256 ELEMENTARY BIOLOGY. [CHAP. 



mesenteric septa. Those of the crop-gizzard 

 region are liable to considerable variation. 



E. The excretory or segmental organs. 



1. Take the largest worm you can find and open up its 

 anterior third along the dorsal middle line. Pin down 

 the two flaps of the body-wall and remove the ali- 

 mentary canal, being especially careful to avoid 

 stretching the mesenteric septa more than is neces- 

 sary. Wash until quite clean and examine in water 

 under a lens. With a little care there will be seen 



a. The segmental organ or nephridium, a delicate tube, 

 coiled upon itself and suspended to the mesenteric 

 septum by a special fold or mesentery of its own. 



The whole organ is thrown into a series of loops. 

 At its inner end, close to the nervous axis, a very 

 conspicuous internal loop will be seen; it projects 

 backwardly and outwardly. 



b. Examine the region of this internal loop with care. 

 The nephridium will be seen to perforate the mes- 

 enteric septum with which it is connected, and to 

 open into the somite in front by an enlarged 

 pendant extremity or nephrostome. 



2. Take a similar but freshly killed worm, and open up 

 as before. Remove as much as possible of a mesen- 

 teric septum (preferably from somite 15 or there- 

 abouts, where the calibre of the alimentary tube is 

 least and the reproductive organs do not overcrowd) 

 with one of its related segmental organs. Examine 

 under a low power, and restrict your attention to that 

 portion of the tube in which ciliary action is going on. 

 Follow it along until a point is reached at which it 



