Yerkes, Behavior of Gonionemus. 461 



represent the reaction of the medusa at the transition line between 

 light and shadow the dotted line should incline slightly toward the 

 shaded end of the dish, for the black card which covered the right 

 end of the dish permitted the sunlight to illuminate a larger area at 

 the surface of the water than at the bottom. As a consequence of 

 this condition a Gonionemus when it reacted to the sunlight, as it 

 passed into it, by ceasing to swim, turning over and sinking pas- 

 sively, usually came to rest on the bottom of the dish, unless it 

 resumed its swimming before reaching the bottom, partly in the 

 sunlight and partly in the shaded region. In this position it was 

 unequally stimulated on opposite sides, and as a result the bell 

 contracted in such a manner as to turn the animal toward the 

 shaded region. 



In my repetition of this experiment the importance of this 

 marked inequality of illumination has been demonstrated for I 

 found that when the animals swam so far into the sunlit region 

 before turning over that they were entirely in the sunlight when 

 they came to rest on the bottom of the dish, they moved away from 

 the region of shadow about as often as toward it. When they 

 came to rest with a part of the body in the shadow they turned 

 back toward the shaded region in about 70 per cent of the cases. 

 With one group of animals there were forty-four movements 

 toward the shade to twenty away. 



The effect of suddenly increasing the illumination of a portion of 

 the bell by allowing sunlight to fall upon it was noted. The result 

 apparently depends upon the intensity of light to which the organ- 

 ism happens to be adapted at the moment of the experiment. If it 

 has been in weak light, the brilliant illumination of one side of the 

 bell in more than two cases out of three, according to my recent 

 observations, brings about movement toward the region of lower 

 illumination. Lack of material compelled me to leave this work 

 uncompleted, but in reply to Mr. Morse I may say that my experi- 

 ments indicate that Gonionemus turns from the region of strong 

 stimulation in as definite a manner as the structure of the organism 

 and method of locomotion permit in the cases of all stimuli which 

 are injurious. Light is not an exception to this rule, nor have I 

 discovered any, although I do not doubt that some such exist. 



While repeating my experiments I noticed for the first time 

 another form of local reaction to light. A medusa partly in sun- 

 light and partly in shadow was drawn from the light into the 



