THE BONY FISHES IO5 



solved this problem by drinking sea water and absorb- 

 ing both the salt and water in the gastrointestinal tract; 

 and then they excreted the bulk of the salt through the 

 gills, leaving free water in the body available for urine 

 formation. 



The teleost can live in salt water not because of any 

 superiority of the kidney but because the gills can trans- 

 fer salt from the blood into the sea water that bathes 

 them with virtually no loss of water. In this operation 

 the animal is, in effect, concentrating the sea itself, an 

 operation that requires physiological work and the ex- 

 penditure of valuable energy in proportion to the quan- 

 tity of salt thus transferred and the quantity of water 

 gained in the body. Consequently the formation of urine 

 is reduced to a very low level, generally to the minimal 

 level set by the quantity of metabolic products requiring 

 excretion. The common eel, Anguilla, for example, which 

 can live in both fresh and salt water, has a urine flow 

 in fresh water ranging up to 150 cc. per kg. per day, 

 and this urine is very dilute; but when in salt water, 

 the urine flow is generally less than 5 cc. per kg. per 

 day and the urine is as concentrated as it can be made 

 within the limitations of the fish kidney. These figures 

 are roughly typical of all fresh- and salt-water fishes that 

 have been studied. 



This drastic requirement for water conservation in the 

 marine teleost has had the most marked consequences 

 on the structure and function of the kidney of any event 

 in vertebrate history. With no excess water to be ex- 

 creted, extravagant glomerular filtration is disadvanta- 

 geous because every volume of filtrate formed requires 

 the extensive reabsorption of both salt and water in the 

 tubules. Hence the filtration rate is reduced to minimal 

 levels by what appears to be an active inhibition of 

 glomerular activity, probably by constriction of the 

 glomerular arterioles. To compare the fresh- water cat- 

 fish Ameiurus nebulosus with the marine sculpin Myoxo- 

 cephalus octodecimspinosuSy for which fairly accxirate 



