vi VASCULAR SYSTEM 337 



The earthworm, like the frog, possesses a series of blood- 

 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 (Figs. 81, dors, v, vent, v, 

 and 83). In addition to these there are three smaller longi- 

 tudinal trunks in relation to the nerve-cord, which, as we shall 

 see, extends along the ventral side of the ccelome : these are a 

 median subneural and two lateral neural vessels (Figs. 81 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 (Allolobophora) pairs of large vessels connecting the 

 dorsal and subintestinal trunks ; and the dorsal .and sub- 

 neural 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 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 for- 

 wards, and the large commissural vesselsy^often spoken of as 

 " hearts "which connect it anteriorly with the subintestinal 

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



PKACT. ZOOL. Z 



