Il6 MORPHOLOGY OF THE ORGAXS OF VERTEBRATES. 



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The discharge of the organ is under control of the will, and 

 varies in strength according to the size of the organ and its con- 

 dition of fatigue : in the torpedo 

 and electrical eel it is sufficient to 

 knock a man down, but in the 

 others it is much less in amount. 



UROGENITAL ORGANS. 



The excretory and ttie repro- 

 ductive organs of the vertebrates 

 are so closely related to each other 

 that it is impossible to treat them 

 separately. The excretory glands 

 (nephridia), reproductive organs 

 proper (gonads), and the ducts to 

 carry away the nitrogenous waste 

 and the reproductive elements, 

 stand in close relation to each 

 other in position, development, 

 and function. Hence we speak of 

 urogenital organs. 

 Nephridia. — Under the head of nephridia are to be included 

 three different structures which appear in the vertebrates, — a 

 pronephros ( ' head kidney ' of older writers) ; a mesonephros or 

 Wolffian body ; and a metanephros, which is the functional kid- 

 ney in the amniotes. Pronephros and mesonephros appear only 

 as embryonic structures in the amniotes ; but in the lower groups 

 the pronephros is usually functional for a time, the mesone- 

 phros assuming its work in the adult. ^ These organs have a 

 regular succession in time, and hence in our account we follow 

 the order of development and begin with the pronephros. 



It will be remembered (p. loi) that the walls of the meso- 

 thelial cavities on either side are divisible into three zones, and 

 that segmentation only affects the dorsal and middle of these, 

 the hypomere being unsegmented. The nephridial structures 



Fig. 125. Diagrammatic sec- 

 tion of electrical apparatus, from 

 Wiedersheim. The arrow points 

 dorsally or anteriorly. BG, con- 

 nective tissue framework; EF, 

 electrical plates ; G, gelatinous tis- 

 sue ; X, nerves entering through 

 the septa; yy, terminations of 

 the nerves. 



1 In the elasmobranchs the pronephros is never functional, while apparently in Bdd 

 lostoma (a cyclostome) the whole excretory organ is pronephric. 



