DIMINUTION OF REACTION 141 



to previous stimuli. A single stimulus when repeated with 

 sufficient frequency sooner or later brings about a change 

 of response. The most usual modification is a gradual 

 cessation of the reaction. Light mechanical stimulation of 

 the anterior end of Loxophyllum causes at first a ready 

 response; several repetitions in close succession diminish 

 the responsiveness so that a much stronger stimulus often 

 fails to produce any noticeable effect. In a few seconds, 

 however, recovery is apparently complete and the animal 

 is as responsive as before. In Stentor, as we have seen, 

 after a few light contact stimuli no further contraction takes 

 place. Hydra, according to Wagner, if subjected to a 

 slight mechanical stimulus such as is caused by tapping on 

 the object on which it rests, usually contracts completely. 

 "As the tapping continues this state of contraction is 

 maintained for several seconds, sometimes even from one- 

 half to one minute; but sooner or later, in spite of continuous 

 stimulation, the Hydra slowly expands. When it has reached 

 its normal state of expansion it remains in that position 

 as long as the stimulus is not increased, and even when it 

 is slightly increased. ... If the interval between the 

 stimuli is considerably increased so as to allow the Hydra 

 to expand fully after each contraction, the tap being given 

 the moment expansion ceases, the result is a different one. 

 There is in this case no change in the reaction after repeated 

 stimulation. . . . Recovery from the acclimatizing effect 

 must, therefore, be very rapid." 



Walter found that a planarian, if lightly jarred during 

 its gliding movements, would halt momentarily and then 

 continue its course. If the stimulus was repeated at inter- 

 vals of a second the worm would halt with less and less 

 certainty and finally glide along undisturbed. The effect 

 of the stimulation was very evanescent for after a minute 



