EXCRETORY SYSTEM. 75 



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 

 follows a long and very slender portion, to which succeeds a 

 shorter and wider part ; the terminal portion is still wider 

 and has muscular walls, and opens distally to the exterior at 

 the aperture already seen on the surface. 



1. Examination of a nephridium in situ (fig. 27). 



The worm should be opened immediately after being killed 

 with chloroform and dissected in salt solution. Stretch the 

 segments thoroughly and examine with a hand-lens. 



The nephridia are seen as semi-transparent 

 loops lying along the inner surface of the body-wall, 

 and extending from near the mid-ventral fine almost 

 to the mid-dorsal fine. Each is in relation with 

 two segments, the funnel and a very short length of 

 the tube lying in the anterior of the two segments, 

 and the rest of the tube with the external opening in 

 the posterior segment. 



Examine one of the nephridia more closely, noting its parts. 



a. The nephrostome is a minute funnel lying in the 

 body-cavity close to the mid-ventral line, and 



