MICHIGAN ACADEMY OF SCIENCE. 



1-27 



that an oblique band of light would be more decisive (c. f. Fig. 3) as one side 

 of the up-swimming medusa would be stimidated in advance of the other. 

 In this case fifty per cent of the animals turned away from the band of sun- 

 light, forty-one per cent toward it and nine per cent did not respond. 

 I believe that the experiments confirm the conclusion that the medusae can 

 be turned by strong light. A more striking result was obtained with relaxed 

 medusae floating down and striking an oblique band of sunlight, turning 

 to swim up several times. 



In these experiments it was noticed that the up-swimming medusae would 

 not always come to the top and some turned back after they had passed the 

 band of sunlight. I do not believe that this is an after stimulation of the 

 light-band, I think it is the counter stimulus of change to lower intensity 

 of light. This view is supported, by the results I got on excluding all possible 

 sources of light; these were reflections from debris in the water, and from a 

 light substratum on which the aciuarium stood. The water was filtered and 

 the aquarium made so dark that the medusae could scarcely be seen outside 

 the band of light. Now many of the medusae swam back before reaching 

 the top in their upward course. 



Many observations indicate that medusae that have been exposed to light 

 of low intensity will collect temporarily, at least, in strong light. This, 

 I believe, is the explanation of the apparent discrepancy between Morse's 

 [(3) p. 452] and Yerkes' [(3) p. 459] observations on the collecting of medusae 

 in sunlight, Yerkes maintaining that when medusae are first placed in a 

 glazed, white, earthenware dish half of Avhich is shaded they first collect 

 in the sun. His original statement [(6) p. 446] does not specify sunUght. 



If then the movements of Gonionemus are not regularly directed by light 

 but it gets into faA'orable environment liy random movements being turned 

 each time it gets into strong light, it would seem a priori, that ray-directing 

 is out of the question. However Morse [(4) p. 684] has decided from an ex- 

 periment with oblique illumination over the shaded end of an aciuarium, 

 because the medusae collect in the ray-direction end of the aquarium rather 

 than in the shaded end, that "the direction of the ray of light is the important 



3. Jour, of Comp. Neurol. Psychol., 1900, Vol. XVI. 



4. American Xat.. 1907, Vol. XLI. 



0. American Jour. l'liy.siol., 1U02, Vol. VI. 



