20 COLLEGE ZOOLOGY 



take in food which is digested and assimilated, that is, dissolved, 

 absorbed, and changed into protoplasm. Oxygen is also taken 

 in during respiration; this unites with protoplasm (oxidation), 



OXYGEN ^^"^ ^\ EXCRETIONS 



Fig. 6. — Metabolism. Diagram showing the various metabolic 

 activities of animals. 



furnishing energy and producing waste products or excretions. 

 Animals are primarily katabolic organisms, being unable to 

 manufacture organic compounds from simple inorganic substances. 



CARBON DIOXIDE ^ >V OXYGEN 



STARCH 



Fig. 7. — Metabolism. Diagram showing the manufacture of food 



by plants (photosynthesis). 



Plants or other animals are therefore absolutely necessary for 

 their existence. 



Plants, on the other hand, are able to manufacture food from 

 inorganic matter by a process known as photosynthesis (Fig. 7). 



