266 FBOM FISH TO PHILOSOPHER 



elasmobranchs and teleosts is possibly an archaic opera- 

 tion which was inherited by the vertebrates from their 

 invertebrate ancestors. As an ion transport mechanism, 

 the gill may have primitively been incapable of osmotic 

 work with reference to water. Had it been otherwise, one 

 may ask why the marine elasmobranchs had to turn to 

 the m^ea-retention habitus in order to separate water 

 from sea water, or why the ostracoderms failed to es- 

 tablish themselves in the sea. 



An equally cogent consideration, however, is the fact 

 that to absorb water directly from sea water, the mem- 

 brane (or cell) involved must operate specifically on 

 water molecules in separating water from salt. Though 

 many unicellular organisms can pimip water out of the 

 cell, there is as yet little evidence that any unicellular 

 organism, or any cell or organ in any animal, can pump 

 water into the cell against an outward osmotic gradient, 

 and it may be that this is a trick that protoplasm has 

 never solved. The mechanism of making a concentrated 

 urine in the birds and mammals is now interpreted in 

 terms of the active transport of sodium chloride. (Chap- 

 ter X.) 



The process of drinking sea water, as practiced by the 

 marine teleosts, is not a very efficient one. Sea water con- 

 tains, in addition to sodium chloride, considerable quan- 

 tities of the divalent ions, calciimi, magnesium, and sul- 

 fate, all of which are poorly absorbed by the intestinal 

 mucosa (as is demonstrated by the famiUar use of mag- 

 nesium sulfate or Epsom salts as a 'saline' or osmotic 

 cathartic). As sodium chloride is absorbed, these divalent 

 ions are in large part left behind in the intestinal residue 

 and draw water from the body fluids to produce a solu- 

 tion which approaches the blood in osmotic pressure; 

 on opening the intestine of a marine fish, one finds in it 

 large quantities of this unabsorbable Epsom salts mix- 

 ture, and this clear fluid is frequently discharged from 

 the anus with vigor when the fish is handled. Some cal- 

 cium, magnesium, and sulfate are, however, absorbed 



