284 'Journal of Co?npa7'ative Neurology and Psychology. 



however, certain disagreements. First, it is certainly true that 

 bringing the black screen parallel to the longitudinal axis of the 

 larva frequently determined a rolling of the larva on its own lon- 

 gitudinal axis, whatever the original orientation may have been. 

 But in Case 21, certain orientations of the larva were noted in 

 which these rollings did not occur. It is true, moreover, that the 

 progressive orientation often took place in that direction in which 

 the back was directed. But several instances were observed 

 wherein the orientation to the black screen resulted merely from 

 a swinging of the longitudinal axis of the larvae so that the head 

 was directed toward the screen and where consequent progressive 

 orientation was either a movement backward or forward, head 

 foremost or tail foremost, as in positive or negative phototaxis. 



We have now examined somewhat in detail the effects of sudden 

 illumination and of sudden shading, the effects of white screens 

 and of black. If we now compare the detailed results of these 

 studies, we note that the effects produced by introducing a white 

 screen are comparable with those obtained by suddenly admitting 

 illumination, while the results brought about by black screens are 

 comparable to those determined by suddenly cutting off the light. 

 In other words, the larvae appear to respond to the influence of 

 screens of black and white by reactions which are dependent 

 upon the same simple forms of response observed under the con- 

 ditions of direct lighting and shading. 



In view of this correspondence in the nature of reaction to direct 

 lighting and to screens of black and white, it may be considered 

 probable that the screens and backgrounds are instrumental in 

 determining the behavior of the larvae, only in so far as they are 

 themselves the source of (reflected) illumination. Thus, when the 

 black background causes a swing of the larva, as a result of which 

 it comes to face the screen, we cannot say that the primary factor 

 is the blackness of the screen; but rather that the small amount 

 of light reflected from the screen permits rays of light from other 

 directions to become effective. The larva "heads" to the black 

 screen because his eyes encounter no light rays coming from this 

 direction; and he turns away from the white screen because his 

 eyes encounter stronger reflected light from this than from any 

 other direction. 



The effect of backgrounds — The question of the influence of 

 backgrounds in determining the orientation of crustacean larvae 



