REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 185 



SUMMARY 



1. The larvae of Amaroucium are much Uke amphibian tad- 

 poles in shape. They have but one eye, which is located laterally 

 at the posterior end of the body near the base of the tail. The 

 eye contains a lens, a pigment cup and optic nerve-endings on 

 the inner surface of the cup. 



2. The reactions to light in the tadpoles of Amaroucium con- 

 stellatum and Amaroucium pellucidum are essentially the same. 



3. The tadpoles orient fairly precisely. When they emerge 

 from the colonies they are strongly photopositive, but they 

 remain positive only a few moments, after which they become 

 photonegative and remain so until they become attached and 

 begin to metamorphose. 



4. In swimming the tadpoles rotate rapidly and continuously 

 on the longitudinal axis, counter-clockwise as seen from the 

 rear. Locomotion is produced by means of lateral strokes of the 

 tail, rotation on the axis, probably by twisting of the tail during 

 the lateral strokes. 



5. They have alternate periods of rest and activity. If the 

 light is rapidly reduced the resting specimens, both positive and 

 negative, respond by becoming active, and the active specimens 

 respond by changing their direction of locomotion, the positive 

 specimens turning toward the abocular and the negative ones 

 toward the ocular side. 



6. Increase in illumination has no effect on resting specimens 

 regardless as to how rapid or extensive it may be, but active 

 specimens respond by changing the direction of motion. This 

 was ascertained with certainty only in negative specimens. 

 These turn toward the abocular side. 



7. The reaction time is so short that if the hand is moved up 

 and down in front of the microscope as rapidly as possible, 

 alternately increasing and decreasing the luminous intensity, 

 the tail in attached specimens swings from side to side in harmony 

 with the movement of the hand. 



8. If the illumination is gradually changed in either direction 

 there is no response regardless as to how extensive the change 

 may be. The responses to light are dependent upon the time- 



