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 nitrogenous 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 Platyel- 

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

 in the Rotifera excretory organs of a similar form to those of the 

 Platyelminthes 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 

 connective-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. 



