CHAPTER IV 



MODES OP NUTRITION 



IT is thus easy to show that the life-processes of 

 marine organisms are directly affected by changes in 

 the external conditions under which they live : that 

 is to say in the chemical composition of the sea- 

 water ; in the temperature of the sea ; in the intensity 

 of solar radiation other than heat ; in the pressure 

 of the sea-water and so on. But each species of 

 organism feeds in some manner and its density and 

 range of geographical distribution must be affected 

 by changes in the abundance of its food. Now most 

 marine animals nourish themselves by eating other 

 animals or plants ; and most marine plants by 

 assimilating certain substances which are contained 

 in solution in the water of the sea. We have there- 

 fore to consider what are the precise ways in which 

 these nutritive processes are carried out, and how 

 the food-stuffs are distributed in the sea. 



The majority of marine animals are predatory, and 

 the history of the evolution of their bodily form is to 



