EEACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 179 



involved in the process of orientation and in the retention of the 

 oriented axial position? 



The movements of the tail of the tadpoles are normally so 

 rapid that it is impossible to see directly how orientation is 

 accomplished. These movements can readily be reduced in rate 

 by means of lowering the temperature or by adding narcotics. 

 But unfortunately it was found that when, under these conditions, 

 they become slow enough so that they can be seen, orientation 

 ceases. It was consequently not possible to ascertain directly 

 what occurs during this process. Orientation can, however, be 

 fairly satisfactorily explained by the application of the facts 

 concerning photic responses presented in the foregoing pages. 



We shall consider orientation in negative specimens first. 

 In these it was found, 1) that a sudden decrease in the illumina- 

 tion of the nerve-endings on the inner surface of the pigmented 

 cup of the eye induces the tail to bend toward the ocular side, 

 resulting, in free swimming individuals, in a sharp turn of the 

 anterior end toward this side; 2) that an increase in illumination 

 induces, in active specimens, bending of the tail and sharp turning 

 of the anterior end in the opposite direction; 3) that the former 

 reaction may be induced by a change in the direction of the 

 rays of light or by a change in the axial position of the body from 

 one in which the inner surface to one in which the outer surface 

 of the cup is directly exposed to the light; 4) that the latter 

 reaction may be induced by a change precisely opposite in 

 character, and 5) that these reactions alternate in harmony 

 with alternate decrease and increase in illumination of the optic 

 nerve-endings. 



If a tadpole in a horizontal beam of light is so directed that 

 the Hght strikes it at an angle of, e.g., 90° with the longitudinal 

 axis, the inner and outer surfaces of the pigment cup of the 

 eye are alternately turned toward the light, owing to the rotation 



results in turning from the light. In position (5) the retina becomes fully illu- 

 mmated. This causes bending of the tail toward the abocular side, which again, 

 owing to the rapid rotation, results in turning from the light. Thus the tadpole 

 continues to turn from the light each time the retina becomes shaded or illumi- 

 nated until it is oriented and rotation on the longitudinal axis no longer produces 

 sufficient changes in the illumination of the retina to induce reactions. 



