CHAPTER LXVI 



BEHAVIOR OF ANIMAL ORGANISMS 



Behavior, as already defined, is the sum total of an animal's move- 

 ments. Reference has been made to various modes of behavior in con- 

 nection with different phyla, but it is desirable to review the whole 

 subject in one chapter. Before beginning the discussion, however, it 

 may be said that no other subject in zoology invites so much speculation 

 as does this and in no other is such a variety of opinions held. 



518. Memory. — Memory is due to the persistence of some modifica- 

 tion in a nerve cell resulting from its activity. Just what is the nature of 

 such a modification is not known. In the lowest animal organisms 

 the effect of a stimulus is exceedingly transitory, passing away in a short 

 time. In a noncentralized nervous system, also, it does not persist for 

 any great length of time, although it remains very much longer than in 

 any one-celled organism. One of the characters of a centralized nervous 

 system is a longer memory, and the development of the brain permits 

 memory to last even throughout the lifetime of a long-lived animal. How- 

 ever, even in animals which possess a brain, the effects of stimulation 

 may not remain long. What makes the difference is not exactly known, 

 but one thing is clear and that is that the effect of a very marked stimulus 

 remains for a greater length of time than that of one which is inconsider- 

 able. It is also true that frequent repetition of the stimulus increases the 

 duration of the effect. Association of ideas, too, tends to prolong mem- 

 ory. From what has been said it is evident that memory is the outgrowth 

 of a property which, like irritability, conductivity, and contractility, 

 belongs to all living matter, though it is only shghtly developed in undif- 

 ferentiated protoplasm. It plays a steadily increasing role in the dif- 

 ferent modes of behavior from the lowest to the highest. 



519. Types of Animal Behavior. — The various types of animal behav- 

 ior can be clearly interpreted only when studied in the light of the struc- 

 ture involved. When so studied six general modes of action may be 

 recognized: direct response, simple reflex action, instinct, habit, intelli- 

 gent behavior, and reasoning. These will be taken up in turn. 



520. Direct Response. — This term has generally been applied to 

 the type of behavior which involves action by the same cell that receives 

 the stimulus. It is the mode of action of protozoans and sponges and is 

 seen in the action of the cnidoblasts of the coelenterates, which are some- 

 times called independent effectors. All movement in the coelenterates, 



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