CHEMICAL STRUCTURE AND METABOLISM 99 



marine fishes swallow ("drink") sea water, and the latter is absorbed, 

 salts and all, into the blood (Fig. 5.4B). That practice restores the needed 

 water, but it also gives the body a greater quantity of salts than is needed. 

 Thus there is the further problem of ridding the body of excess salts. The 

 kidney is of little help here, since its function has been much reduced in 

 the interest of water conservation. The problem is solved by secretory cells 

 in the sills which have the function of passing salts from the blood into 

 the surrounding water (Fig. 5.4). Note that they function in just the op- 

 posite way from the secretory cells in the gills of fresh-water fishes. 



The method described above of solving the osmotic problems of ma- 

 rine life is that of bony (teleost) fishes. For cartilaginous fishes (sharks, 

 dogfishes, etc.) the problem was solved in an entirely diff"erent manner. 

 Their internal concentration was raised to meet the concentration of the 

 surrounding sea water and thus prevent unfavorable osmotic eff"ects. This 

 change was brought about in a most curious way. The salt concentration 

 was not substantially increased; rather the urea concentration was in- 

 creased. Whereas most fishes excrete their nitrogenous wastes as ammo- 

 nia, sharks and their allies convert the ammonia to urea and then retain 

 a high concentration of the latter within the body. This is a unique means 

 of protecting these fishes from the desiccating eff'ect of water loss to the 

 surrounding sea water through exposed membranes. 



Fresh-water fishes also gave rise to amphibians, the first terrestrial verte- 

 brates, ancestors of reptiles, birds, and mammals. Earlier in the chapter 

 we noted that water conservation is a primary problem of land dwellers. 

 This being true, a kidney excreting large quantities of dilute urine would 

 be as much a detriment to land dwellers as it is to marine teleost fishes. 

 Accordingly, the amphibian kidney has some ability to absorb water from 

 the urine back into the blood. Some of the salt supply is also salvaged in 

 this way. In addition salt is obtained in the food. But amphibians are only 

 incompletely terrestrial animals; many of them live their lives in fresh wa- 

 ter and are much like fresh-water fishes in matters of osmotic regulation. 

 Other amphibians must stay in a moist environment, never having evolved 

 skin coverings efl'ective against desiccation. As noted previously, however, 

 these amphibians no longer excrete most of their nitrogenous wastes as am- 

 monia, as did their fresh-water fish ancestors. They convert the ammonia 

 to urea and use the kidney as the principal means of ridding the body 

 of the urea. 



What was the original function of the vertebrate kidney? We have two 

 opposite points of view; at the present time we can not decide which is 

 correct. 



