CHAPTER II 

 THE UROGENITAL SYSTEM 



EVOLUTION OF THE UROGENITAL SYSTEM 



The evolution of urinary and reproductive systems may be conven- 

 iently discussed as if the two were independent. 



A. Phylogenesis of the Urinary System 



Metabolism furnishes the necessary chemical foundation of life. But 

 the oxidation of nitrogenous compounds, essential to the process of 

 metabolism, results also in poisonous nitrogenous wastes, which must 

 be removed. It is, therefore, necessary that all animals have mecha- 

 nisms for removing the ashes of life, intra-cellular mechanisms in the 

 Protozoa, Porifera, and Coelenterata, multicellular in higher animals. 



Protozoa. The characteristic excretory organ of Protozoa is the con- 

 tractile vacuole. Usually each cell contains a single vacuole, but there 



-» FLAME CELLS 



PROTOPLASMIC 

 PROCESSES'^--- 



A.CANAL SYSTEM B. FLAME CELLS 



Fig. 260. — Excretory organs in flatworms. A shows the branching excretory canals 

 terminating in flame-cells. B shows three flame cells enlarged. By the action of the 

 cluster of cilia (flame) a current carries wastes away from the excretory cell into the 

 canals. (Redrawn after Benham.) 



may be two in protozoans with elongated bodies. The position of the 

 vacuole just beneath the ectosarc is constant and its external orifice per- 

 manent. Liquid wastes formed within the cytoplasm stream toward the 

 vacuole from all directions. When the tension of liquid within the vacuole 

 reaches a maximum, the external orifice is forced open and the liquid passes 

 to the outside. The rate of contraction is a function of the temperature, 

 warmth increasing and cold retarding excretion. There is, of course, no 

 genetic connexion between the intracellular organs of Protozoa and the 

 multicellular organs of Metazoa. 



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