CHAPTER XV 

 THE SLUG: A STUDY IN ANIMAL BEHAVIOR 



Every movement that an animal makes is a response to 

 some stimulus applied to its contractile substance. This 

 stimulus may be applied directly to the contractile substance; 

 as, for example, when a muscle is pinched. Thus if we pinch the 

 muscle of a frog's leg after the leg has been removed from the 

 body, the muscle that has been irritated will contract. The 

 stimulus may, however, be applied indirectly, as when we 

 dodge a ball moving near us. In this case the sight of the ball 

 stimulates the eye, a message is sent to the brain and from 

 the l^rain to the muscles which produce the dodging. In either 

 case it is clear the movement results from some stimulus. 

 With the exception of a possible origination of a stimulus in 

 the iDrain itself all movements are reactions to some external 

 stimulus. Now the stimuli which affect us are those changes 

 in the external world for the appreciation of which our body 

 is provided with certain parts called sense-organs. We have 

 a sense-organ for receiving the stimulus of light, the eye. We 

 have sense-organs for api^reciating the contact quality of bodies 

 including vibration, the organs of touch and of hearing. We 

 have organs for testing the chemical qualities of substances, the 

 organs of taste and smell. We have organs for detecting 

 changes of temperature. These lie scattered in the skin over 

 the whole surface of the body. We have organs for appreciat- 

 ing pressure, both its direction and its amount, and especially 

 we can appreciate gravity when we move up or down. By 



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