134 THE OCEAN 



the herbivores are in turn the prey of carniv- 

 orous animals, just as on the land-surfaces. 

 Beneath the photic zone living plants are 

 not present. However, the dead remains of 

 the algae which inhabit the photic zone in 

 falling to the bottom supply food for animals 

 in the intermediate waters, for those fixed 

 organisms which catch the small organic 

 particles as they setfle on the bottom, and 

 for the echinoderms and other invertebrates 

 which crawl along the bottom and eat the 

 oozes, muds, and clays. These in turn are 

 preyed on by the carnivores which are also 

 present. All marine animals derive their food 

 primarily from marine algae, to which must be 

 added the nutritive material carried into the 

 ocean by rivers. 



Remembering the great depth of the ooean, 

 that plants may function at a depth of 3000 

 or 4000 feet, and that animals exist through- 

 out the whole ocean, we may conclude that 

 the total quantity of living matter in the ocean 

 greatly exceeds that on the land- surfaces of 

 the globe. 



As has been pointed out in previous chapters, 

 the physical conditions in the ocean to which 

 organisms have adapted themselves are most 

 varied. Some marine organisms in the Arctic 

 and Antarctic regions live the whole year 

 round in water having a temperature below 



