208 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



with the types of reaction to black screens which have been reported 

 by Bohn for the European lobster. 



We have now examined rather briefly the effects of sudden illu- 

 mination and of sudden shadow, the effects of white screens and 

 of black; and if we now compare the detailed results of these studies, 

 w-e note that the effects produced by introducing a white screen are 

 very comparable to those obtained by suddenly admitting illumina- 

 tion, while the effects brought about by black screens are comparable 

 to those determined by suddenly cutting off the light. In other 

 Avords, it becomes clear that the larvse respond to the influence of 

 screens of black and white by reactions which are dependent upon 

 the same simple types of behavior that have been previously described. 



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

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

 probable that the screens are instrumental in determining the 

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

 of (reflected) illumination, or, in the case of the black background, a 

 light-absorbing agent. Therefore, when the black background causes 

 a swing of the larva, as a result of which he comes to face the screen, 

 we cannot say that the primary factor is the blackness of the screen; 

 but rather, that the total light absorption and consecjuent non- 

 reflection of light rays from a particular direction allow rays of light 

 from other directions to be particularly effective; and we know from 

 our previous experiments that the final orientation of the larva is 

 determined by the relative intensities of light striking the eyes from 

 different directions. Therefore, the larva "heads" to the black 

 screen because his eyes encounter no light rays coming from this 

 direction; and he "heads" away from the white screen because his 

 eyes do encounter a stronger reflected light from this than from any 

 other direction. 



The Effect of Backgrounds. — The question of the influence of back- 

 grounds in determining the orientation of some crustacean larvae 

 has been ably brought forward by Keeble and Gamble.* The whole 



*The Color Physiology of Higher Crustacea. Philosophical Transactions, Royal Society of 

 London, Series B, Vol. CXCVI, p. 295-388. 



