V VASCULAR SYSTEM 329 



vessels, containing red blood, the whole of which form a 

 single closed vascular system, there being no com- 

 munication between them and any of the other cavities of 

 the body. The main trunks have a longitudinal 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 (Fig. 78. dors, v, vent. v). 

 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 coelome : these are a 

 median suinet^ral (sui. n. vess) and two lateral neural vessels. 

 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 pair of lateral cofnmissural vessels : 

 in the region of the gullet there are about five or six 

 pairs of large vessels connecting the main dorsal and 

 ventral 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. 



Notice that there is here no distinction into arteries and 

 veins, as in the frog (p. 27), and also that there is no heart. 

 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 rhythmical, 

 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 ventral 

 trunk, from above downwards, so that the blood passes for- 

 wards in the dorsal, backwards in the ventral vessel. The 



