HYDRA FUSCA 139 



arise, some of which are useful, while others are waste products. 

 The result of the combination of food with oxygen is, that a 

 certain amount of force is liberated in the same way that heat 

 is liberated from coal when it is burned. This force varies 

 according to the amount of activity of the animal life. The 

 whole process of chemical change by which the food is used is 

 called metabolism. Two distinct phases of it may be recognized : 

 anabolism (Gr. ana = up) , the process by which complex sub- 

 stances are built out of simpler ones; and katabolism (Gr. kata = 

 down), the process by which complex substances are torn down 

 into simpler ones. In animals the latter are more extensive than 

 the former. 



Excretion. The function of getting rid of the waste products 

 of metabolism is called excretion. These products are no longer 

 valuable but act as a direct poison to the body if allowed to 

 remain. These waste products are solid, liquid, or gaseous. 

 The gases are excreted by respiration, as just described. In 

 higher animals the liquids are carried off by the lungs, by the 

 skin, and by special organs called kidneys. It must be remem- 

 bered that excretion does not refer to the passage from the 

 intestines of the undigested food. This undigested food has 

 never become part of the body and its passage from the intes- 

 tines is not strictly excretion. There is apt to be confusion in 

 the use of the terms, as the undigested food which passes through 

 the intestines frequently goes by the name of excreta. In the 

 strict sense, however, the excreta or faeces are not excretions. 



Motion. Practically all animals possess some power of mo- 

 tion and have special organs adapted for bringing it about. 



Support. The living parts of an animal (protoplasm) are 

 made up of a soft, jelly-like substance, too non-resistant to 

 have the power to hold any particular shape. If the animal is 

 small the resisting power of the jelly may be sufficient to preserve 

 its shape; but in large animals it is necessary to have some hard 

 support for holding the soft parts. This hard supporting sub- 

 stance may be in the form of a skeleton or shell. 



