REACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 175 



obtained were all consistent. Without going into details, 

 they may be stated categorically as follows: a) If the lamp 

 is moved from a position in which the inner surface of the cup, 

 the retina, is illuminated to one in which it is shaded (fig. 8), resting 

 specimens immediately become active and the tail bends toward the 

 ocular side. (Only negative specimens were studied.) This occurs 

 even if the light is brought nearer so that the illumination of the 

 field becomes stronger when the retina is shaded, b) If the 

 lamp is moved from a position in which the retina is shaded to 

 one in which it is illuminated there is no reaction in resting speci- 

 mens, but in active specimens the tail turns toward the abocular 

 side even if the lamp is moved away so as to decrease the intensity 

 of the light in the field, provided it is not moved too far. c) If 

 the lamp is moved back and forth so as to alternately illuminate 

 and shade the retina, the tail swings back and forth in harmony 

 with the lamp, even if the lamp is moved as rapidly as it can be 

 with the hand. 



It is surprising how little movement of the lamp is required in 

 certain positions to induce these reactions. They were re- 

 peatedly obtained by movement back and forth over a distance 

 of less than 2 cm. 



Second method. The results obtained by the second method 

 show that reactions similar to those described above are induced 

 if the light remains stationary and the axial position of the tad- 

 pole changes. 



If a number of tadpoles are put into a drop of water on a 

 slide they tend to swim along the edge of the drop without rotat- 

 ing on the longitudinal axis. Thus given individuals frequently 

 swim several times around the drop changing their axial posi- 

 tion through 360° each time without rotating on the axis. Dur- 

 ing this process it is evident that if they are exposed in a hori- 

 zontal beam, different surfaces of the eye become successively 

 exposed to the light, so that in one position the retina is fully 

 exposed, while in others it is shaded by the opaque cup of the 

 eye (fig. 9). Now, it was found that when the retina becomes 

 shaded as the tadpoles proceed along 'the edge of the drop, they 

 suddenly turn sharply and very rapidly toward the ocular side. 



