EXCRETORY SYSTEM. 65 



removing excretory elements is not confined, even in the 

 highest animals, to a single organ. Thus the respiratory ap- 

 paratus may receive some of the inorganic elements, such as 

 carbonic acid and water; other excreta, as in certain air- 

 breathing animals, may exude either through the skin (ex- 

 halation), or its appendages, viz., the perspiratory and anal 

 glands ; whilst others may find their way into the intestines, 

 to be voided with the ejecta. 



Of necessity closely associated with channels of nutrition, 

 excretory organs, wherever seen, are unusually vascular. It 

 is partly from the recognition of this feature that systems 

 of contractile vessels, having corpusculated contents, like 

 blood, yet opening upon the exterior, like excretory tubes, 

 have recently been assigned a position midway between the 

 circulatory system and the emunctories. (See p. 55.) 



Conspicuous among these is the WATER-VASCULAR SYSTEM. 

 This is an arrangement of vessels seen in certain aquatic and 

 entozoic articulates, which contains a corpusculated fluid 

 communicating with the external medium by a simple open- 

 ing. The main trunk of the system arises from this point, 

 and divides into branches which are distributed through the 

 tissues. The walls of the vessels are well defined; and by 

 their pulsation, a to-and-fro motion is given to the contents, 

 permitting water to enter the body and excrementitious mat- 

 ters freely to escape. 



The water- vascular system is best developed in the Ento- 

 zoa, q. v. 



Rotifera. The external aperture is situated at the posterior 

 end of the body, and communicates with a large sac which 

 rhythmically contracts. From this, ciliated trunks arise, 

 which usually give off short lateral branches, to terminate 

 in the trochal disk of the animal. 



KIDNEY. 



The kidney is that organ which, being specially devoted to 

 excretion, elaborates a crystalline product. This is constantly 

 uric acid or guanin, either free or combined. With many 



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