476 ZOOLOGY 



tical problems with great success, until the war put a 

 stop to the cooperative plans. In the efforts to trace 

 the migrations of fishes, very elaborate methods have 

 been devised. Thus it has been possible to learn much 

 about the mackerel and herring by carefully measuring 

 thousands of individuals, in order to distinguish the 

 slightly different races. Also, especially in the case 

 of the salmon, the seasonal markings on the scales have 

 afforded a clew to the habits and movements of the 

 fish. The movements of the smaller animals, such as 

 the Crustacea, also prove to be important, since these 

 furnish food to fishes. 



The benthos 6. On the floor of the ocean is the assemblage of 

 organisms called the benthos. It occupies different 

 levels and environments, according to the depth. The 

 deep-sea animals feed on one another, but existence on 

 this basis alone would be as difficult as that in the 

 mythical island where all the inhabitants got a living 



FIG. 205. Foraminifera tests from the bottom of the North Atlantic Ocean; 

 magnified about 10 diameters. 



