EXCRETOEY SYSTEM. 69 



2. Microscopical structure of a nephridium. 



■ Carefully remove by fine pointed forceps, an entire nephri- 

 dium with a very small part of the septum which it traverses. 

 If the worm has been killed with chloroform, the ciliated 

 portions of the nephridium may be still luorMng. Examine 

 in salt solution. 



Examine with low and high powers. 



a. The funnel is ciliated, its border consisting of a row 



of large cells with very distinct nuclei. The 

 nephrostome or open mouth of the funnel can 

 be easily seen : it is often partially blocked up 

 by a small mass of ccelomic corpuscles. 



b. The first part of the tube, or ' narrow tube,' is very 



slender and its relations are difficult to deter- 

 mine. Starting from the funnel it runs straight 

 back through the septum into the posterior of the 

 two segments with which the nephridium is con- 

 nected. In this segment it runs in a somewhat 

 irregular course along both limbs of the short 

 loop, and along the proximal limb of the long 

 loop, it then bends back sharply on itself, and 

 returns, following the same path, to the base of 

 the short loop, where it passes into the second 

 part of the tube. The duct of the ' narrow tube ' 

 is an intracellular one, traversing the individual 

 cells, which are placed end to end like drain-pipes. 

 The ' narrow tube ' is ciliated along part, but not 

 the whole of its length. 



c. The second part of the tube, or 'middle tube,' 



forms the proximal limb of the long loop. The 

 duct is here also an intracellular one : it is much 

 wider than that of the ' narrow tube,' and is 

 ciliated along its whole length. 



d. The third part of the tube, or 'wide tube,' com- 



mences with a dilated portion of the ampulla, at 

 the apex or bend of the long loop : beyond this 



