CHAPTER IX 



SPATIALLY DETERMINED REACTIONS AND SPACE PERCEP- 

 TION (continued) 



63. Class III : Reactions to a Moving Stimulus 



SPECIALIZED response to a stimulus in motion, that is, 

 one which successively affects several neighboring points on 

 a sensitive surface, is also frequently met with in animal 

 behavior. Its usefulness is obvious : a stimulus in motion 

 is very commonly a living creature, hence either an enemy 

 or food. In any case it must be reacted to with extreme 

 promptness. Reactions of this class may be distinguished 

 as tactile or visual according as the moving stimulus is 

 mechanical or photic. 



We find good examples of specialized reactions to motile 

 touch in the ccelenterates. The sea-anemone Aiptasia gives 

 its most violent reaction, involving all the tentacles at once, 

 when touched by a moving object (521). The medusa 

 Gonionemus makes, in the case of a moving mechanical 

 stimulus, its single exception to the rule of responding by 

 the feeding reaction to edible substances only. The tenta- 

 cles are wound corkscrew fashion about a glass rod 

 drawn across them, they bend in toward the mouth, and 

 the bell margin bearing them contracts ; the feeding reac- 

 tion goes no further, however. But the response is dif- 

 ferentiated from that to any other form of stimulation by 

 its greater speed : the reaction time is from .3 to .35 of a 

 second, compared with .4 to .5 of a second for other stimuli 



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