v] THE SOURCES OF FOOD 127 



Their elements are the same as those which make up 

 the greater mass of the animal body, but they have 

 passed out of circulation so far as the nutrition of 

 the latter is concerned. 



But the degradation products of the animal body 

 are the sources of food-stuff for the plant organism, 

 and just where the destructive processes of the animal 

 end, there the constructive processes of the plant 

 begin. The simple compounds, carbonic acid, nitrate 

 or ammonia, are utilised by the algae, the diatoms, 

 the peridinians and other protozoa which possess 

 chlorophyll, and by all the higher animals which also 

 possess green cells in their tissues. These substances 

 are those on which all the life in the sea depends for 

 they are employed in the constructive work of the 

 plants. They are the raw materials for the production 

 of living substance the ultimate food-stuffs of the sea. 



It is important then that we should have a know- 

 ledge of the proportions in which these substances 

 exist in sea- water, and it must be confessed that our 

 knowledge on this point is not so full as is desirable. 

 Now it is to be noted first of all that not all the 

 proteid and carbohydrate and fat of animal or plant 

 tissues suffer, at once, the profound degradation 

 which we have mentioned above. Many organic 

 carbon and nitrogen compounds enter the sea in 

 the water of the rivers, or pass into solution from 

 organisms living and dying in the sea. These carbon 



