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THE URO-GENITAL SYSTEM 381 



peritoneal cavity until discharged through these abdominal 

 pores. The urinary organs have no direct connection with 

 this system other than through the nephrostomes which open 

 into the peritoneal cavity, and these are not specialized to re- 

 ceive the free germ cells. 



There is thus shown the original condition of gonads and 

 their ex current ducts, slightly modified by the fusion of all the 

 gonads into one and the reduction of the gonadic ducts to a 

 single pair. Otherwise the primitive physiological functions 

 are carried on as they were before the gonadic cavities became 

 converted into a metacozle. 



It will be noticed that in the above description the ne- 

 phrostomes open directly into the ccelomic cavity and thus 

 suggest the possibility of the use of nephridia for the exit of 

 the germ cells. Such is actually the next stage in the history 

 of these organs, for in the selachians certain of the nephridia 

 are so employed while the pori ab dominates, although they 

 still exist, are no longer used for their original purpose. In 

 the male the testes lie in close proximity to the anterior por- 

 tion of the kidneys, and enter into direct connection with the 

 nephridia of this region through the development of a series 

 of tubes, the vasa eiferentia, which extend from the testes and 

 enter the nephridia a little beyond the nephrostomes. The 

 original function of this part of the kidneys is not impaired, 

 and during the greater part of the time it exercises the urinary 

 function alone; but during the periods of sexual activity the 

 nephridia involved become filled with the spermatic" fluid and 

 deliver it directly from the testes to the mesonephrotic duct 

 and thence to the cloaca. From there it is received into a 

 channel formed by the approximation of the inner modified 

 portions of the ventral fins, and delivered within the cloaca 

 of the female by an internal copulation, an unusual method 

 among aquatic animals. That there is no genetic connection 

 between this act and that developed among terrestrial verte- 

 brates may be seen from the employment of very different 

 organs for the purpose in the two cases and from the fact of 

 the interposition, in the direct line of descent, of forms that 



