14 AN INTRODUCTION TO ZOOLOGY 



changes taking place during combustion. Respiration supplies 

 the oxygen for such metabolic activities, and also eliminates cer- 

 tain gaseous excretory products. The waste products formed 

 by the breaking down of living substance are cast off as excretions. 

 These should not be confused with faeces, which have never actu- 

 ally become constituents of living matter. 



Figure 2 is intended to indicate the relations of the various 

 metabolic activities. It shows that an animal requires food and 



FIG. 2. Diagram showing the relations of the various metabolic activities 



of animals. 



oxygen; that part of the food is digested and assimilated, while 

 the rest is cast out as faeces; and that the oxidation of the living 

 substance results in the production of certain excretions which 

 are eliminated. Urea is the most important of these excretory 

 products. 



Growth has been said to represent the excess of anabolism 

 over katabolism. The power of growth becomes gradually less 

 throughout the life of any individual, and all animals pass 

 through certain well-marked stages, youth, maturity, and old age. 

 Youth is generally characterized by great vigor and activity as 

 well as rapid growth. At maturity anabolism and katabolism 

 are about equally balanced. During this adult period little 

 change is observable in external form or gross weight. Maturity 

 gradually gives way to old age, a period of decline, when the body 



