i66 



Why Living 



Weok 



Salt 

 Water 



Fig. 252 How do parainecia re- 

 spond to salt water and weak 

 vinegar? 



Protozoa respond to many other stim- 

 uli. When a paramecium, in the course 

 of its swimming, meets some soHd object 

 other than food, it reverses its direction. 

 It is responding to a mechanical stimulus. 



Things Behave As They Do unit v 



A change in the intensity of light or in 

 its color also acts as a stimulus to a 

 Paramecium; so does a change of tem- 

 perature, or a change in the chemical 

 composition of the water. Food is a 

 chemical stimulus. And paramecium re- 

 acts to electrical stimuli as well. 



Why protozoa seem to show intelli- 

 gence. If you have ever watched the re- 

 sponses of protozoa you have perhaps 

 wondered at their intelligence. The or- 

 ganism is almost always performing acts 

 which are useful to it. But if it has no 

 intelligence why does it make such cor- 

 rect responses? It is because organisms 

 making responses unfavorable to them- 

 selves quickly die and leave no offspring. 

 They survive only if their protoplasm 

 makes useful responses. It is for this 

 reason that all protozoa capable of con- 

 tinued existence will always seem to 

 show intelligence. Now review this prob- 

 lem by doing Exercise 7. 



Questions 



1. What does the biologist mean by behavior? 



2. Name four places in the human body where sense organs are found. 

 What is a receptor? 



3. What is an effector? Name some stimuli and the responses made to 

 them by the human body. 



4. Define a reflex. Give two examples of human reflexes that you have 

 seen occur. Describe two internal reflexes. 



5. In which group of organisms do reflexes make up a larger proportion 

 of the behavior, vertebrates or invertebrates? Defend your answer. 



6. How does an instinctive act differ from a reflex act? Describe an in- 

 stinctive act performed bv some animal. How do you know that 

 instinctive acts are unlearned? 



7. Describe an experiment which illustrates that an instinctive act is not 

 an intelligent act. 



8. How are the words "instinctively" and "reflex" often misused? Ex- 

 plain why the words are incorrect when used in that wav. 



