REPORT OF COMMISSIONERS OF INLAND FISHERIES. 209 



question of screening which we have discussed in the preceding 

 section is probably but a single phase of this problem of backgrounds. 

 By the term background, as it is used in the present case, is meant 

 the permanent color-tone of the surrounding walls (as a whole or part) 

 which confine the young larvae. This condition was somewhat 

 different from that determined by the use of screens that were mov- 

 able and could be placed at any angle with reference to the body 

 axes of the larvae. In the present treatment, backgrounds were 

 employed in several different ways. They were sometimes repre- 

 sented by the black or white lining of the reaction boxes ; again by 

 the ground upon which the glass dishes or tubes rested, or in still 

 other cases by the outer covering of these dishes or tubes. The ques- 

 tion of the effect of background may be considered under two heads : 

 (1) the effect of backgrounds in connection with the purely photo- 

 pathic responses; (2) their effect in determining the "choice" of a 

 region of particular light intensity when phototaxis is also operative. 



From the result of experiments on the effect of backgrounds in 

 connection with purely photopathic responses, it appeared that the 

 nature of the background had no important influence upon the sign 

 of the photopathic response of the nearly stage larvae of the lobster, 

 although the intensity of the light did appear to have been of value 

 in determining the nature of the reaction. On the other hand, we 

 note from other experiments that larvae which were positive on a 

 white background were negative (under the same conditions of light) 

 when upon a black background. It is true that, when these experi- 

 ments were performed, the value of knowing the exact age and history 

 of the larvae under observation was not fully appreciated by the 

 writer. But these experiments were nevertheless suggestive of a 

 certain effect of background in modifying the sign of the photopathic 

 reaction. 



A study of the effect of background in connection with both the 

 phototactic and photopathic response received greater attention. 

 Under this head we may consider those conditions of experiment 

 which, although they are no doubt chiefly productive of phototactic 



