UROGENITAL SYSTEM T 35 



and the variously formed canals which penetrate into the cloaca 

 of certain classes of the Holothurians. 1 There is sufficient 

 ground for the assumption that these act as primitive excretory 

 organs, even though the nature of the excreta be not as yet 

 fully ascertained. The higher we rise in the organic scale the 

 greater is the importance assumed by these organs and the 

 more clearly are the excreta to be recognised. In the worms 

 these latter take the form of granules and concrements corre- 

 sponding chemically to the matter excreted by the kidneys of 

 the higher animals and of man. 2 



" According as the body is articulate or inarticulate the 

 excretory apparatus is complex or simple." 



Where the body-cavity is not clearly defined the ends of 

 the tubes as well as the finest ramifications of the canals are 

 closed, otherwise the canals, simple or ramifying, open on to 

 the surface of the body, and, where the body-cavity is very 

 clearly defined, into the interior also, when a special ciliated 

 organ is seen ; indeed, in the Trematoda and the Radiata a 

 common contractile bladder is found. The Nematoda exhibit 

 a pore in the linea alba through which the contents of the two 

 united lateral canals are excreted ; the Annelida have two 

 canals regularly divided and almost invariably provided with 

 an internal, ciliated orifice and an external orifice at the opposite 

 extremity of the body ; the Gephyrea, lastly, possess two 

 separate organs of excretion, the walls of the canals being of a 

 glandular character ; one organ acts as a kidney, the other 

 as a reproductive organ. Thus early do we find indications 

 of the urogenital system of the higher animals. 



Among the Arthropoda little is definitely known as to the 

 urinary organs of the Crustaceans. It has been ascertained 

 that the larvae of the Copepoda possess a temporary excretory 

 organ in connection with the extremity of the large intestine, 

 and the excretory nature of the so-called testaceous glands in 

 the double shell of the Daphnia, in the head-shield of the 

 Apus and in the shell-valves of the Limnadiacea cannot be 

 doubted, but it is still uncertain whether the looped canals of 

 certain Crustaceans are true organs of excretion or not. On 

 the other hand, the so-called Malpighian vessels, occurring in 



J C. Gegenbauer, Vergl. Anat., p. 342. z lbid., p. 253. 



