REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 167 



respectively, 3.03 hm., 1.04 hm., and 6.74 hm. These readings 

 were unfortunately all made on different individuals, so that the 

 results obtained are not so conclusive as they would be if they had 

 all been made on the same individual. They strongly support, 

 however, the conclusion reached after numerous casual observa- 

 tions, i.e., that the position of the tail is held considerably longer 

 after increase than it is after decrease in illumination. 



Under either condition, however, the bent position of the tail 

 is held longer when the illumination which induced it is con- 

 tinued than it is when this is not continued. That is, if the 

 reduction in light is immediately followed by an increase, then 

 the tail, after bending toward the ocular side, immediately bends 

 toward the abocular side and vice versa. Thus by moving the 

 hand in front of the microscope so as to change the luminous inten- 

 sity, the tail can be made to swing back and forth in harmony 

 with the hand as rapidly as it can be raised and lowered. A 

 number of readings were taken in reference to this rate, and it 

 was found to average 20 complete oscillations per 8 hm. 250 per 

 minute, or 0.4 hm. per oscillation. This would indicate that the 

 tail under these conditions holds the bent position in either 

 direction less than 0.2 hm. 



These results bring out in a striking way the difference be- 

 tween the activating response and the bending response. The 

 former occurs only in resting specimens. It is induced only by 

 reduction in illumintion and it is independent of the illumination 

 which follows the reduction. The latter occurs in active speci- 

 mens. It can be induced by increase as well as by decrease in 

 illumination, and it depends in character upon the illumination 

 which follows that by which it is induced. 



Since the extent of the lateral movements of the tail toward 

 the ocular side when the light is decreased and toward the 

 abocular side when it is increased is greater than it is in the 

 oscillations under constant illumination, and since the lateral 

 positions of the tail are held much longer when the illumination 

 which induced them is maintained than when it is changed, it 

 seems evident that both decrease and increase in illumination must 

 be considered as a stimulus; the former causing contraction in 



