REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 165 



In tadpoles that are at rest the tail is usually bent considerably 

 toward the abocular side. The extent of this bending varies 

 with age. In very young tadpoles it is hardly noticeable, but 

 in older ones it is marked (fig. 1). Now the question arises as 

 to whether the tail, in bending toward the abocular side after 

 the illumination has been increased, returns merely to the rest- 

 ing position or whether it goes beyond this position. The 

 results obtained in numerous observations clearly show that it 

 does go beyond this position, and they show, moreover, that, 

 after the bending effect of decrease in illumination has ceased 

 entirely and the tail is vibrating in the resting position, increase 

 in illumination will still cause it to bend toward the abocular side. 

 These facts seem to prove that increase in illumination does not 

 merely inhibit the effect of a decrease in illumination, but acts 

 also as a stimulus. 



The observations referred to above were made on specimens 

 of both species studied, some attached, others free. There was 

 no observable difference in the reactions of the tw^o species. 

 Since these observations have a bearing on a number of prob- 

 lems aside from that stated above, I shall present in some detail 

 the results obtained in three individuals, all in vaseline enclosures 

 under cover-glasses; one with the anterior end firmly attached 

 to the vaseline, the others free but limited in their movements. 



In the attached specimen studies, Amaroucium pellucidum, 

 there were alternate periods of rest and rapid vibration in the 

 tail. When at rest the tail was very nearly straight, the curva- 

 ture in it being unusually small (fig. 5, a). When the light w^as 

 reduced by bringing the hand down in front of the stage, the tail 

 immediately turned toward the ocular side and vibrated rapidly 

 (fig. 5, b). This position was, however, held only a short time, 

 2 to 6 hm. Then, vibrating continuously, it returned to the 

 resting position and sometimes slightly beyond : remained here 

 2 to 6 hm., then turned toward the ocular side again, etc., until 

 it came to rest. If' the hand was removed so as to rapidly in- 

 crease the illumination while the tail was at rest there was no 

 response, it remained at rest; but if it was removed before the 

 tail came to rest, i.e., while it was still vibrating, no matter in 



