III. HOW THEY PERFORM THE 

 PRIMARY FUNCTIONS 



{Breathing; feeding and excretion; reproduction) 



Every living thing, animal or plant, is a miniature chemical manu- 

 facturing plant. Raw materials are taken in, in the form of food, to- 

 gether with large volumes of oxygen, and the resulting chemical 

 reactions serve three purposes. They produce the proteins and other 

 substances needed to repair wounded or worn tissues of the body : they 

 build new tissues, enabling the animal to grow, and to advance towards 

 maturity (to metamorphose)', and they release energy which produces 

 power for the internal and external movements of an animal. 



An animal, therefore, is dependent on four primary functions: 

 obtaining an adequate supply of oxygen (breathing); taking in a steady 

 supply of food {feeding) ; getting rid of the waste materials that are left 

 over from its chemical processes {excretion)', and providing against the 

 day when its body is finally worn out by producing offspring to take its 

 place {reproduction). 



Breathing 

 Terrestrial or air-breathing animals get oxygen from the air round about 

 them; aquatic animals either take down with them an air-bubble of 

 some kind, or extract their oxygen from that which is dissolved in the 

 surrounding water. Vertebrate animals, including ourselves, extract 

 oxygen from the air by means of a lung, or from water by means of a 

 gill, and absorb it into the blood, usually with the assistance of a special 

 substance called haemoglobin, which takes up and releases oxygen very 

 easily. The blood is then pumped round the body by the action of the 

 heart, and as it passes through the various tissues and organs these are 

 able to take up as much oxygen as they want, and to release carbon 

 dioxide and other waste products. 



Insects also have a body-fluid that is called blood, and which carries 

 away waste products from the tissues, but the insect blood practically 

 never has any special substance for absorbing oxygen. It holds no more 

 oxygen than would any other watery liquid, and as far as is known it 

 plays only a minor and incidental part in the breathing of the insect. 



Insects breathe by carrying the oxygen direcdy to the tissues where 

 it is needed, nearly always through an elaborate system of branching 



34 



