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. Whenit hasreached 
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 
