REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 161 



is increased. In Euglena and other similar forms, as is well 

 known positive specimens respond in a specific way to a 

 decrease, while negative specimens respond in precisely the same 

 way to an increase in light. 



Let us now consider the question as to the cause of the direc- 

 tion of movement of the tail when the tadpoles become active 

 in response to shadows. Why does the tail strike toward the 

 ocular side when the tadpoles are negative and toward the 

 abocular side when they are positive? Is the direction of the 

 stroke of the tail dependent upon the localization of the stimulus? 



As previously stated, the tadpoles of Amaroucium have alter- 

 nate periods of rest and activity which are not associated with 

 any observable changes in the environment. These periods are 

 therefore probably related to changes in internal processes. 

 Numerous observations were made on both species in attempt- 

 ing to ascertain precisely the nature of the response as they be- 

 come active. These observations were unfortunately all made 

 on negative specimens. It was found in these specimens that 

 the first stroke of the tail is invariably toward the ocular side, 

 provided they have been at rest fifteen seconds or longer. If 

 they have been at rest for a shorter period of time, the stroke 

 of the tail may be in either direction depending upon the direc- 

 tion of the last stroke preceding the rest period. If this stroke 

 is toward the ocular side, then the first stroke in the active period 

 will be toward the abocular side and vice versa. Now this is 

 essentially what was found in the response of negative speci- 

 mens to shadows. That is, when the tadpoles become active in 

 response to reduction in illumination they act just as they do 

 when they become active in the absence of any change in the 

 environment. However, when a tadpole becomes active with- 

 out any change in the environment there can be nothing in the 

 nature of what is ordinarily understood by localization of the 

 stimulus. If the direction of the stroke of the tail in these 

 reactions is not related to the localization of the stimulus, it 

 must be determined by the structure and physiological states 

 of the organism. It would consequently appear that, since 

 the action of the tail in response to shadows is like the action 



