92 LIFE IN THE SEA [OH. 



difficultly assimilable cellulose, and that of the 

 herbivorous marine animals contains in place of 

 cellulose the calcareous or siliceous shells or skeletons 

 of the food animals. Generally speaking the food 

 of the marine carnivores and herbivores corresponds 

 to that of their counterparts on the land, but bulk 

 for bulk their food is not so rich in the tissue-forming 

 constituents, nor in the heat-producing fats or carbo- 

 hydrates. 



The processes of digestion must be clearly 

 distinguished from those of assimilation an im- 

 portant distinction which is made far more clear by 

 the vegetable physiologists than by writers on marine 

 biology. The tissues which compose the bodies of 

 plants and animals are made up of cells modified in 

 innumerable ways to carry on vital processes. 

 Organic cells form about them many other structures : 

 fibrous structures, calcareous and siliceous structures 

 in the form of shells, etc., and other hard parts ; 

 they also lay down in themselves substances such as 

 fats, sugars, oils and starch, which can be used for 

 the production of heat or other forms of energy by 

 being oxidised or burned. The living substance of 

 the organic cell may be called bioplasm : we do not 

 know what this is, but when it is killed we recognise 

 it as protoplasm. The bioplasm of the cells is 

 continually dying or wasting away and it must be 

 renewed ; under its control the hard parts of the 



