RENAL FUNCTION 265 



cannot be maintained for long periods of time by the animals, but represent 

 observations of maximum rates which are useful. But the high rates of 

 urine flow impose a severe task of salt reabsorption. Potts (1954c) has 

 based an interesting computation of the amount of work required to bring 

 about the reabsorption of salt on the analyses of Anodonta urine from 

 Pickens' work and his own determinations of the salt content of Anodonta 

 blood. He concludes that a lowering of the blood salt level may have been 

 essential to the adaptation of fresh-water lamellibranchs to their new 

 habitat because of the great surface which they expose to the medium. The 

 move through brackish water may not only have increased the ability of the 

 animals to take up salt but allowed for the adaptation of their tissues to the 

 much lower salt content of this group. For an interesting comparison with 

 the fresh-water crustaceans Potts' ( 1954a,b,c) papers should be examined. 



In gastropods the collection of urine is relatively easy but we do not 

 have, unfortunately, any comparable experiments on salt conservation. 

 Freezing-point determinations show the urine of Achatina fulica, the 

 giant African snail, to be hypotonic to the blood ; the freezing point of blood 

 was —0.462, that of the urine was —0.285. There has, however, been 

 a careful proof of the reabsorption of glucose (Martin, Harrison, and 

 Stewart, 1953). Urine obtained from the snail kidney even at higher rates 

 of flow than can be considered normal shows a glucose concentration con- 

 siderably below that of the blood. Values for the blood sugar of eleven 

 animals ranged from 1.2-67.5 mgm% with an average of 20.7 mgm% and 

 the corresponding urine-reducing substances in nine cases ranged from 

 to 35.0 mgm% with an average of 10.7 mgm^. 



When an animal producing urine which shows this difference in glucose 

 concentration from the blood is treated with phlorizin, the urinary glu- 

 cose rises until it reaches the level of the blood glucose. If water reabsorp- 

 tion is taking place the urinary glucose may then rise above the blood 

 glucose level, but simultaneous determination of inulin concentrations 

 shows that the glucose does not exceed the inulin in excretion rate, and 

 the result is best interpreted as a simple filtration followed by water re- 

 absorption by a mechanism stable to phlorizin, but with the glucose reab- 

 sorption mechanism thrown out of action. 



Another type of experiment was devised by my colleague, Stewart. 

 In Achatina it is possible to tie a tube into the kidney lumen high on the 

 structure, so that a material perfused into it will have to pass over many 

 trabeculae of the kidney on its way to the excretory duct and may have 

 adequate exposure to cells concerned with glucose reabsorption. When 

 glucose solution is allowed to perfuse into the kidney in this way, or 

 through the pericardial sac, the normal kidney extracts the added glucose, 

 but after phlorizin the added glucose may be recovered quantitatively. 



