RENAL FUNCTION 273 



prising is the observation that phenol-red secretion was stopped by a 

 modest back pressure which simultaneously stopped the secretion of water. 

 In view of the well-known capacity of secretory cells to secrete against high 

 gradients and high pressure, this is a very interesting observation. 



Annelids 



Evidences for filtration and reabsorption in certain annelids have been 

 described from the work of Bahl and Ramsay. The difficulties of quanti- 

 tative blood and urine collection, and the relative certainty that secretory 

 mechanisms exist, have so far prevented analyses of rates of dye excretion. 

 Cordier ( 1934) published an extensive cytological study on the athrocytic 

 excretion of various dyes injected into the coelomic cavity of the earth- 

 worm. His studies showed that a great deal of activity was involved in the 

 processes, but no measurements of rate could be made. It is to be hoped 

 that search will reveal forms suitable for quantitative work and that the 

 knowledge of the excretory capacity of the annelids may be brought to a 

 par with that of the other higher invertebrates. 



Summary 



This review is limited to a consideration of physiological studies in which 

 some degree of knowledge of simultaneous concentrations in blood and 

 urine is available. For the sake of brevity, studies on insects and studies 

 on athrocytosis have been omitted. 



Reliable data have accumulated for a filtration origin of urine in the 

 molluscan classes Cephalopoda and Gastropoda, and a clear suggestion of 

 filtration in Pelecypoda. In each class the filtrate undergoes chemical 

 changes as it passes through the lumen of the kidney, with the reabsorption 

 of salts or glucose or water established for at least one class of the phylum. 

 There is active secretion into the urine of such foreign substances as 

 phenol red and para-amino hippuric acid. 



Appropriate studies of the Arthropoda are confined almost entirely to 

 crustaceans. Although very satisfactory evidence for filtration has been 

 presented for some marine members of this group, the fresh-water forms 

 pose a very particular problem because of the claim that inulin, as well as 

 all other ingredients of the urine, is secreted by the kidneys of the cray- 

 fish. If substantiated this would represent the first case in which such an 

 activity of kidney cells had been demonstrated and would serve as a serious 

 criticism to many conckisions based on studies with this substance. In the 

 marine groups filtration is obviously accompanied by an active process 

 of reabsorption of substances important to the economy of the animal, and 

 several dyes are known to be secreted. 



With the exception of some elegant studies by Ramsay, modern methods 



