EXCRETORY SYSTEM. 67 



pair, and driving the blood from the dorsal to the 

 ventral vessel. In the fresh state each has frequently 

 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. 79). 



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. 68). 



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. 



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 smaller vessels from it. 



D. The Excretory System. 



This consists of a series of tubular nephridia or ' segmental 

 organs,' arranged in pairs corresponding to the segments 

 along the whole length of the animal, excepting the first three 

 and the last segments, from'which they are absent. They 

 are largest in the region of the oesophagus. 



Each nephridium is a long tube thrown into loops, which 

 are closely applied together and suspended in the body-cavity 

 close to its wall. The tube commences with a minute ciliated 

 mouth — the nephrostome— opening into the body-cavity ; then 



f2 



