152 



ANIMAL LIFE OF THE DEEP SEA BOTTOM 



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Siliceous oozes, too, are of both animal and 

 vegetable origin. There is a broad belt of these 

 running right across the Pacific near the Equator, 

 in which the skeletons of the small unicellular 

 animals radiolarians predominate. Far more im- 

 portant than these, however, are the frustules of 

 the unicellular siliceous algae. Diatoms, which 

 dominate the bottom in a broad belt running 

 round the earth at the Antarctic, as well as in a 

 narrower strip across the northern Pacific; that 

 is to say, in both cases in the cool oceanic regions 

 where every spring the siliceous algze make 

 enormous growth. 



Deep-sea clay and radiolarian ooze are unlike- 

 ly to occur at depths short of about 4,000 metres, 

 and the average depth of the expanses which 

 they cover may be put at 5,400 and 5,300 metres 

 respectively. Diatom ooze and globigerina ooze 

 lie at average depths of approximately 3,900 and 

 3,600 metres, so that, in fact, they form the 

 transition to the continental slope. Pteropod 

 oozes are only occasionally found as deep as 

 about 3,500 metres and are altogether of minor 

 importance; even in the Atlantic, from where the purest typical deposits 

 are known, they form only a small percentage of the total area. The pre- 

 dominant deep-sea deposits, therefore, are deep-sea clay (38 per cent.), 

 calcareous oozes, especially globigerina ooze (48 per cent.), and siliceous 

 oozes, especially diatom ooze ( 1 4 per cent. ) 



From studies of shallower water we have a good deal of evidence to 

 show that the nature of the bed plays a considerable part in the distri- 

 bution of the fauna; we may speak of a sand-bed fauna and a mud-bed 

 fauna, or of animals which inhabit rocks and reefs. But we are a long way 

 from being able to say anything about the importance of the various 

 types of deep-sea oozes to deep-sea fauna; nearly all the animals, to use 

 a term taken from shallow water, may be called soft-bottom forms, those 

 which require a solid foundation being poorly represented. True, there 

 are expanses where it may be supposed that the bottom is pure rock, 

 but they are very limited, one reason being that the deep ocean current 

 is very slight, so that sedimentation can take place where in shallower 

 water it would be swept away. Obviously, such hard expanses are difficult 



Bottle with two sea-anemones 

 from the deep sea ojf South- 

 west Africa, 3,620 metres. 



