94 LIFE IN THE SEA [CH. 



ways, and that the products of these decompositions 

 are always substances called ammo-acids, which 

 though very much simpler in composition than the 

 proteids are still very complex chemical compounds. 

 We also know that the amino-acids can be re- 

 constituted to form proteids, but this reconstitution 

 can only be effected by the agency of a living cell, 

 and all attempts to make this synthesis outside the 

 living substance have hitherto failed. 



The food-stuffs of an animal are the proteids, fats, 

 and carbohydrates of some other animal or plant and 

 they are always different from those of its own body, 

 and cannot be directly utilised by it as food. Before 

 these substances can be assimilated they must be 

 broken down into their constituents and then the 

 latter must be resynthesised to form the specific 

 proteids, fats, or carbohydrates of the animal. This 

 decomposition and recomposition make up the 

 processes of digestion in the body of an animal. In 

 our own case the proteids may be those of lean 

 meat, of white of egg, of cheese, of the gluten of 

 wheat bread, milk-albumen, etc. The carbohydrates 

 may be the starches or sugars of various grains, and 

 the fats are also usually different from those of our 

 own body. 



Digestion of the food is carried out by substances 

 called ferments, or enzymes, elaborated by the 

 digestive glands. The proteolytic enzymes are those 



