58 FOOD 



nutrition. In any case, they come mthin the compass of one 

 definition of food, "whatever sustains and supports growth." 



One of the most perfect foods for mammals is milk, which 

 contains all these compounds in proper proportions, and all 

 young mammals live for a time on milk. Eggs are almost as 

 perfect a food, but they do not contain quite enough carbo- 

 hydrates for a mammal. Vegetable foods, such as rice, potato, 

 wheat, contain an almost excessive amount of carbohydrates, 

 whilst animal food, such as mutton, beef, pork, chicken, 

 contains an excess of proteins for man, who has as a rule a 

 mixed meal. "Loaves and fishes" with some oreen vegetable 

 — for the vitamins — form a light and w^holesome diet. For 

 a human being the vegetable and animal foods should be 

 mixed in suitable proportions. 



The process of building up the dead food into the living 

 protoj^lasm of the cells is known as anabolism^ and the process 

 of breaking down the protoplasm of the cells into the various 

 waste products is known as katabolism. The two together, 

 the building up and the breaking down, are often referred to 

 as metabolism. 



Variety of Foodstuffs and Feeding Habits 



As we shall see, the food of animals is very varied. Many 

 animals are fixed, or move about with difficulty. x\mongst 

 the molluscs the fresh-water mussel, common in our rivers, 

 moves chiefly at night and rests by day, but it does not move 

 very far, perhaps only fifteen feet in the course of twenty- 

 four hours, or about a mile a year. The marine or edible 

 mussel is anchored to rocks, etc., by stout threads, and the 

 oyster by one valve of its shell, and in all these cases the food 

 is brought into the neighbourhood of the mouth by the action 

 of cilia; these set up a stream which carries inside the shell 

 not only food but oxygen. Cilia also bring sea-water laden 

 with nutritive matter into the bodies of sea-squirts (Ascidians) 

 and sea-anemones — "those flowering stomachs," as George 

 Meredith calls them, "wiiich open to anything and speedily 

 cast out what they cannot consume" — and into many other 

 aquatic animals. The minute plants and animals which form 

 the food of jelly-fish are entangled in mucus secreted along 



