parietal layer 

 of coelomic epithelium ; vent. long. mus. ventral longitudinal muscle ; vent.vesx. ventral vessel ; 

 vise., peri, visceral layer of ccelomic epithelium. (The entire extent of the ccelomic epithelium is 

 not represented.) 



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, 

 wss), 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 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 



