162 MARINE ANIMALS 



The waters of the ocean have been divided into three strata with 

 reference to the amounts of light present. Employing the names intro- 

 duced by Chun these are: 



1. The euphotic stratum, rich in light, rich in phytoplankton and 

 in herbivorous animals, extending from to 80 m. according to Chun; 

 from to 30 m. according to Lo Bianco; 67 from to 100 m. according 

 to Murray and Hjort. 



2. The dysphotic stratum, weakly lighted; 80-200 m. according to 

 Chun; 30-500 m. according to Lo Bianco; down to 500 m., Murray 

 and Hjort. Above 200 m. this corresponds to the "Shadow Flora" of 

 Schimper. 



3. The aphotic stratum, below 200 m. according to Chun; below 

 500 m. according to Lo Bianco; below 1700 m., Murray and Hjort; 

 it is lightless, and without herbivores; only detritus-eaters and pre- 

 daceous forms are present. 



The more nearly the poles are approached, the shallower is the 

 euphotic stratum, and the closer to the surface is the main mass of 

 marine life. This perhaps explains why the sea birds, dependent on 

 marine life for their food, are so abundant in both individuals and 

 species in the polar seas, where their food is both accessible and con- 

 centrated. 



i 



BIBLIOGRAPHY 



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 & Hjort, op. cit., p. 291.— 23) Agassiz, A., 1888, Bull. Mus. Comp. Zool., 14, 

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