282 'Journal of Comparative Neurology and Psychology. 



went a forward rotation through an arc of 180°, and assumed 

 the normal swimming position, B' , with the back uppermost and 

 the head facing the screen. This orientation was maintained 

 with a greater or less degree of constancy so long as the conditions 

 of light remained the same. If, on the other hand, the screen 

 was so placed, or the larva had such a position, that the plane of 

 the screen was not exactly parallel to the vertical plane passing 

 through the two eyes of the larva, a different reaction was expe- 

 rienced. In this instance the first response was a revolution 

 on the longitudinal axis, usually through 180°. This resulted in 

 bringing the back of the larva uppermost, and was usually fol- 

 lowed by a swinging of the longitudinal axis, which brought the 

 head to face the screen. The direction of this side swing (to the 

 left or the right) was determined by the angle which the longitu- 

 dinal axis of the larva made with the screen. For instance in 

 Fig. 15, the larva designated y/' would swing to the right, while 

 the larva designated C' would swing to the left, each in the direc- 

 tion indicated by the arrows. In other words we may say that the 

 larva would swing in that direction which brought the head, by the 

 shortest course, to face the screen. But the two reactions men- 

 tioned above might, as in previous cases, be blended to form a 

 composite reaction, which differed from either of its simple com- 

 ponents. 



Case 25 — In the present instance the larva was oriented lying on 

 its back and the screen was introduced from before. Under these 

 conditions, as in Case 19, there was no modification in the body- 

 position. In certain instances the larva underwent a revolution 

 through 180° on its longitudinal axis and assumed a position with 

 back above and head still directed toward the black screen; 

 but in the great number of cases the orientation remained un- 

 changed. 



Case 26 — In case the larva was oriented with the back below 

 and the screen was made to approach from the side the reactions 

 were as follows. The larva experienced a rolling or revolution on 

 its longitudinal axis, in consequence of which the back moved 

 away from the screen through an angle of 90°, occasionally 

 more. At the same time there was a swinging of the longitudinal 

 axis, itself, so that the larva came face to face with the screen, 

 eventually with the back uppermost. During this reaction the 

 larva often departed from the screen. As in Case 21, mentioned 



