Morse, Behavior of Gonionemiis. 453 



be said at once that the same results were reached regardless as to 

 whether the light was sent parallel to the surface of the water, at 

 right angles to it, or obliquely. In each case, when the medusa 

 moved from the shaded region to the light portion, it immediately 

 ceased pulsating and sank to the bottom of the jar, or to some 

 intermediate depth. This is entirely in agreement with Romanes' 

 ('93) statement for Sarsia and Yerkes' ('03) statement for 

 Gonionemus. This inhibitory action of light is as striking as the 

 stimulating effect of that factor as both Romanes and Yerkes 

 have shown. If, for instance, light is suddenly thrown upon a 

 medusa resting in the dark it will respond by pulsations of the bell. 

 The character of the pulsations vary from a few ineffectual con- 

 tractions to a swimming bout more or less prolonged. As a rule, 

 strong sunlight simply causes a series of ineffectual pulsations. 



With respect to the passage of the medusa from a region of 

 illumination to one of darkness, the case is different, judging from 

 the experiments of the present writer. The jar just mentioned 

 was used as follows : One-half of the top of the jar was covered so 

 that the sun, reflected by means of a large mirror perpendicularly 

 downward, illuminated one-half of the vessel, the boundary 

 between the light and the dark zones being vertical. In this case, 

 a medusa swimming from the light area to the dark would, on 

 reaching the transition line, exhibit no reaction whatever and very 

 frequently it would swim for a dozen pulsations within the dark 

 area. However, the pulsations became less vigorous and less 

 frequent until they ceased altogether. The same result was 

 obtained when the border from light to darkness was horizontal, 

 and also when it was oblique. Yerkes' experiment of the white 

 porcelain dish was repeated and the results just mentioned were 

 again obtained. . 



It seems quite evident, then, that we have no inhibition of 

 movements in passing from light to darkness. In the dark the 

 stimulating effect of light is absent and hence the movements ulti- 

 mately cease. Obviously the case is materially different from the 

 movement from darkness into sunlight. 



Yerkes ('03) has observed that if a number of medusae are 

 placed in a vessel, one-half of which is darkened and the other 

 half is exposed to the direct sunlight, the medusae in the sunlight 

 begin to swim toward the shadow, enter it and come to rest, as we 

 have just described. The response is definite and direct, according 



