MOVEMENT TOWARD LIGHT IN COELENTERATES 165 



enough to warrant definite conclusions regarding the me- 

 chanics involved in this process. 



These creatures are only about i mm. in diameter. They 

 have four prominent reddish brown spots symmetrically 

 situated on the margin of the bell. From the tissue sur- 

 rounding each of these spots there project three short 

 tentacles which are much contracted when the medusae 

 swim. The medusae are negative in their reactions to 

 gravity, and positive to light of intensities ranging from 

 weak diffused sunlight to intense direct sunlight. This 

 causes them to collect at the surface of the water in the sea 

 and to swim toward regions of highest light intensity. 



In swimming toward the source of light they frequently 

 turn to the right or left rather sharply so as to produce a 

 zigzag course. The turning from side to side indicates that 

 light does not act as a constant directive stimulation. If 

 however the ray direction is changed through 90° the 

 medusae turn directly toward the source of light without 

 any preliminary trial movements. It may be then that 

 they are stimulated only when they turn a certain amount 

 and that, owing to the power of differential response to 

 localized stimulation, they always turn toward the light 

 after such stimulation and consequently remain oriented, 

 in a general way. If orientation is due to differential 

 response to localized stimulation the stimulation may be 

 caused by an increase of intensity on the illuminated side 

 or a decrease on the shaded side. We have however no 

 evidence bearing on this question. 



If exposed to light from two sources they swim toward a 

 point between them. The location of this point depends 

 upon the relative intensity of light from the two sources. 



