74 Causes of Emigrations 



Many animals when transferred directly from the ocean into air or 

 fresh water live longer in the former than in the latter (Borradaile, 

 1903; Pearse, 1929; Barnes, 1932). According to their toleration 

 for varying salinities aquatic animals may be classified as stenohalin 

 or euryhalin (Schlieper, 1933) . The former are limited to narrow 

 ranges, and the latter endure various salinities. Not all animals fall 

 sharply into one class or the other (Beadle, 1932a) , but there are 

 species or even individuals in a single species which have interme- 

 diate tolerations. Nearly related marine species, as the worms in 

 the genus Nereis, may have quite different degrees of toleration for 

 fresh water; young individuals may differ markedly from old in- 

 dividuals of the same species. Some species have narrowly restricted 

 optima; others can change their organization so as to carry on 

 their activities without difficulty through a wide range of salinities. 

 Along the shores of Puget Sound the species of barnacles on the 

 shore indicate the degree of salinity. Some of the species are larger 

 and more definitely divided into zones in high salinities but in low 

 salinities are smaller and less sharply segregated (Rice, 1930). 

 Along the coast of Japan the eggs of each species of oyster require 

 peculiar conditions of salinity for optimum development. Littoral 

 oysters are euryhalin; brackish-water species are adapted to low sa- 

 linities; and a sublittoral species is stenohalin (Amemiya, 1928) . 

 Plants along the seashore show rather definite segregation in rela- 

 tion to the salinity of the water (Gessner, 1931). 



Reproductive activities are often related to or limited by salinity. 

 The Baltic Sea contains less salt than most oceans. Some adult ma- 

 rine animals are able to persist, but their offsprings cannot mature; 

 others have been favored by low salinity. For example, local spe- 

 cies of Nereis and Cyanea have increased in numbers, and Aurelia 

 has decreased (Reibisch, 1926) . The eggs of Nereis and other lit- 

 toral animals may be fertilized and may develop in greatly diluted 

 sea water, but the eggs of such a stenohalin type as Echinarachnius 

 will not survive such treatment (Just, 1930, 1930a). Cannon 



