Hadley, Behavior of the Americarr Lobster. 279 



degrees of the same reaction) resulted: (i) The larva would 

 swing the head upward as shown in Fig. 20, B', and maintain this 

 swimming position so long as the Hght condition remained un- 

 changed, or (2) it might, on the other hand, experience this same 

 reaction in an exaggerated form, i. e., there might result a back- 

 ward rotation through 180°, which reaction would cause the 

 larva to fall to the bottom and to assume a position with the 

 back below and with the head directed upward at a slight angle 

 as shown in Fig. 20, C', Usually, however, this form of orienta- 

 tion resulted only when the light was of greater intensity, such as 

 that secured in cases of direct illumination. 



Case 18 — When the larva was oriented in the normal swimming 

 position and the white screen was made to approach from above, 

 the reaction was similar to that described for Case 6, p. 270. 

 The one difference was that while the direct lighting often caused 

 a number of complete rotations through 360° before the final 

 body-orientation was assumed, the white screen, on the other 

 hand, usually acted by changing the swimming position directly 

 to that of Fig. 17, B'. This difference in response was probably 

 due to the difference in the intensity of Hght (direct or reflected) 

 coming from above. 



The black screen — The method of conducting the experiments 

 with the black screen was almost the same as that for the white 

 screen. There was one point of difference. It was found that, in 

 order that the black screen should determine any reaction of the 

 larvae, it was necessary to have a slightly greater illumination with- 

 in the dark box. The following report of cases shows the result 

 of making the screen to approach, from various directions, the 

 larvae diversely oriented. 



Case ig — When the larva was in the normal swimming position 

 and the back screen was presented opposite the head, and at right 

 angles to the longitudinal axis, the orientation was not changed, 

 but was retained constantly so long as the screen remained in 

 position. 



Case 20 — When the larva was in the normal swimming position 

 and the screen was made to approach from behind, so that its 

 plane was parallel to a vertical plane passing through both eyes of 

 the larva, there usually resulted a forward rotation of 180° in 

 the arc of a circle. This reaction brought the back of the larva 

 below, and the head toward the black screen. This position was 



