CHAPTER IX 



SPATIALLY DETERMINED REACTIONS AND SPACE PERCEPTION 



(continued) 



74. 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 be- 

 havior. 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 prompt- 

 ness. 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 (291). 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 tentacles 

 will be 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 reaction goes no 

 further, however. But the response is differentiated 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 (451). Special vigor and speed 



190 



