REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 159 



definite way to a decrease in light, while they give no response to 

 an increase, and that photonegative specimens respond in pre- 

 cisely the same way to an increase, but give no response to a 

 decrease in light. In the tadpoles of Amaroucium there is, 

 both in the positive and in the negative states, a definite res- 

 ponse to a reduction and no such response to an increase in 

 illumination. In other words, resting specimens become active 

 when light is reduced, no matter whether they are positive or 

 negative, but not when it is increased. There is, however, 

 a difference in the character of the response in these two states. 

 If the specimens are photonegative the first stroke or the first 

 series of strokes of the tail is always toward the ocular side; 

 if they are photopositive it is always toward the abocular side. 

 The former was fully established in both species, the latter in 

 pellucidum only. In constellatum the photopositive period is 

 so short that the precise nature of the response could not be 

 ascertained with certainty. 



The difference in the activating response of the tadpoles in 

 the positive and the negative states was particularly striking 

 in one individual studied on August 27th in a vaseline enclosure 

 under a cover-glass. This individual, a specimen of Amarou- 

 cium pellucidum, was mounted immediately after it had left 

 the colony. It was strongly positive on the slide under the 

 microscope and swam directly toward the window in front of 

 which the observations were made. But when it reached the 

 limit of the enclosure the tail became entangled in vaseline to 

 such an extent that it was held fast. The body, however, could 

 freely move back and forth laterally. After the tadpole came to 

 rest the light was reduced by passing the hand in front of it. 

 It immediately became active and it could be very distinctly 

 seen that the first rnovement of the body was toward the abocular 

 side (fig. 3). This was repeated many times with the same 

 results. The light was also increased in various ways, but in no 

 case was any reaction obtained, provided the specimen was 

 at rest when the increase occurred. Ten minutes later the 

 specimen was again shaded in precisely the same way in 

 which it had been earlier, and now it responded by turning 



