Hadley, Behavior of the American Lobster. 283 



above, these two reactions might occur at the same time, and then 

 the resulting reaction was a composite. 



Case zy — In the present case the larva was oriented with back 

 below and the black screen was introduced from above. Under 

 these conditions it usually underwent a slight forward rotation 

 with a consequent rise from the bottom, and came into a new swim- 

 ming position with the longitudinal axis directed somewhat upward 

 as shown in Fig. 20, 5^ 



Case 28 — In this instance the larva was oriented with back below 

 and the black screen was introduced from beneath. The reac- 

 tions were usually as follows. The larvae underwent a revolution 

 of about 180° on its longitudinal axis, and assumed practically 

 the normal swimming position, with the back uppermost and the 

 head bent downward at an angle of about 30°. In other cases, 

 however, this new position was brought about by a different sort 

 of reaction; namely, a backward rotation through an arc of 180°. 

 This resulted in throwing the larva again into the normal swim- 

 ming position. 



Generally speaking, we may say that, when black or white 

 screens were made to approach larvae of any one of the first three 

 stages, diversely oriented, the larvae manifested two forms of re- 

 sponse. First, a motor reflex, which tended to place the longitu- 

 dinal axis in a certain relation to the plane of the screen; secondly, 

 and subsequent to the first response, a progressive orientation, 

 toward or away from the screen, as the luminosity of the screen, 

 the physiological state of the larvae, and other conditions of the 

 case, determined. When the white screen was used, the larvae 

 commonly became oriented with the head directed away from the 

 screen. In the case of black screens, on the contrary, the head 

 was directed toward the screen and the back more or less away. 

 These reactions occurred whether the screens were made to 

 approach from above, below, behind, or the side. After body- 

 orientation had taken place, the larvae might approach or recede 

 from the black or the white screen, according as they were reacting 

 positively or negatively. 



The mechanics of reaction upon which orientation to the screens 

 w^as found to depend, agree, for the greater part, with the types of 

 reaction to black screens reported by Bohn (1905), who has made 

 a careful study of the effects of causing a black screen to approach 

 the larvae of Homarus vulgaris, diversely oriented. There are, 



