ANIMAL LIFE OF THE DEEP SEA BOTTOM 



151 



Fishing at night. The sledge- 

 trawl frame has been bent by 

 an obstacle on the bottom. 



distribution extending from coastal regions right down to the oceanic 

 trenches. Examples will be given when we come to the fauna of these 

 trenches; meanwhile, suffice it to say that special adaptation to the high 

 pressures must be assumed, even though the individual species may occur 

 over an astonishingly wide range of depths. The difficulty of judging the 

 significance of pressure in individual cases is complicated by two other 

 important factors. 



First, there is the nature of the bottom material. This is always very 

 fine and can roughly be classed as clayey. Yet it varies a great deal, as the 

 basic substance — deep-sea clay — may be mixed with a number of ele- 

 ments of organic origin. These may consist partly of the excrement of 

 animals, of vegetable material which has drifted far out to sea before 

 sinking to the bottom and being broken down by bacterical activity, and 

 of the skeletons of small organisms which may occur in such vast quan- 

 tities as to dominate the nature of the bed entirely. When organic elements 

 make up at least one-third of the bed it is known as deep-sea ooze. In 

 tropical and subtropical regions the predominant types are calcareous. 

 The skeleton parts may here derive from the unicellular animals globi- 

 gerinas, the commonest members of the group of Foraminifera which mostly 

 Ywe as plankton in the surface layers of the sea. From other layers of water 

 come the skeletons of other small unicellular creatures, the CoccoUtho- 

 phorida, along with the shells of pteropod snails. 



