TECHNICAL NOTES AND BIBLIOGRAPHY 265 



water, has the congenital features characteristic of a 

 marine habitus. 



The peritubular blood supply of the aglomerular kid- 

 ney could conceivably have been derived from the renal 

 artery, but this would have required the evolution of 

 some device (analogous to the glomerulus) to reduce 

 the pressure of this arterial blood before it entered these 

 capillaries; otherwise water would be filtered out of the 

 capillaries into the interstitial space between the tubules, 

 with no way of removing it. 



Keys has shown by means of a heart-gill preparation 

 that branchial excretion in the eel can move chloride 

 against a concentration gradient of nearly 3 to i. Our 

 assumption that ion transport is eflFected by the undiffer- 

 entiated respiratory epithelium has been challenged by 

 him and by Copeland and others, who attribute the 

 process to isolated cells that occur not only in the gills 

 but also in other regions of the head, especially on the 

 inner surface of the operculum. Opposed to this inter- 

 pretation is the belief of Bevelander that these special- 

 ized cells are mucous cells and have no special relation 

 to ion transfer. The hterature of the subject is cited by 

 Copeland {108}, Bums and Copeland {105}, and Get- 

 man {116}. The problem is under further study by sev- 

 eral investigators at the present time. 



It is theoretically feasible for the marine fish to ab- 

 sorb water (without salt) directly from sea water through 

 the gill membranes, since the same quantity of work 

 would be involved in the separation of salt from water. 

 There are two possible explanations why the circuitous 

 method of drinking sea water is used. It has been noted 

 that in the invertebrates the respiratory epithelium is ap- 

 parently concerned in the regulation of the ionic com- 

 position of the body fluids, even where the animal's 

 blood is isosmotic with sea water {54, p. 92}; hence the 

 excretion of sodium chloride by the gills in both the 



