BEHAVIOR OF CCELENTERATA 215 



Thus the reaction of the animal depends on its physiological state; 

 when excited it moves toward the light, otherwise it is indifferent or 

 gathers in the shade. In the flatworms we find a parallel condition of 

 affairs, but with the relations reversed. It is not unlikely that the 

 tendency of the medusa to go toward the light when disturbed is related 

 to its usual method of life, and has a functional value. The animal 

 when at rest is commonly attached to the vegetation of the bottom. 

 When disturbed by a large animal foraging among the plants, it would 

 move toward the light, hence out into the free water and upward, thus 

 escaping the enemy. 



Thus far we have considered the behavior under light of constant 

 intensity. Let us now see the effects of sudden changes in intensity 

 of illumination. Here we find again that the effect of a given change 

 depends on the state of the animal. If the medusa is at rest on the 

 bottom, a sudden marked increase in the intensity of the light usually 

 causes a sudden contraction of the bell. As a result the animal, of 

 course, swims away from its first position. Sometimes, however, an 

 increase of light merely causes an animal that is at rest with the sensitive 

 concave surface up to turn over, so as to bring the sensitive surface 

 against the bottom, where it is little affected by the light. In a case 

 described by Yerkes, increase of light caused regularly this turn with 

 bell up, while decrease caused a return to the "bell down" position. 



A decrease of light usually has no effect on a resting Gonionemus. 

 But sometimes it causes contraction, so that the medusa swims away. 

 In such specimens an increase of light usually causes no reaction. Some- 

 times, however, a given specimen reacts both to increase and decrease 

 of illumination. 



Thus the reaction of a resting medusa to a change of illumination is 

 variable, depending on the individual. Doubtless in a given individ- 

 ual it varies with the physiological state and past history of the animal. 



In the swimming Gonionemus, usually both an increase and a de- 

 crease of light cause the animal to expand, cease swimming, and sink to 

 the bottom. Here it usually remains for a time, then resumes activity. 



If a vessel containing a number of the medusae is divided by a line 

 x-x into two regions, one brightly illuminated, the other shaded, the 

 animals usually behave as follows: A specimen swimming about in 

 the light region crosses in its course the line x-x, passing into the shade. 

 It at once ceases swimming and sinks to the bottom. Here it remains 

 for a short time, then continues to swim about in the shaded region. 



If a specimen swimming in the shaded region crosses the line x-x 

 into the light, it likewise sinks to the bottom and remains quiet for a 

 time. Now, upon resuming activity, it swims in such a way as to pass 



