Excretion 129 



Excretion 



"All organisms which have the power of regulating the osmotic 

 pressure of their body fluids are provided with an excretory organ 

 corresponding to a kidney" (Wardlaw, 1931). Probably the primi- 

 tive function of renal organs was to regulate the osmotic pressure 

 of internal fluids, and only secondarily have they assumed the elimi- 

 nation of metabolic wastes. All animals which live in fresh water, 

 where the external medium is hypotonic to their body fluids, readily 

 permit the return of water from their bodies to the outside. A 

 Paramecium in 24 hours may eliminate water equal to 31 to 700 

 times its own volume, depending on temperature (Hesse, 1920) . 



The water-eliminating activities of kidneys are as a rule greater 

 in fresh water than in the sea or on land because animals there live 

 in solutions that are hypotonic to their body fluids. Crabs usually 

 do not eliminate water through their kidneys, but crayfishes, fresh- 

 water fishes, and amphibians do. Most marine crustaceans have 

 urine which is isotonic with sea water, but fresh-water amphipods 

 have larger kidneys than those that live in the ocean and excrete 

 more water through them (SchUeper, 1933) . The green gland of 

 the land hermit crab Coenobita is reduced and lacks a terminal 

 vesicle, thus difl'ering from comparable marine crustaceans (Borra- 

 daile, 1903) . "The glomerular development of the kidneys of ver- 

 tebrates is related to water excretion. The protovertebrate kidney 

 was at one stage probably aglomerular, and the glomerulus was 

 evolved as an adaptation to a fresh-water habitat. In the lower 

 vertebrates remaining in fresh water (dipnoans, ganoids, and fresh- 

 water teleosts) , and those still in intimate dependence on it (Am- 

 phibia) , the glomerular development is good; but with the secondary 

 assumption of marine habitat (marine teleosts) or with the assump- 

 tion of terrestrial life in which water-conservation becomes a neces- 

 sity (arid-living reptiles and birds) the glomerular development is 

 poor" (Marshall & Smith, 1930; Baldwin, 1937) . Mammals have 



