268 'Journal of Coynparative Neurology arjd Psychology. 



tion; and that rays coming from below (plane C)more often deter- 

 mined a forward rotation. 



Case J. Larva lymg zuith the side doivmvard ; head toward light 

 — In this case, the larva was oriented with one side uppermost and 

 the head turned toward the source of light. The conditions may 

 be represented by Fig. 14, if it be imagined that for the present 

 case the larvae are lying in a horizontal plane rather than in the 

 vertical as originally intended in this figure. The arrows A, B 

 and C represent rays in the same vertical plane, while (a), {h) 

 and {c) represent them in a horizontal plane. When the light was 

 admitted to a larva so oriented, the reaction was similar to that 

 described und^r Case 2. In the present instance, however, when 

 the rays had the direction {a), the backward rotation was more 

 likely to occur than when the rays had the direction A as in Case 2. 

 Rays in the direction {b) or (c) almost invariably determined a for- 

 ward rotation, in which, if the larva was fatigued, it would merely 

 turn through 180° in the same plane, and become oriented, still 

 lying on the side, but with its head away from the source of light. 

 If, however, the larva was fresh and active at the end of the rotation 

 of 180° in the arc of a circle {A'), it would rotate through 90° on 

 its longitudinal axis and come into the normal swimming position 

 with the back uppermost and the head directed away from the 

 source of light. 



Case ^. Larva oriented with back above; head directed away 

 from the source of light — When the larva was thus oriented and 

 the light was so introduced that the rays streamed in a direction 

 parallel to the longitudinal axis of the larva, no change in the body- 

 orientation took place. The progressive orientation, however, 

 might continue as either positive or negative. In case, however, 

 the light came from the sides a or c (Fig. 15) the larva reacted by 

 swinging (pivoted on the middle or end of its longitudinal axis) 

 to either one side or the other, and it might then undergo positive 

 or negative progressive orientation. If the direction of the rays 

 changed through the series, a, b. c, the larva could likewise be 

 made to swing as regularly as a pendulum and for long periods of 

 time, according as the light came from one side or the other. 

 Indeed the animal was quite at the mercy of the influence of light. 



In case the light came somewhat from above as shown in Fig. 

 16, A, the larva would incline itself farther forward, the num- 

 ber of degrees of rotation depending upon the degree of the angle 



