64 THE EAETHWOEM 



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. 25). 



Bemove an mch or so of the intestine, so as to expose the 

 nephridia fully. Wash gently under the tap, and then 

 examine under spirit with a pocket lens or dissecting micro- 

 scope. 



The nephridia are seen as opaque white 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 nephxostome is a minute funnel 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 septutQ 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 longest and reaches almost 

 to the mid-dorsal line ; while the outer one is the 

 widest and leads to the external aperture. 



2. Microscopical structure of a nephridium. 



Carefully remove an entire nephridium with a very small 

 part of the septum which it traverses. Stain it, dehydrate 

 with alcohol, clear with oil of cloves, and mount in balsam. 



Examine with low and high powers. 



