Hadley, Behavior of the Amei-ican Lobster. 285 



has been brought forward by Keeble and Gamble (1904). Aside 

 from the effects of screening, the more general problem of back- 

 grounds did not receive especial attention in the course of the pres- 

 ent investigation, but, as we shall see, the question of screening 

 w^hich we have discussed in the preceding section is probably only 

 a single phase of the problem of backgrounds. The following 

 experiments which were performed more or less at random in 

 connection with other experiments, but which deal with the ques- 

 tion of backgrounds, may, however, be presented. 



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 in part) which confined the young larva?. This condi- 

 tion was somewhat different from that determined by the use of 

 screens which were movable and could be placed at any angle 

 w^ith reference to the body-axis of the larvae. Backgrounds were 

 employed in several different ways. They w^ere 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, and in 

 still other cases by the outer covering of these dishes, or tubes. 

 The subject may be considered under two heads: (i) the effect 

 of backgrounds in connection with the purely photopathic response; 

 (2) their effect in determining the "choice" of a particular region 

 of light-intensity when phototaxis also is operative. In view of 

 the fact that the investigation of the first phase of this problem 

 was not undertaken in the present work, we may pass directly to 

 the consideration of the second point stated above. 



The effects of backgrounds in connection with both the phototactic 

 and the photopathic response — Under this head we may consider 

 those conditions of experiment, which, although they be chiefly 

 productive of reactions to the directive influence of the light, never- 

 theless were modified by response to the intensity of the light. 

 These conditions were secured by the use of Y-tubes. The fol- 

 lowing experiments serve to show why, in the case of the larval 

 lobsters, the tendency to gather in the brighter areas (assumed 

 positive photopathy) is often associated with positive phototaxis; 

 and why a tendency to gather in the darker areas (assumed nega- 

 tive photopathy) may be associated with negative phototaxis. 

 In the diagrams of Fig. 21 are represented the Y-tubes as set up 

 for experiment. Those whose arms are above were arranged for 

 experiment with larvae having positive phototactic reaction; those 



