280 'Journal of Comparative Neurology and Psychology. 



at once further modified by a revolution of 160° on the long 

 body-axis, either to the left or right (determined by the nature of 

 the lateral or secondary illumination), and the larva again assumed 

 the normal swimming position, but with the head directed toward 

 the black screen. In case the plane of the screen was not exactly 

 parallel with the vertical plane passing through the eyes of the 

 larva, the reaction was not represented by the simple forward rota- 

 tion, but was modified by side movements. 



Case 21 — ^When the larva was in the normal swimming position 

 and the black screen approached from the side, several reactions 

 might occur. Most commonly the larva underwent a swing of 

 its longitudmal axis so that the head was brought to face the screen. 

 Another reaction sometimes observed was a rolling, or revolution, 

 on the long body-axis, in such a manner that the back moved 

 away from the screen. At the same time there occurred a swing 

 of the longitudinal axis which caused the head to be directed toward 

 the screen. These; two reactions might occur simultaneously, 

 and the resulting reaction be a blending of the tw^o components 

 mentioned above. The rolling on the longitudinal body-axis was 

 seldom over 90° from normal (back above), usually less. Yet in 

 cases w^here the illumination in the dark box was greater, or when 

 the screen was introduced suddenly, the rolling motion might 

 exceed 90°, and the larva fall to the bottom of the container. 



Case 22 — In this instance the larva oriented in the normal swim- 

 ming position and the screen was made to approach from above. 

 This combination produced several forms of reaction. In cases 

 where the general illumination in the container was not great, the 

 larva merely experienced a slight change in the direction of the 

 longitudinal body-axis; the head assumed a superior position, 

 so that the long axis of the body was nearly horizontal, or even 

 directed upward at a small angle, rather than bent downward 

 at an angle of 30° from horizontal, as in the normal swimming 

 position. On the other hand, if the illumination was greater, 

 the larva might undergo a rotation on its own longitudinal axis 

 through 180° and fall, back downward, to the bottom. What- 

 ever reaction occurred, it could be explained as an effort of 

 the larva to turn the head toward the black screen, and the 

 degree to which this was attained depended very much upon the 

 intensity of illumination throughout the container. The type of 

 reaction mentioned above was demonstrated to better advantage 



