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THE AMERICAN NATURALIST 



[Vol. XLI 



was tried. Care was taken to have the impinging ray as nearly 

 as possible parallel to the oral-aboral axis of the animal. When 

 thus illuminated, the medusa changed its course, moving away 

 from the axis of light so that the path formed an acute angle with 

 the ray. 



Attention was then directed to the movements of Gonionemus 

 when swimming freely in an aquarium illuminated from one 

 direction. Figure 2 explains the arrangement. The sunlight was 

 reflected through the aquarium from side to side {xy). A jelly-fish 

 was freed at the point a, and it at once sank to the bottom. 

 Within a few seconds it began to swim and finally reached the top 

 of the water. The path, however, was not vertical, but was in- 

 clined away from the light as shown by the path Ah. On reaching 

 the surface, the ordinary reaction took place whereby it inverted 

 and sank in the vertical line hB. The process was repeated so that 

 the resultant of the whole was the direction Ah. In this way it 

 will be seen that the medusa ultimately reaches the farthermost 

 point, as a result of the light acting exactly as in the simpler experi- 

 ments in unilateral stimulation. In one case, that of a strong 

 swimmer, the path followed was not broken by frequent inversions, 

 inasmuch as the animal did not reach the surface until it had 

 passed to the opposite side of the aquarium, a distance of about 

 70 em. 



That it is the direction of the ray of light that is the important 

 factor in orientation, is made evident by the following experiment 

 (Fig. 3). It will be seen that the light was thrown upon the 

 aquarium at the angle indicated by the arrow, so that the end 

 abc, lying nearer the source of light, is dark, the opposite end being 

 illuminated. When a medusa starts at h in the light, it rises to the 

 top and performs the actions just described, so that it reaches 

 ultimately the end d. By this means we find an accumulation 

 of jelly-fish in the end farthest from the light. Here they will 

 remain until they die, or, as is often the case, they begin to swim 

 regardless of the direction of the light and ultimately reach the 

 shaded area, in which they setde down as described in my previous 

 paper. 



Yerkes ^ has described a very interesting response in Gonionemus 

 1 Yerkes, R. M. Concerning the Behavior of Gomonemus. Journ. Comp. 

 Neur., 1906, vol. 16, p. 457-463. 



