MOVEMENT TOWARD LIGHT IN COELENTERATES 163 



by the trial method, as has been assumed by some investi- 

 gators. 



While the planulae of Eudendrium may undoubtedly 

 orient by differential response to localized stimulation it 

 is at present impossible to say whether such stimulation is 

 due to changes of light intensity or to constant intensity. 

 The question as to the importance of lateral movements of 

 the anterior end in orientation, and the cause of stimulation 

 will be referred to more in detail under the reactions of fly 

 larvae and earthworms. 



3. Eudendrium Hydranths 



After the planulae are a few days old the anterior end 

 becomes attached to the substratum and they soon develop 

 into hydranths, which bend toward the source of light as 

 they grow. In order to study this process of bending 

 toward the light, I placed on the stage of the compound 

 microscope, a small aquarium containing hydranths which 

 had bent so that the distal end was nearly horizontal, and 

 turned it so that one side of the organisms faced the light, 

 and then projected a selected specimen with a camera 

 lucida. This same individual was projected later from 

 time to time, and in this way its movements were definitely 

 recorded. It was found that the hydranths turn directly 

 toward the source of light. There was no indication of cir- 

 cumnutation movements. The bending takes place only 

 in the region of growth. All sides elongate but the shaded 

 side elongates more than the illuminated side. The organ- 

 isms bend toward the source of light very slowly. In all 

 the individuals studied it required 48 hours or more to turn 

 through 90°. 



In the orientation of this organism It seems probable that 

 light acts as a constant directive stimulation. But the 

 knowledge we have concerning the process hardly warrants 

 even a suggestion as to the probable mechanism involved. 

 The bending may possibly be due to contraction as sug- 



