CHAPTER II 



THE UROGENITAL SYSTEM 



EVOLUTION OF THE UROGENITAL SYSTEM 



The evolution of urinary and reproductive systems may be conveniently 

 discussed as if the two were independent. 



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, not surprising that all animals have mecha- 

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

 protozoa, porifera, and coelenterates, multicellular in higher animals. 



FLAME CELLS 



PROTOPLASMIC 

 PROCESSES >^--- 



A.CANAL SYSTEM B.FLAME CELLS 



Fig. 328. — 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.) 



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

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

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

 vacuole just beneath the ectosarc is constant and its external orifice 

 permanent. 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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