54 Physiology of the Kidney 



body by excreting a highly concentrated urine, but in prac- 

 tice they cannot do this for the fish kidney is unable to elab- 

 orate a urine which is osmotically more concentrated than the 

 blood. Their lot would be as unhappy as that of the Ancient 

 Mariner were it not that, unlike that thirsty man, they have 

 the happy advantage of possessing gills, and the gill is the 

 only organ in the lower vertebrates capable of doing hyper- 

 tonic osmotic work. Had the Ancient Mariner possessed 

 such a marvelous organ he could have lived like a fish by 

 drinking the briny sea; he could have separated the salt from 

 the water by excreting the salt out of his gills in a concen- 

 trated form, leaving the water free for his tissues, or for the 

 formation of urine. But with the limitations of the fish kid- 

 ney he still would have had cause to deplore his lot, since for 

 every liter of urine formed he would be forced to concentrate 

 a liter of sea water by 66 per cent. It is not surprising that 

 the marine fishes, rather than spend their precious energy in 

 making more concentrated sea water out of the already con- 

 centrated sea, naturally became conservative in the matter of 

 urine formation and excreted no more urine than was required 

 to remove waste products from the body.^^ When the bony 

 fishes migrated from fresh water to the sea, the high-pressure 

 filtering device of the glomerulus was no longer an asset, but 

 a liability. They shut the filtering bed down as far as possible, 

 and with the passing years the glomeruli grew smaller and 

 smaller, fewer and fewer; to examine the glomeruli in a 

 series of marine teleost kidneys reminds one of the old-fash- 

 ioned Herpecide advertisement: Going — going — gone! 

 Nearly all the marine teleosts show some evidence of glom- 

 erular degeneration, and in certain of them (the toadfish, 

 midshipman, goosefish, batfish. sea horse, pipefish, and in 

 certain deep sea fishes) the kidney has become entirely agio- 



