126 ELEVENTH REPORT. 



SOME LIGHT REACTIONS OF GONIONEMUS. 



Louis Murbach. 



Its peculiar behavior and its availibility have brought this Httle jellyfish 

 Gonionemus into quite general use for experimental purposes at Wood Hole, 

 Mass. (1). In the following notes on behavior toward light it is my purpose 

 to record such observations as I have made after finding that the subumbral 

 papillae are not specially sensitive to light (2); and such other observations 

 as seem desirable to decide some points on which other authors are not • 

 agreed. ~ 



My own experiments have led me to believe that the responses of Gonione- 

 mus to light were not as direct as has been maintainetl. neither that the light, 

 as such, acted directly but that change of intensity, such as would be encount- 

 ered in the natural environment, is a stimulus, that random movements 

 tend to bring the medusa into a favorable environment. More intense light 

 stimuli such as may be used in laboratory experiments may act directly 

 on muscles and bring about an "avoitling reaction," or inhibition which 

 may -bring the animal to rest in an unfavorable environment, such as strong 

 sunlight . 



When the medusae are at rest, shading one with an opacjue object such as 

 the hand, is sufficient to start movement in a few seconds; again by throwing 

 stronger light on an animal it can be made to move about. Indeed the one 

 or the other of these ways was generally used in my experiments to get 

 motor reactions. These observations confirm Yerkes' (3) recent assertion 

 that "change in the intensity of light stimulates the medusa." 



From a carefully planned experiment for the i)urpose of finding the effect 

 of illumination from one direction Morse (4) p. 684, has found one way in 

 which Gonionemus may get from an unfavorable to a more favorable location 

 when disturbed. Each time on its upward course a medusa in the fight end 

 of an aquarium was seen to bend its path a little farther aw^ay from the 

 direction of light, and thus ultimately it got to the farther side of the 

 aquarium. The explanation seems promising, since it is based on the natural 

 up-swimming habit peculiar to these medusae when disturbed. But ]\Iorse 

 also records a case where the animal swam more directly toward the darker 

 end of the aquarium. On repeating the experiment I found that a]:)Out 

 one-fourth of the medusae would swim to the farther end away from the 

 light cfirection, the others swimming about irregularly, coming ultimately 

 to the darker region, and some even coming to rest in the brighter end of 

 the aquarium. 



A test was made using only a horizontal band of sunlight, the width of the 

 aciuarium. Darkening the aquarium momentarily was the means of starting 

 the medusae to swim up through the band of light. Forty-three per cent 

 turned away from the source of strong light, thirty-three jier cent toward 

 the source ami twenty-four per cenr swam straight through the l)and. This 

 was simply to learn whether sunlight really causes the medusa to turn as 

 Yerkes (5) p. 284, and Morse (4) p. 683, have maintained. Now it was thought 



1. The experiments on which the-e notes are based were made at the Marine Biological Laboratory, 

 at intervals between other work in the summer. Acknowledgment is due the Director of the Labora- 

 tory for continuing to place the necessary faciUties at my disposal. 



2. American Jour. Phvsiol.. 1903, Vol. X. 



.3. Jour, of Comp. Neurol Psychol , 1906, Vol. XVI. 



4. American Nat , 1907, Vol. XLL, 



5. American .Jour. Physiol , 1933, Vol. IX. 



