PROBLEM Z What Makes Complex Behavior Possible 



in Many-celled Aiiimals? 



Properties of protoplasm. All animals con- 

 sist of protoplasm, and protoplasm has 

 several properties which make it differ- 

 ent from lifeless matter. One of these 

 properties is irritability , the ability to re- 

 spond to stimuli. It is believed that proto- 

 plasm has the property of irritability be- 

 cause it is so complex physically and 

 chemically. Changes in the environment 

 may easily cause changes within it. The 

 physical and chemical changes that take 

 place in the structure of living matter 

 are the direct cause of the responses 

 which the animal makes. 



When you see streaming in the ameba 

 or lashing of cilia in the paramecium, the 

 protoplasm is exhibiting another of its 

 ipro^&TU&s — C07itractility. The single- 

 celled protozoan can perform reflex acts 

 because it has these two properties, but 

 for you or any other complex animal to 

 perform a reflex act requires more than 

 simple irritability and contractility. In 

 animals that consist of many parts there 

 is a nervous system which is useful in 

 making the many parts of the animal 

 work together. Reflex acts, as well as all 

 learned acts, in complex animals are pos- 

 sible because of the nervous system. Let 

 us study this important system. 



The nervous system. You know what 

 happens when someone accidentally puts 



his outstretched hand into a flame. He 

 pulls his hand out of the flame. But he 

 also feels pain and probably has a sudden 

 fright together with a rapid heart beat, 

 a catching of the breath, and perhaps a 

 tight feeling about the abdomen. How 

 does it happen that so many organs re- 

 spond to the one stimulus? 



Fig. 253 Vor- 

 ticella, a tiny 

 one-celled ani- 

 mal, is attached 

 by a contractile 

 stalk. One Vor- 

 ticella has just 

 been touched 

 by a glass rod. 

 How did it re- 

 spond? What 

 two properties 

 of its proto- 

 plasm 7nalie this 

 possible? 



Contractile 

 fibrils 

 in stalk 



