LIGHT-ADAPTATION IN THE PLANARIAN 231 



9.0, 5.5, and 3.5 cm., respectively, and with three diameters, 

 drawn 60 degrees apart. (See figure.) An enlarged duplicate 

 of this ruling was pasted upon a sheet of glass, 23 by ^^ cm., 

 which formed the top of the tracing box. The tracing- box was 

 an ordinary wooden box, containing an electric light and so 

 arranged that when a blank piece of paper was placed upon its 

 glass cover the diagram upon its top could be seen through the 

 paper. Since the diagram reproduced that beneath the tank, it 

 was possible to trace quite accurately on a paper upon the box 

 the movements of the planarian in the tank. The top of the box 

 was about 15 cm. above the bottom of the tank, and the light 

 coming through the paper and glass was small, so that the illum- 

 ination in the box had practically no effect upon the illumina- 

 tion in the tank. When the tracing box alone was lighted, the 

 observer was but just able to make out the contour of the tank. 



METHOD 



The procedure, in general, was as follows. A rested, but 

 hungry, individual was removed from the dark and placed in 

 about 1 cm. of water in the tank. Then, with only the normal 

 illumination furnished by the light above the apparatus, its 

 course for five minutes was traced as accurately as possible on 

 the tracing box. The subject was then left in the dark for two 

 minutes, after which one of the lower lights in the circle, oppo- 

 site one side and slightly in front of the animal, was thrown 

 in at the keyboard. As the subject was brought into activity 

 by the light and turned away from it, another light was thrown 

 on in place of the first one, so that the same side of the animal 

 as before was lighted. Thus the planarian was kept constantly 

 turning and turning, while the source of illumination, controlled 

 from the keyboard, kept following it about. As the head is 

 more sensitive to light than the rest of the body, it was found 

 advisable to anticipate the turn, in order to keep the planarian 

 away from the sides of the tank, by throwing on each light about 

 20 degrees before the long axis of the animal became perpen- 

 dicular to the radius passing through the light. The light was 

 then left on until the animal had passed 40 degrees beyond 

 the radius. This procedure was possible only in slow, consistent 

 movements; for when the subject turned rapidly it was possible 

 only to follow it hurriedly with the light, often skipping a single 



