CHAPTER XVI 



THE URINOGENITAL SYSTEM 



The urinary and genital systems are considered as a unit in 

 vertebrate anatomy because of their close association. Physio- 

 logically they are entirely different; but certain structures are 

 common to both systems, and during the course of evolution 

 some excretory (urinary) ducts became a part of the reproduc- 

 tive system. In the lower chordates there is no connection be- 

 tween the two systems, but in the cyclostomes an association 

 begins and becomes progressively closer in the more specialized 

 classes. 



The essential structures of excretion and reproduction are (1) 

 nephridia which collect wastes from the body and pass them to 

 the outside, and (2) gonads in which the reproductive cells (ova 

 and spermatozoa) develop. With the exception of the lower 

 chordates there are always ducts for the conduction of w^astes 

 and reproductive cells to the outside. In the higher groups struc- 

 tures may develop (1) for the storage of wastes; (2) to permit 

 the fertilization of the ova while they are still in the maternal 

 body; and (3) for the retention of the fertilized ova during their 

 development. 



The nephridium is the functional unit of excretion. There is 

 no evidence of homology between the nephridia of invertebrates 

 and those of the vertebrates, although there is a functional 

 similarity. In the primitive condition each is an independent 

 structure composed of a head within the coelomic cavity, a 

 short tubule for the conduction of wastes, and an excurrent pore 

 opening on the outside of the animal. The head has a funnel- 

 shaped opening, the nephrostome, surrounded by cilia which beat 

 in toward the lumen of the tubule. The nephridia are paired, 

 metameric structures. This completely primitive condition is not 

 found in any of the living chordates, but is present in some in- 



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