CIRCULATOEY SYSTEM 63 



2. The ventral vessel is a median vessel running backwards 



below the alimentary canal the whole length of the 

 animal. 



3. The ' hearts ' are five pairs of large vessels lying in seg- 



ments VII. to XI., and connecting the dorsal with the 

 ventral vessel. During life they are rhythmically 

 contractile, the contractions passing along them from 

 above downwards, and commencing with the hind- 

 most pair. In the fresh state each has frequently a 

 moniliform appearance. 



4. Neural vessels. These are in close relation with the 



nerve-cord which runs along the floor of the body- 

 cavity in the mid- ventral line (see p. 74). 



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. 



6. The nephridial vessels supply the nephridia or excretory 

 organs (see p. 64), 



a. The nephridial arteries arise from the ventral vessel, 



one pair in each segment, and run outwards to 

 the nephridia. 



b. The nephridial veins return the blood from the 



nephridia to the subneural vessel. 



6. The parietal vessels. In each segment the subneural 

 and dorsal vessels are connected by a pair of parietal 

 vessels, which run on the inner surface of the body- 

 wall, and receive branches from it. 



D. The Excretory System. 



This consists of a series of tubular nephridia or ' segmental 

 organs,' of which each segment, except the first three and the 

 last, contains a pair. They are largest in the region of the 

 oesophagus. 



