CHAPTER VI 

 ANIMAL PHYSIOLOGY 



Motions and locomotion. — Our attention is usually first 

 attracted to an animal by the movements it makes. These 

 are the plainest proof that it is alive. For the animal itself 

 the ability to move is essential to existence. Most animals 

 move in search of food, to escape from their enemies, to find 

 and build their homes, to seek their mates, and care for 

 their young. In the higher forms the organs of motion 

 constitute the great bulk of the body. The shape and 

 size of such an animal are .determined largely by these 

 organs. 



The heart and blood-vessels, the lungs and digestive sys- 

 tem, are principally concerned with supplying the organs of 



Fig. 15. Scorpion walking, showing the successive positions of body 

 and legs. (After instantaneous photographs by Marey.) 



motion with materials necessary for their working, and by 

 far the larger part of the work of the sense organs and ner- 

 vous system is to put these organs into action, and to direct 

 and control them. We can see therefore that they have 

 much to do with both the structure and physiology of ani- 

 mals. Indeed the most marked usual difference between 

 animals and plants is the possession by the former of the 

 organs of motion and their nerves. True, plants have the 



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