214 BEHAVIOR OF THE LOWER ORGANISMS 



(2) Reactions to Light in Gonionemus 



The relation of the behavior of the medusa Gonionemus to light, as 

 studied by Yerkes (1902 a, 1903), is exceedingly complex; it can by no 

 means be expressed by any simple formula. In examining the matter 

 it will be well to consider first the relation of the light to the amount 

 of activity shown by the animal; then the nature of the activities in 

 constant lights of various intensities; then the effects of changes of 

 illumination. 



In ordinary daylight, Gonionemus continues its usual activities, 

 swimming about by rhythmical contractions, and pursuing its usual 

 occupation of "fishing" (p. 192). It is not clear that the direction of 

 its movements has any relation to the direction of the rays of light, so 

 long as all conditions remain uniform. If the light comes from below 

 instead of above, Yerkes (1903) found that Gonionemus continues to 

 swim to the top and float to the bottom, as before. 



If the light is cut off, the medusa usually comes to rest after one to 

 five minutes. By covering the vessel containing them, it is thus possible 

 to bring the animals to rest for experimental purposes. In continued 

 darkness the animal is much less active than in the light. 



In strong sunlight the animal becomes very active. At first it swims 

 toward the source of light, thus rising under natural conditions to the 

 surface of the water. Later its reaction changes ; it stops coming to the 

 surface, begins to avoid the light, and swims toward the bottom. It 

 may now persistently strike against the bottom in its efforts to swim 

 away from the source of light. Sometimes in a strong light it places 

 the more sensitive subumbrellar surface against the bottom and comes 

 to rest. At times its activities become, under the action of direct sun- 

 light, uncoordinated; it moves upward in its contraction, downward 

 in its expansion. 



In a moderate light coming from one side the behavior of Gonio- 

 nemus is at times very peculiar. When the conditions are quite uni- 

 form, as we have seen, its movements often show no relation to the direc- 

 tion of such a light. But when the light first begins to act, as when a 

 jar containing medusae is placed near a window, they at first swim 

 toward the source of light. The medusae thus gather at the lighted side 

 of the vessel. But after a time, if undisturbed, they cease to react to 

 light, and may scatter throughout the vessel. If there are regions of 

 light and shade, the animals now usually gather in the shaded region. 

 But if they are again disturbed in some way, as by stirring up the water, 

 they swim toward the light again, later scattering as before, when 

 the conditions become uniform. 



