EXCEETOEY OEGANS OF VEEMES. 



173 



be continued from them requires to be more closely examined. The 

 relations of the canals, which are known as the water vascular 

 system, are more exactly known in most Platyhelminthes. They 

 have not been observed in the land Planarians. In the Trematoda, 

 and many Turbellaria, two excretory canals, one on each side, 

 ramify in the body; the chief trunks giving off fine branches 

 which traverse its parenchyma (Fig. 80, A B). Long cilia are 



Fig. 80. Diagram of the excretory organs of Platyhelminthes, showing the 

 different forms, which may be derived from one another. 



distributed on the walls of the fine canals. The principal trunks, 

 which are as a rule somewhat enlarged, still open in many 

 forms in the anterior region of the body (Fig. 80, A) (Ti'istoma 

 papillosum). The orifice (porus excretorius) is most commonly 

 placed towards the posterior region (D), where the two vascular 

 trunks approximate and unite at a common orifice. In this way a 

 terminal tract, common to both canals, is formed. This tract, which 

 is generally enlarged, has the form of a contractile vesicle {E). 

 Vesicles of this kind may be formed even on trunks which open 

 separately. They form a third division of the apparatus. 



In the Cestoda, the fusion of the excretory canals into a single 

 porus excretorius, placed at the end of the scolex-body, which in the 

 Other Platyhelminthes is only an acquired character, has apparently 

 become typical. A contractile vesicle generally foi-ms the point of 

 union. There are generally a larger number of principal trunks — that 

 is four, six, or eight — which either unite with one another by loops 

 in the head, or merely curve round it, and passing backwards branch 

 again ; in this case their more special characters are similar to what 

 are found in other Platyhelminthes. When the scolex-form is 

 divided into metameres, the terminal portion of this canal system is 

 apportioned to the oldest proglottid, and the succeeding proglottides 

 contain portions of the canals. At the termination of the metameres 

 the longitudinal trunks are, in many forms, united by a circular 

 canal. As the proglottides break off, a new porus excretorius must 

 be formed every time. 



That part of these organs, which is formed of the finest canals, 

 contains a clear fluid only. In the Tape-Worms, on the other 

 hand, there are calcareous concretions at the enlarged points, which 



