EXCRETORY SYSTEM 63 



I 



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. 



Each nephridium is a long tube thrown into loops, which 

 are closely applied together and lie in the body-cavity close to 

 its wall. The tube commences with a minute ciliated mouth 

 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. 18). 



Open a freshly killed worm along the mid-dorsal line ; 

 pin out the body-wall, and remove an inch or so of the intes- 

 tine, so as to expose the nephridia fully . Wash gently under 

 the tap, and then place the worm, pinned out, in spirit for a 

 quarter of an hour. Wash it, and examine under water with 

 a pocket lens or dissecting microscope. 



The nephridia are seen as white opaque loops 

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

 extending from near the mid-ventral line almost 

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

 two segments, the funnel and a very short length of 

 the tube lying in the anterior of the two, and the 

 rest of the tube with the external opening in the 

 posterior. 



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



a, The funnel is a minute knob lying in the body- 



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

 nected by a short stalk the first part of the 

 nephridial tube with the septum behind it. 



b. The looped portion of the tube lies in the segment 



behind that containing the funnel. Three limbs 

 can readily be made out in the loop, of which 

 the middle one is the largest and reaches almost 



