134 LIFE IN THE SEA [OH. 



waters, or at the surface of the sea ; and that the 

 proteid, carbohydrate, and fat of the bodies of the 

 organisms falling to the bottom may remain for some 

 time not much altered in composition. They thus 

 form the food of the abyssal animals, and the latter 

 must be, to some extent, carrion-eating creatures. 

 In the end, however, the carbon and nitrogen 

 compounds of the bodies of organisms at the sea- 

 bottom will be transformed to products like urea, 

 and, of course, carbonic acid ; and the urea, or similar 

 substance probably passes into the form of ammonia 

 by the action of enzymes. Then these substances 

 must be removed from the sea-bottom, for otherwise 

 the bottom water would be richer in organic matter 

 than it actually is. 



This removal is effected by the agency of currents. 

 In the main the system of oceanic circulation consists 

 of a bottom-drift from about the region of the 

 temperate seas to the equator. This cold bottom 

 water rises up to the surface at the equator, and 

 then becoming heated flows away to the north and 

 south. There is a similar bottom-drift from the 

 temperate regions towards the poles, and then this 

 water rises to the surface to take the place of that 

 which flows to the south or north after it has become 

 lighter by freezing. Thus there is a continual 

 streaming of water along the sea-bottom and this 

 water rises to the surface in the equatorial and polar 



