OBSERVATIONS OF THYONE BRIAREUS. 285 



stantly passing between it and the window. After this animal 

 had been allowed to remain in a quiet situation in another room 

 for a week however it had again become extremely sensitive to 

 shadows, jars, currents of water and other gentle forms of stim- 

 ulation. 



Thyone sometimes modified its behavior after a stimulus had 

 been repeated several times and a new form of response occurred. 

 On one instance an individual which had been used in previous 

 experiments was stimulated by gently sticking a glass rod among 

 the tentacles as it was feeding. At first all the tentacles were 

 withdrawn as soon as the rod touched one of them, but after the 

 fourth trial they were no longer retracted, and when the rod was 

 pressed gently against the mouth the anterior end was turned 

 to one side but not withdrawn. This change in response was 

 brought about in half an hour. 



2. LiJiibition. — As has been stated, the shadow reaction was 

 one of TJiyonc s most constant and characteristic types of response 

 but it would not take place if certain stimuli were present. To 

 give some specific instances : This reaction was inhibited when the 

 temperature of the water fell below 10° C, when the posterior 

 end was greatly elongated toward the surface on account of stag- 

 nant water, and after the respiratory movements had been pre- 

 vented from occurring for some time. In all these cases indi- 

 viduals gave characteristic shadow responses before and after the 

 inhibiting stimulus was present. Another characteristic reaction 

 was locomotion away from the light, but, when an animal was 

 against the side of a glass vessel it often moved at right angles 

 to the direction of the light rays, the thigmotactic stimulus being 

 more potent than the light. Furthermore, if an individual was 

 laid on its dorsal surface with the median plane inclined slightly 

 toward the source of the illumination, it often moved one or two 

 centimeters toward the light in righting itself. These instances 

 are typical of others which might be given and they show that 

 though Thy one's responses are largely of a stereotyped nature, they 

 are interrelated in such a way that one may inhibit another. 



