162 



THE BIOLOGY OF AN ANIMAL. 



on the alert, retreating at the slightest alarm. In winter or in 

 seasons of drought it burrows deep into the earth. 



Income. As in the case of other living things, the life of 

 Lumbricus is carried on by a continual interchange of matter 

 and energy with the environment ; but the character of the ex- 

 change differs widely from that of the fern, chiefly for the reason 

 that the animal cannot directly utilize the kinetic energy of light 

 as a source of vital power. Heat is directly absorbed, it is true, 

 but this is not available except as a favorable condition for vital 

 action. The sole source of energy must therefore be ready-made 

 food (carbohydrates, proteids, etc.), which has been manufactured 

 by plants from crude materials such as carbon dioxide and water. 

 Moreover, proteid matter is an essential constituent of this food. 

 We do not know why the earthworm must have proteids, but it 

 is a fact ; and Luinbricus is a type of all animals in this respect. 

 The worm also requires free oxygen, salts, and water, but in this 

 it agrees with all living things. Probably it lives best upon 

 a mixed diet containing proteids, carbohydrates, fats, w r ater, 

 salts, and free oxygen. It is difficult to determine the exact in- 

 come of Lunibricus, but it may be set down approximately as 

 follows : 



INCOME OF LUMBRICUS. 



