1913] on Life in the Great Oceans 62!) 



tlitin ill the layers nearer the surface and nearer the bottom, and 

 are mostly deep black or ])right red in colour. 



The bottom-living- animals (Benthos) are most abundant in shallow 

 water, and decrease in mimbers towards the deep water far from land, 

 excejit perhaps in the deei) water of the great Southern Ocean. 

 (Characteristic assemblages of benthonic animals are found indifferent 

 regions and at different depths, and various schemes have been de- 

 vised to express their distribution. In European waters we may 

 generally recognize : (1) the littoral zone, extending from high- 

 water mark to a depth of about 20 fathoms, which is sub-divided 

 into areas and belts according to latitude and to the materials cover- 

 ing the bottom ; (2) the sublittoral or shallow-water zone extending 

 from about 20 fathoms to about 100 fathoms ; (3) the archibenthal 

 zone, occupying the continental slopes beyond 100 fathoms down to 

 1700 fathoms (the mean sphere level)* ; and (4) the deep-sea or 

 abyssal zone in depths beyond 1700 fathoms. 



Many characteristic deep-sea forms have long stalks or long legs, 

 lifting the bodies of the animals out of the ooze ; others have well- 

 developed tactile organs ; others are delicate in structure v/hen 

 compared with their shallow-water allies, especially those that secrete 

 calcium carbonate. Phosphorescent light plays an important role in 

 deep-sea life, and is emitted principally by coelenterates, which have 

 freipiently been brought up from great depths in a light-giving con- 

 dition. The subdued colouring of deep-sea animals, mostly red and 

 ])rown monotones, is doubtless correlated with the faint gleams of 

 phosphorescent light, but it is difficult to explain the variation in the 

 size of the eyes observable among some deep-sea forms, the same 

 haul of the trawl sometimes bringing up creatures with large eyes 

 along with others having small eyes or totally blind. Since the low 

 temperature prevailing in the abyssal zone retards the action of the 

 digestive enzymes, the food requirements of the deep-sea animals 

 must be less than those of animals living in the warmer waters of the 

 ocean. 



The colonizing of the deep sea seems to have been effected by 

 successive migrations from the shallower reaches of the ocean, 

 especially from the vicinity of the mud-line, but apparently more 

 frequently from cold than from warm regions. We may suppose 

 that, if the waters of the ocean had at one time a nearly uniform 

 temperature throughout, the deep sea would be unsuitable for animal 

 life from the lack of circulation and hence of atmospheric oxygen, 

 though benthonic animals might occupy the whole of the shallow- 

 water zone. When cooling commenced in the polar regions the 

 forms with pelagic larval stages would be killed out or forced to 



* Recent researches show that the fish-fauna in this zone of the north-east 

 Atlantic is very uniform from the Faroes to the Canaries, several species being 

 common to these two localities. 



