Yerkes, Behavior of Hydroides. 447 



after a few such experiences, the shadow, cast before, as it were, by 

 the coming event, more frequently produces a reaction. The 

 repetition of both stimuH many times in succession causes this 

 effect to wear away temporarily, since the touch is not sufficiently 

 injurious to force reaction. 



Two other interesting facts have appeared in the course of these 

 experiments. One is shown clearly in the record of one day's 

 experiments given in Table II. In this both stimuli were given; 

 the responses to shadow are shown in the first column, those to 

 touch in the second and the length of time during which the 

 animal remained retracted into its tube in the third. 



The period of retraction is short the first three times — 19 to 34'' — 

 but the fourth time it is nearly four minutes. For the next 

 thirteen times it janges from eighteen to ninety-three seconds; 

 then comes another period of nearly four minutes followed by 

 nineteen contractions which last from twelve to eighty-five seconds 

 each and then a contraction of nearly twelve minutes' duration. 

 Thus after the fourth, eighteenth, thirty-eighth and sixtieth trials 

 the animal remained contracted for a relatively long period, vary- 

 ing from four to twelve minutes, whereas the intervening contrac- 

 tions seldom lasted more than one and a half minutes and are 

 usually less than thirty seconds. This rhythm is very marked in 

 all the series which were carried on long enough each day to show 

 it, and in the short series there is usually one period of retraction 

 noticeably longer than any of the others. 



The occurrence of unusually long periods of retraction in the 

 case of another individual. No. 41, for the first five and the tenth 

 days of the experimentation is shown in Table III. It is worthy 

 of note that the frequency of the prolonged retraction periods is 

 much greater on the tenth day than on the first. 



These periodic "rests" might be attributed to fatigue, but they 

 do not seem necessarily due to that. Being a tube-dwelling 

 animal, Hydroides is narrowly limited in the kind of reactions 

 possible to it. It responds to mechanical stimulation repeated at 

 frequent intervals in the most apparent way, by a contraction into 

 the tube. If the behavior of the animal were modifiable we might 

 expect after a number of repetitions of this stimulus a change in 

 reaction comparable to that of Stentor^ when, after repeated faint 



'Jennings, H. S. Loc. c/V., pp. 173-175. 



