LIGHT REACTIONS OF THE MEDUSA GONIONEMUS. 363 



to start movement in a few seconds ; again, by throwing stronger 

 light with a small mirror on any medusa it can be made to move 

 about. Indeed, the one or the other of these ways was gen- 

 erally used in my experiments to get motor reactions. Next 

 some experiments were made to determine the height to which 

 Gonionemits would swim after a single stimulation by change of 

 light intensity. Eight medusae were placed in a hydrometer jar 

 72 cm. deep and 9 cm. in diameter. When they had come to 

 rest in the bottom of the jar they were stimulated by dark- 

 ening the room until the medusae could just be seen. They swam 

 to the top, at intervals, somewhat irregularly. The shades were 

 now quickly raised and thus the top of the jar illuminated with 

 strong diffused daylight. Three medusae swam downward so 

 directly as not to touch the s-ides of the aquarium --not varying 

 9 cm. from their course. Five floated down part way and then 

 two turned and swam the last third of the way down. In another 

 trial, out of three at the top one swam down 31 cm. without 

 touching the sides of the glass jar. This, it would seem to me, 

 is in the nature of proof that change of light intensity starts and 

 gravity directs the downward course. 



SOME MOOTED POINTS. 



In regard to the question whether the decrease or the absence 

 of light (darkness) is a stimulus for motion or inhibition there 

 is difference of opinion. Yerkes in his first paper 24 on Gonioncimis 

 says " Romanes's statement that change from light to darkness is 

 inhibitory of action is not very apt." He adds, it " is merely the 

 absence of any motion producing stimulus." In a later paper 

 on this medusa, however, he 25 says "decrease in light intensity 

 temporarily . . . inhibits activity." Morse 21 from practically 

 the same experiment that Yerkes used concludes that " we have 

 no inhibition of movement in passing from light to darkness. 

 In the dark the stimulating effect of light is absent and hence the 

 movements ultimately cease." Yerkes Tl does not agree with this 



u Amer. Jour. P/iysiol., 1902, Vol. VI., p. 445. 



25 Amer. Jour. Physiol , 1903, Vol. IX., p. 282. 



^Jour. of Comp. Neural. Psycho!., 1906, Vol. XVI., p. 453. 



21 Jour, of Comp. Neural. Psycho!., 1906, Vol. XVI., p. 460. 



