264 INVERTEBRATE PHYSIOLOGY 



bloods ranged in NaCl-equivalent concentrations from 0.06% to 0.14%, 

 mean 0.103% ; the pericardial fluid from 0.03-0.13%, mean 0.091% ; the 

 urine from 0.04-0.11%, mean 0.071%. It may be seen that the means are 

 rather close together and do not appear to justify the use of the term 

 "markedly," since it is clear that much salt will be lost via the urine at 

 these salt concentrations in view of the large daily urine volume. 



Admitting the difficulty of collecting pure urine in these forms, the im- 

 portance of the reabsorption of salt in meeting the osmotic problem is so 

 great that one wishes there might be an extension of this work following 

 the pattern suggested by Krogh (1939, pp. 61-64) : 



As they stand Pickens' experiments would place the kidney tubules as being much 

 less effective than the cells in the surface, but I venture to predict that this is only 

 because they were not put to a crucial test. When animals are prevented from absorb- 

 ing salts from outside, the kidneys will probably be able to produce a urine which is 

 almost salt free. Otherwise the process of washing out with distilled water would be 

 much more rapid than is actually the case. 



Florkin and Duchateau (1949) have added some confirmatory evidence 

 for reabsorption in Anodonta. Their chemical comparisons of blood and 

 ampullar fluid were concerned with chloride, calcium, and inorganic phos- 

 phate, all of which proved to be less concentrated in urine than in the 

 blood. The data speak clearly for reabsorption, particularly since the 

 difference in CI is marked, the urine containing only a little more than 

 one-half the amount present in the blood. It is worthy of mention that the 

 protein nitrogen of the urine was considerably higher than that of the 

 blood. No interpretation of this observation was given, and it is possible 

 that it represents a merocrine type of glandular excretion which is con- 

 tributing protein to the urine in the process of urine formation. The ad- 

 vantage of collecting urine and conducting inulin clearance studies on this 

 form must be pointed out, since it is possible that the uptake of salt is 

 accompanied by the uptake of water. This would account for the observed 

 increase in protein. The analyses of Potts for inulin cannot answer the 

 question, since he did not collect urine but only assayed the water in which 

 the animals were kept. 



It has long been clear that fresh-water animals face a much more serious 

 problem of ridding their bodies of water without losing salts than do 

 brackish-water or marine animals. Reports of rates of urine flow, or rates 

 of filtration, have been gathered together from the literature and are pre- 

 sented as Table 1. The ordinary rate of urine flow in the marine animals 

 does not much exceed 5% of the body weight in a day, while that in fresh- 

 water and terrestrial organisms can easily reach 50% of the body weight 

 per day and, when filtration is allowed to take place unchecked, the very 

 high rates represented by Pickens' figure may be obtained. Such rates 



