Sensory Discrimination : Vision 125 



consciousness. Yerkes found that this jellyfish, unlike Sarsia, 

 reacts in the same manner in passing either from sunlight to 

 shadow or the reverse. In both cases it stops swimming and 

 sinks to the bottom. A sudden change of illumination, there- 

 fore, checks its activity. On the other hand, if when the 

 light falls upon it the animal is at rest, it becomes active 

 again ; but sudden decrease of illumination has no effect upon 

 the resting animal. The inhibitory effect of strong light fall- 

 ing upon the jellyfish while in motion Yerkes explains as a 

 special adaptation. For one case of such increase of illumi- 

 nation occurs when the animal swims, bell upward, to the 

 surface on being disturbed; the light of the surface is of 

 course normally stronger than that in the lower regions. The 

 inhibition of activity resulting causes the animal, after turning 

 over, to sink slowly, bell downward, with expanded tentacles. 

 This is a position that gives it a better chance of catching food 

 and carrying it to the lips than is offered by the right-side-up 

 posture, where food would have to be carried downward 

 against the upward current occasioned by the sinking of the 

 animal. Light is not the only factor in producing the inver- 

 sion at the surface, however, for it will occur in darkness. 

 When swimming, Gonionemus moves toward the light if the 

 latter is fairly intense, but comes to rest in the shaded portions 

 of the vessel containing it. The reaction time to light is 

 much slower than that to other stimuli, but the animal re- 

 sponds most promptly when certain pigmented bodies at the 

 base of the tentacles are exposed to the stimulus. If the mar- 

 gin of the bell containing these bodies is cut off, no reaction 

 to light can be obtained (451, 458, 470). A great variety of 

 structures apparently sensory in function is found on the bell 

 margin of different genera and species of Medusae. Some 

 of them are statocysts. Others suggest a visual function, 

 and in the Cubomedusae there are fairly well developed eyes. 



