Yerkes, Behavior of Hydroides. 449 



from base to summit and the reverse so forcibly as to turn the 

 whole crown of filaments over to one side or the other in a horizon- 

 tal position. The contraction after such treatment this particular 

 time lasted only ten seconds. This change in behavior could 

 certainly not be due to fatigue since the resting condition is surely 

 that of withdrawal into the tube and in this case the animal not 

 only endured the stimulus repeatedly but after a contraction which 

 lasted a remarkably short time was ready for it again. This 

 occurred many times with several specimens when series of twenty 

 or more trials were given in succession. In many other cases there 

 was only a partial retraction into the tube and the crown of branchiae 

 would remain half out of the tube until the next expansion took 

 place. 



SUMMARY 



1. To a photic stimulus consisting of a decrease in the intensity 

 of light, repeated at short and irregular intervals, Hydroides re- 

 sponds at first by contraction but later gives no reaction. 



2. When the stimulus occurs at longer intervals the responses 

 are more frequent. 



3. To tactual stimulation the animal responds almost uniformly 

 by a contraction into the tube. 



4. When the photic stimulus is followed immediately by the 

 tactual the worms gradually respond more frequently to the former 

 alone than they did previous to this training; i.e., they learn to 

 react to the shadow. 



5. The time of retraction after the tactual stimulus is usually 

 short but in any long series a retraction of much longer dura- 

 tion occurs periodically as a possible variation in the method of 

 response to repeated stimulation. 



6. After many repetitions of shadow and tactual stimuli the 

 animals no longer react normally to either but require frequently a 

 large number of tactual stimulations to induce contraction. 



7. The behavior of Hydroides is thus eminently modifiable 

 since it varies with repetitions of a stimulus or when two stimuli of 

 different reactive value are repeatedly given in succession. 



Woods Hole, Mass., 

 September, 1906. 



