THE LUNGFISH 8l 



quently per gram of body weight lost in fasting he ob- 

 tains 4.0 calories to the lungfish's 1.4, thus making him 

 a better candidate for fasting, per unit of body weight 

 lost, by a ratio of 2.5 to 1.0. But being warm-blooded, 

 man spends his store of energy in thirty to sixty days, 

 while the limgfish, because it is cold-blooded and be- 

 cause of the remarkable reduction in its metabolic rate 

 in the fasting state, can spread its reserves over several 

 years. 



In the dry mud the lungfish has no water available 

 to it except the small quantity of so-called metabolic 

 water formed by the oxidation of carbohydrate, fat and 

 protein, and an additional small quantity Uberated from 

 the tissues as they are degraded. Nothing is known about 

 renal fimction during estivation except that no urine is 

 formed (and it is rather difficult to study any physio- 

 logical function when it is zero). The cloaca, into which 

 the ureters open, is sealed tight by the cocoon, and there 

 is no evidence of any urine excretion after the cocoon 

 is formed. This would be impossible in any case: we may 

 be certain that the blood flow to the kidneys, as in all 

 other parts of the body, is reduced to minimal mainte- 

 nance levels; the blood pressure must decrease as the 

 heart slows and the process of glomerular filtration must 

 be wholly suspended. A mere trickle of blood through 

 the renal-portal system and the glomeruli suffices to keep 

 the renal tubules alive. 



With excretion wholly arrested, all the nonvolatile 

 products of metabolism must accumulate in the body. 

 The one produced in largest amount is the urea, which 

 is formed by the metabolism of body protein. It has 

 been emphasized that urea is a relatively nontoxic sub- 

 stance, and where it accumulates slowly in the body, as 

 in the estivating lungfish, it probably has no effect what- 

 ever on internal salt and water distribution or the func- 

 tion of any organ. At the end of the first year of estiva- 

 tion, the urea content may reach 2 per cent of the total 



