78 Causes of Emigrations 



normal if the fish can Uve. Salts and water leave the body of a fish 

 through its gills in dilute solutions, and the loss of essential salts 

 may thus cause death (Sumner, 1906) . In marine invertebrates and 

 elasmobranchs osmotic swelling is proportional to the diminution 

 of the osmotic pressure of the blood (Hukuda, 1932) . Changes in 

 weight and salt content take place in teleost fishes when the salinity 

 of the medium varies, but they are not directly proportional. 

 "The osmotic pressure of the internal medium fluctuates within a 

 much narrower range than that of the external medium" (Sum- 

 ner, 1906). 



Osmotic pressure is like solution pressure of a dissolving solid, 

 vapor tension of an evaporating liquid, or gaseous pressure of a 

 permanent gas. Each molecule goes as fast and as far as it can. 

 Osmosis exerts uniform pressure in a confined space. It is of course 

 modified by ionization, adsorption, electrical conditions, etc. In a 

 living organism a change in osmotic pressure indicates that work 

 is being done in a cell. The body fluids of animals may be main- 

 tained in equilibrium with the environment by the kidneys (Haw- 

 thorne, 1930; Schlieper, 1929a) , or they may be regulated by some 

 other "active living process" (Margaria, 1931). 



"The action of all the mechanisms which regulate the chemical 

 relations of the organism is essentially to control the exchange of 

 material which takes place between the organism and its surround- 

 ings. In its crudest form this mechanism acts simply by abolishing 

 interchange between organism and environment when the characters 

 of the latter become unsuitable. . . . The differences observable 

 among different species of fish are due to the fact that they possess 

 adjusting mechanisms of different degrees of efficiency and not to 

 the maintenance of specifically distinct levels of osmotic pressure" 

 (Wardlaw, 1931). The changes that animals have undergone to 

 become adapted to various salinities will be considered in the next 

 chapter. 



