CHAPTER XII 



THE BIOTIC DIVISIONS OF THE OCEAN: 

 THE BENTHAL 



The variety in the environmental conditions in different parts of 

 the ocean produces corresponding differences in the adaptation of the 

 inhabitants. In consequence, the animal communities which are ex- 

 posed to a given set of conditions, in spite of being composed of animals 

 from groups separated in zoological classification, bear the stamp of a 

 similarity produced by a number of adaptive resemblances in structure 

 and habits. Such common characters appear the more plainly the more 

 numerous the adaptations required by the environment. At the hypo- 

 thetical original site of the development of life, in the coastal areas 

 of warm seas, where all conditions approach the optimum, the possi- 

 bilities for variety of organization are greatest. Every structural plan 

 found in the animal kingdom is represented by a variety of develop- 

 ments and transformations so that the number of species is literally 

 bewildering. Every change in the environment in the direction of the 

 unfavorable brings limitations with it, and these become more sharply 

 denned the more extreme the conditions, as in the littoral area in 

 polar seas, in the deep sea, and in brackish seas. At such places the 

 number of species is diminished and the variety of structural types is 



reduced. 



In the ocean as a whole, two main divisions may be contrasted, the 

 sea bottom and the open sea, the "benthal" and the "pelagial" (Fig. 

 16). Each falls vertically into two parts, a lighted zone and a lightless 

 one. The lighted zone of the benthal is designated as littoral, the light- 

 less as abyssal. The boundary between the two zones is not sharply 

 defined and is reckoned on the average at about 200-m. depth, coincid- 

 ing with the border of the continental shelf. At many places there is no 

 sloping littoral shelf, and the coast drops steeply off to great depths. 

 The lighted zone in such places must still be called the littoral. Two 

 subdivisions of the littoral are to be recognized, according to the 

 steepness of the coast and the nature of the shore: depositing shore, 

 usually with more or less loose shore material, and eroding shore, more 

 or less steep and rocky. Each of these groups divides into a number of 



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