RENAL FUNCTION 2S9 



claimed in other animals. That the creatinine U/B ratios are lower than 

 those for inulin would be interpreted on the filtration-reabsorption basis 

 simply to mean that creatinine is less well rejected from the inflow of re- 

 absorbed water than is inulin, and since it has a lower molecular weight 

 this appears understandable. Because of the importance of the finding, 

 Forster and Zia-Walrath (1941) carried out some experiments with 

 inulin on Homarus americanus and found no evidence for secretion of 

 inulin, reabsorption of glucose, or secretion or reabsorption of water. To 

 compare a marine animal with a fresh-water one, however, does not 

 answer our question satisfactorily and the reservation should be removed 

 by further investigation of the crayfish. The search might be doubly re- 

 warding if it proves not only that inulin is not secreted but indeed that 

 there is w^ater uptake from the kidney of a fresh-water arthropod. The 

 search could not be negative, since further proof that inulin is secreted 

 would be of general importance. 



The studies with Homarus americanus have gone forward in the hands 

 of Burger ( 1953, 1955a), who has carried out a careful analysis of kidney 

 function in this very suitable animal. He finds U/B ratios for injected 

 inulin of approximately unity, verifying Forster's w-ork, with both urine 

 and plasma concentrations falling with the same slope over periods of time 

 up to 28 hours. In the absence either of a clear structural filter or a sizable 

 pressure gradient. Burger (1955b) feels some reluctance to apply the 

 term filtration to the process, though he agrees that the end result is the 

 same. 



It is not possible to pursue in any detail the phenomena of ionic regula- 

 tion. Some of the studies directed to this end, however, may bring some 

 additional evidence in favor of filtration as a process in the formation of 

 urine in Crustacea. One case may be cited here because of the unusual ca- 

 pacity of palaemonid prawns to maintain their blood ions at a remarkably 

 constant level in salinities equivalent to from to 5% NaCl solutions. 

 Parry ( 1954) says of her results : 



While the inorganic analyses of urine do not necessarily demonstrate the impor- 

 tance of excretion in maintaining the osmotic control of the animal, they do indicate 

 how it maintains ionic control, and make possible some deductions as to the mechan- 

 ism of urine production. 



The analyses of sodium, chloride, potassium, and calcium in urine are, on the 

 whole, very similar to those of blood, and their concentrations would not be incom- 

 patible with the conception of the urine as an ultrafiltrate of the blood. The much 

 higher concentrations of magnesium and sulphate in urine could result from some 

 active excretion of these ions after the formation of an ultrafiltrate. The lower con- 

 centrations of sodium, and the higher concentration of chloride in the final fluid 

 produced, might result from the necessity to balance the fluid ionically after the influx 

 of magnesium and sulphate (and the ammonia which is presumed to be present). The 

 comparatively constant proportion of sodium and chloride in blood and urine, inde- 



