Chap. VII.] CES TO DA ; Ro TA TOR I A. 2$l 



longitudinal vessels which extend through all the 

 joints, which may or may not branch and form a 

 plexus in the head, and which open to the exterior by 

 a single excretory pore, which is placed in the terminal 

 joint ; sometimes the tubes open in the several joints 

 by secondary foramina (Fraipont), and in such cases 

 the terminal pore and vesicle become more or less 

 atrophied. These secondary orifices in a tapeworm 

 are not to be compared with the openings of the 

 " segments! organs " in the earthworm. The calcareous 

 concretions which are so frequently observed in tape- 

 worms have not as yet been certainly shown to have 

 the character of renal excretory products. The course 

 of the fluid in these vessels is directed by the valves 

 which are placed in the region of the head, and which 

 are so arranged as to prevent the fluid from passing 

 forwards ; the canals themselves are devoid of cilia, 

 and, as in the Trematoda, the propelling power is to 

 be sought for in the muscles of the body wall. The 

 fluid is said to contain substances which are chemically 

 allied to xanthin or guanin (Sommer). 



The simple unsegmented body of the Rotatoria 

 presents us with a correspondingly simple condition of 

 the excretory organs, but their relations are here 

 more easily made out, owing to the development of a 

 definite body cavity. There are several distinct 

 ciliated and funnel-shaped openings into the ccelom, 

 and these lead, by short and simple canals, into a 

 longitudinal vessel on either side ; this is more or 

 less coiled on its course, and opens into the cloaca. 

 (See page 119.) 



Similar canals arising from the cloaca, and opening 

 by ciliated infundibula into the body cavity, are 

 found also in the Gepliyrea ; but these forms are 

 most remarkable and interesting for having, in addi- 

 tion to these cloacal outgrowths, others which, by 

 opening on one side into the body cavity and on the 



