130 How Animals Changed 



modified the glomerulus into a filtration-reabsorption apparatus. 

 Cyclostomes and elasmobranchs are like fresh-water fishes in 

 glomerular development. Fishes transferred from sea water which 

 is hypotonic to their body fluids to that which is hypertonic may, if 

 they live, excrete salts through their kidneys (Schlieper, 1933; 

 Smith, 1933) . Frogs in water at 20 C. excrete water at the rate of 

 1.3% of their body weight per hour (Adolph, 1927a). Their urine 

 is always hypotonic to their body fluids. But if the skin is dried or 

 if the body is in hypertonic solutions a frog may absorb water from 

 the bladder into the tissues (Steen, 1929) . A man excretes a fiftieth 

 of his body weight per day through his kidneys; a frog at ordinary 

 temperatures, about a third. 



Kidneys in all animals may perform two functions: (1) the 

 regulation of the concentration of body fluids by conserving or 

 eliminating water and solutes and (2) the excretion of the waste 

 products of metabolism. Elasmobranchs retain urea in their blood 

 to increase osmotic pressure (Scott, 1916; Denis, 1922) ; estivating 

 lung-fishes may retain large quantities of urea in their bodies be- 

 cause the water needed for its excretion is not available (Smith, 

 1931). Many birds and desert reptiles excrete uric acid instead of 

 urea; they thus conserve water, for the uric acid is practically in- 

 soluble. The evolution of the forms taken by nitrogenous wastes in 

 various invertebrates (Delaunay, 1924) and vertebrates (Przylecki, 

 1926) is interesting but not particularly significant in connection 

 with aboceanic migrations except for the fact that changes have 

 been related to environmental conditions. 



Reproduction 



The eggs of some animals require salt in the surrounding medium 

 for their development. "This dependence of the egg on the inor- 

 ganic material of its environment . . . seems to be a significant lim- 

 iting factor, making it impossible for marine animals to colonize 

 fresh water, until, by some chance mutation, perhaps, the capacity 



