Chap. VI T.] 



PL A TYHELMINTHES. 



249 



glandular, and these glands, we may suppose, act on 



the contents of the blood-vessels which are richly 



distributed to the nephridium. The terminal and 



wider portion, the walls of 



which are muscular, may be 



looked upon as analogous to a 



ureter. 



Bearing in mind that we 

 have in the earthworm to do 

 with a form in which meta- 

 meric segmentation is most 

 markedly expressed, and that 

 this metamerism has clearly 

 affected the nephridia, we are 

 prepared to find a very much 

 simpler condition of things 

 among the Platyhelminthes, 

 and, at the same time, to find an 

 arrangement which is more dif- 

 fused. InMonoccelis (Fig. 104), 

 for example, there is a plexus of 

 fine canals, which communicate, 

 on the one side, with large 

 principal canals, of which there 

 are two pairs, one external 

 and one internal, and on the 

 other with funnel-shaped pro- Fig.^ios. A single *: 

 cesses, the entrance to which is 

 guarded by a long cilium ; the "' 

 principal canals are connected 

 with one another by anastomo- 

 sing branches. In the Den- 

 drocoela, as represented by 

 Polyccelis, the fine canals appear to be absent. 



If we take the liver fluke as a type of the 

 Trematoda, we again find that the system of 

 excretory vessels is diffused throughout the whole 



dium of Anachceto - 



gSTes 



K 



