REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 153 



During the free-swimming period the tadpoles are first for 

 a short time strongly photopositive and geonegative. Then they 

 become photonegative and somewhat later geopositive and remain 

 so until they become attached, after which they metamorphose 

 and divide vegetatively to form zooids and colonies. Thus 

 each tadpole may develop into a new colony. The free-swim- 

 ming period is considerably longer in pellucidum than in con- 

 stellatum. In the former it varies with different individuals 

 from twenty minutes to twenty-four hours, in the latter from 

 ten to one hundred minutes. 



In swimming the tadpoles of Amaroucium continuously ro- 

 tate on the longitudinal axis clockwise as seen from the rear. 

 The forward movement is produced, as in amphibian tadpoles, by 

 lateral strokes of the tail. Rotation is probably produced by a 

 twist in the tail during the lateral strokes, owing to the arrange- 

 ment of the muscle fibers. The movement of the tail, locomo- 

 tion and rotation on the longitudinal axis are normally so rapid 

 that details concerning these processes can be ascertained only 

 in specimens in which the activity has been artificially greatly 

 reduced, as, e.g., by reducing the temperature or by confine- 

 ment to a narrow space. In specimens which swim slowly it 

 can be clearly seen that the tail still vibrates very rapidly, so 

 that there are numerous lateral strokes for every complete rota- 

 tion on the longitudinal axis. I am not certain, however, that 

 this holds for specimens swimming freely through the water. 



In the laboratory, especially in small quantities of water, 

 a few drops, e.g., there are alternate periods of rest and activity. 

 The tadpoles swim rapidly about for a few moments, then come 

 to rest, usually in contact with some object or the surface film, 

 remain a few moments, then without any observable change in 

 the environment they suddenly begin to swim about again, con- 

 tinue for a few seconds, then come to rest, etc. If the light 

 is rapidly decreased, practically all individuals which are at 

 rest become active at once and swim about rapidly; but if it is 

 increased there is no observable response.^ 



^ This was discovered independently by Doctor Grave and the author. 



