486 DISTRIBUTION OF LIFE IN THE OCEAN 



types among which are many that possess skeletons of 

 lime. In both regions there are additional species, 

 known as peridineans, without mineral skeletons but im- 

 portant as an article in the diet of many fishes and 

 interesting in that several species are brilliantly 

 phosphorescent. 



Plankton. — Although floating plants normally occur 

 at depths less than 200 feet, their bodies are heavier 

 than water, and at all times great numbers are gradually 



Wb^^S^^^^ 



Fio. 139. — Diatoms, microscopic plants with flinty skeletons. 



settling to the bottom. Hence it is not surprising to 

 find that each of the many different species of animals 

 that directly or indirectly live on this food supply is 

 fitted to live at fairly definite depths anywhere from 

 the surface to the ocean floor. All of these swimming 

 and floating organisms, plant and animal, are collectively 

 known as plankton. In the plankton the plants out- 

 number the animals, but to what extent is not exa^'tly 

 known. In the Baltic sea it has been estimated that 

 the plants make up fifty-six per cent and the animals 

 forty-four per cent of the total amount. It has further 

 been calculated that the volume of plants doubles in 



