74 THE EARTHWORM. 



From the alimentary canal the blood is returned 

 to the dorsal vessel by intestinal vessels, of which 

 there are in the region of the intestine two pairs to 

 each segment, and which are surrounded by the 

 1 hepatic cells.' 



2. The ventral vessel is a median vessel running back- 



wards below the alimentary canal the whole length 

 of the animal : in it the blood flows backwards. 



3. The contractile * commissural ' are five pairs lying in 



segments vn. to xi., and connect the dorsal with the 

 ventral vessel. During life they are rhythmically 

 contractile, the contractions passing along them from 

 above downwards, commencing with the hindmost 

 pair, and driving the blood from the dorsal to the 

 ventral vessel. In the fresh state each has fre- 

 quently a moniliform appearance. 



4. Neural vessels. These are smaller longitudinal vessels 



in close relation with the nerve-cord which runs 

 along the floor of the body-cavity in the mid- ventral 

 line (see p. 84). 



a. The subneural vessel is median, and lies immedi- 



ately beneath the nerve-cord along its whole 

 length. 



b. The lateral neural vessels are a pair of smaller 



longitudinal vessels lying along the sides of the 

 nerve-cord. 



5. The nephridial vessels supply the nephridia or excretory 



organs (see p. 76). 



a. The afferent nephridial vessels arise from the 



ventral vessel, one pair in each segment, and run 

 . outwards to the nephridia, supplying all three 

 loops. 



b. The efferent nephridial vessels return the blood 



from the nephridia to the parietal vessels. 



