368 L. MURBACH. 



the aquarium, leaving a place of high intensity where the band 

 of sunlight fell, but not so high as that where the two overlapped 

 (F' l - 3)- Other portions of the aquarium seemed to be of too 

 low intensity, as the medusae remained in the lighted portions. 

 The effect of relative intensities was then seen by lowering the 

 mirror so that its beam was projected horizontally across the band 

 of sunlight. Again there were more medusae in the corner farthest 

 from the two sources of light (compare Fig. 2). In the final test, 

 sunlight was admitted through a slit near the bottom of the 

 aquarium and the mirror placed back of the aquarium in such a 

 position as to throw reflected light obliquely against the sun- 

 light, as it were (Fig. 4). Now the medusae collected some dis- 

 tance from the back of the aquarium in the region of lesser in- 

 tensity. Finally, that it is relative intensity and not ray direction 

 is also shown by a former experiment (page 359) where medusae 

 turned away from an oblique band of sunlight in a darkened 

 aquarium. They turn nearly as often toward as from the source 

 of light. Ray-directing, seems to me, out of the question. 



SUMMARY. 



1. The medusae do not usually direct their movements to favor- 

 able locations but continue swimming at random until they come 

 into an optimum environment, where they settle down. 



2. Intense light turns medusae away, thus avoiding injury. 



3. Change of light intensity is the stimulus for reactions to 

 light. In pronounced decrease, the change of intensity causes 

 inhibition. 



4. Relative intensity in the field, not ray direction, determines 

 the place of rest. 



5. Light is necessary for the up-swimming activity, though not 

 directive this being due to gravity. 



6. Contact of the bell with air and the accompanying recoil 

 probably causes the inhibition that precedes inversion of the bell 

 at the surface. 



MARINE BIOLOGICAL LABORATORY, 

 WOODS HOLE, MASS., 

 August, 1909. 



