114 How Animals Changed 



(1) they are much more dilute than the blood of higher forms, 

 being about 0.01 per cent total salt; (2) the calcium is quite gen- 

 erally more concentrated than the sodium and the potassium to- 

 gether" (Hetherington, 1932) . Marine animals which attempt to 

 enter fresh water usually are not killed so much by dilution as by 

 loss of essential salts (Sumner, 1906; Adolph, 1925; Hill, 1931). 

 "If we consider any of the natural waters — lake, sea, spring, rain, 

 or spring water — there will be found a general qualitative resem- 

 blance in the inorganic content. The fact that the internal medium 

 of the higher animals has an inorganic composition qualitatively 

 similar to that of the natural waters is scarcely surprising. What is 

 physiologically important is the quantitative pattern" (Conway, 

 1945) . A conger eel may survive for days in 0.1 sea water but dies 

 in a few hours in fresh water (Hill, 1931). 



Sea waters and bloods are complex systems which contain salts, 

 organic materials, colloids, ions, and other substances which may 

 affect organisms. Each substance probably has more or less spe- 

 cific effects on protoplasm and vital processes. Some marine ani- 

 mals contain more than 99% water in their bodies (Gortner, 1933) . 

 Others commonly maintain body fluids which contain such mixtures 

 of salts and organic materials as to make them slightly denser than 

 the sea water. Other marine animals maintain their body fluids in a 

 steady state which is much less dense than sea water. Three types 

 of equilibria occur in animals, and these represent different osmotic 

 relations with the environment: (1) body fluids are isotonic and 

 isohalin with the external medium; (2) body fluids are isotonic but 

 not isohalin with the external medium; and (3) the body fluids 

 differ in osmotic pressure and content from the external medium, 

 and are therefore more or less independent of environment (Fred- 

 ericq, 1922) . Representatives of all these types live in the ocean 

 today. In animals osmotic pressure cannot rise much above that of 

 the environment (Shelf ord, 1929), but in plants it may be two or 

 three times as great (Atkins, 1917) . It has been suggested by some 



