vi VASCULAR SYSTEM 337 



blood-vessels, containing red blood, the whole of which 

 form a single closed vascular system, there being no 

 communication between them and any of the other 

 cavities of the body. The main trunks have a longi- 

 tudinal direction, the chief ones being a large dorsal 

 vessel, running along the dorsal surface of the enteric 

 canal, and a ventral or sub-intestinal vessel, below the 

 canal (Figs. Si, dors, v, vent, v, and 83), In addition to 

 these there are three smaller longitudinal trunks in 

 relation to the nerve-cord, which, as we shall see, 

 extends along the ventral side of the ccelome : these 

 are a median siibnenral and two lateral neural vessels 

 (Figs. Si and 83). All these longitudinal trunks give 

 off branches to the various parts of the body, and 

 certain of them are connected with one another by a 

 series of pairs of lateral commissural vessels : in the region 

 of the gullet there are five (Lumbricus) or six (Allolobo- 

 phora) pairs of large vessels connecting the dorsal and 

 subintestinal trunks ; and the dorsal and subneural 

 trunks are also connected in each segment all along the 

 body by a pair of smaller commissural vessels, running 

 in the inner surface of the body- wall (Fig. 83). 



Notice that there is here no heart and no distinction 

 into arteries and veins, as in the frog (p. 27). The 

 vessels gradually divide up into smaller and smaller 

 branches in the various parts of the body, and 

 then again unite to form larger and larger vessels 

 which eventually open into one or other of the main 

 trunks. 



The circulation of the blood is effected by the rhyth- 

 mical, peristaltic contraction (p. 75) of certain of the 

 larger vessels : thus the dorsal trunk contracts from 

 behind forwards, and the large commissural vessels 

 often spoken of as " hearts " which connect it anteriorly 

 with the sub-intestinal trunk, from above downwards, 



PRACT. ZOOL. ~ 



