362 L. MURBACH. 



havior. 20 While the majority of the medusae in the experiment 

 finally reached the less illuminated end of the aquarium they did 

 so by regularly swimming about, resting longer each time they 

 had progressed farther from the light and a shorter time between 

 swimming intervals that again took them toward the light. A 

 few came to rest in the lighter end of the aquarium, as almost 

 always happens in light experiments. 



Now if the medusae do not swim directly toward weaker light 

 and are not turned definitely in such direction, even after trials, 

 at any time, their collecting in weaker light might still be 

 accounted for by the above explanation, if this could be elevated 

 to the dignity of a method. That is, each time a medusa gets 

 into an optimum light it remains longer, and when in an unfavor- 

 able light field remains a shorter time, and thus more and more 

 of them will get together in these optimum places. 21 



From the foregoing it will be seen that there are several ways 

 in which Gonionemus gets away from the strong light into an 

 intensity best suited to its activities, without the intervention of 

 tropism or "trial." If its only mode of locomotion, or even the 

 chief one, to stimuli were the up-swimming " surface reaction " 

 then it would plainly be " trial," or " motor reaction." 



CHANGE OF INTENSITY AS A STIMULUS. 



In Yerkes's earlier statements 2:! about the relations of Gonione- 

 imis to light the words increase and decrease of light intensity 

 are used, but only in his later answer to Morse's criticisms 23 does 

 he make the statement that he has "abundant evidence that 

 change in intensity of light stimulates the medusa." I had exper- 

 imentally come to the same conclusion. 



When the medusae are at rest darkening the aquarium or 

 shading one with an opaque object, such as the hand, is sufficient 



20 Once I observed a medusa that seemed to follow pretty regularly a movable 

 slit admitting stronger light into a darkened aquarium. Although it was pronounced 

 I could not confirm the reaction in other specimens. 



21 After writing the above 1 find Mast ("Light Reactions in Lower Organisms, 

 II., Volvox, " Jour. Conip. Neural. Psycho!., Vol. XVII. , p. 169) has similarly ex- 

 plained the aggregation of Volvox in optimum light. 



22 Amer. Jottr. PhysioL, 1902-3, Vols. VI. and IX. 

 ^Jonr. of Comp. Neurol. Psycho!., 1906, Vol. XVI., p. 460. 



