CHAPTER XXIII. 

 EXCRETORY ORGANS. 



EXCRETORY organs consist of coiled or branched and often ciliated 

 tubes, with an excretory pore opening on the outer surface of the 

 body, and as a rule an internal ciliated orifice placed in the body- 

 cavity. In forms provided with a true vascular system, there is a 

 special development of capillaries around the glandular part of the 

 excretory organs. In many instances the glandular cells of the organs 

 are filled with concretions of uric acid or some similar product of nitro- 



genous waste. 



There is a very great morphological and physiological simi- 

 larity between almost all the forms of excretory organ found in the 

 animal kingdom, but although there is not a little to be said for 

 holding all these organs to be derived from some common prototype, 

 the attempt to establish definite homologies between them is beset 

 with very great difficulties. 



Platyelminthes. Throughout the whole of the Platyelminthes 

 these organs are constructed on a well-defined type, and in the 

 Rotifera excretory organs of a similar form to those of the Platyel- 

 minthes are also present. 



These organs (Fraipont. No. 513) are more or less distinctly 

 paired, and consist of a system of wide canals, often united into a 

 network, which open on the one hand into a pair of large tubes 

 leading to the exterior, and on the other into fine canals which 

 terminate by ciliated openings, either in spaces between the con- 

 nective-tissue cells (Platyelminthes), or in the body-cavity (Rotifera). 

 The fine canals open directly into the larger ones, without first 

 uniting into canals of an intermediate size. 



The two large tubes open to the exterior, either by means of a 

 median posteriorly placed contractile vesicle, or by a pair of vesicles, 

 which have a ventral and anterior position. The former type is 

 characteristic of the majority of the Trematoda, Cestoda, and Roti- 

 fern, and the latter of the Nemertea arid some Trematoda. Tu the 



U.K. II. 30 



