KEACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 171 



to 180°, but sometimes 360° or more. Then the tail became 

 straight and the swinging stopped, but the vibration usually 

 continued. If, now, the hand was raised so as to increase the 

 illumination, the tail immediately turned toward the abocular 

 side and the posterior end of the body began to swing toward the 

 ocular side. In this direction it continued to swing usually con- 

 siderably farther than it did in the opposite direction after shad- 

 ing. If the hand was removed before the body stopped swinging 

 toward the ocular side in response to a shadow, the direction of 

 swinging was immediately reversed. If it was not removed until 

 after the vibration of the tail as well as the lateral swinging had 

 stopped, there was no response. These experimental observations 

 were repeated many times on this individual and a considerable 

 number of times on another with the same results. 



It is well known that if one of a pair, of antagonistic muscles 

 relaxes after having been contracted for some time, the opposing 

 muscle becomes shorter than it is in the resting condition. May 

 not then the bending of the tail toward the abocular side on 

 increase in illumination be due to the previous bending in the 

 opposite direction and not to the action of light as a stimulus? In 

 the reactions of the first two individuals described above, I was 

 never quite certain that this possibility was excluded, but in those 

 of the third individual it has been excluded, for in this individual 

 it was clearly seen that the bending reaction of the tail toward 

 the abocular side in response to increase in light could be in- 

 duced when the tail was straight and the antagonistic muscle re- 

 laxed, quite as readily as it could when the tail was bent toward 

 the ocular side and this muscle contracted. The results ob- 

 tained in this individual, therefore, seem to prove conclusively 

 that, while an increase in the illumination of the field inhibits the 

 effect of a decrease, it acts also as a stimulus. 



The effect of increase in illumination depends upon the time- 

 rate of change of illumination just as does the effect of decrease 

 in illumination. If the light is gradually increased, there is no 

 response, no matter how extensive the increase may be. 



The magnitude of the response to increase in illumination is, 

 like the response to decrease in illumination, within wide ranges 



