LIGHT-ADAPTATION IN THE PLANARIAN 233 



start to glide along the tank-wall. In these cases it was imme- 

 diately removed by a section-lifter to the center of the tank. 



GENERAL BEHAVIOR 



The behavior of the animal under strong illumination from 

 one side may be described as a continuous, steady glide, occa- 

 sionally interrupted by "wigwag" movements; but in general, 

 either by gradual or abrupt turns, it keeps its negative orien- 

 tation to the light. At times, however, the animal suddenly 

 moves toward the light, often rapidly making two or three com- 

 plete turns in this direction before settling back to its previous 

 course. The behavior of the flat -worm under normal illumina- 

 tion, following strong, directive illumination, is similar to that 

 under intense light, except that the general direction of curva- 

 ture is reversed and that quick, abrupt turning in the direction 

 opposite to the principal one is less frequent. 



DATA 



In measuring the records of the paths traced by the plan- 

 arians the following method was adopted. As the courses were 

 exceedingly tortuous, they were considered as including no 

 straight lines; that is to say, as always inclined either to the 

 right or to the left. All points of inflexion were marked upon 

 the course. In marking these points no separate account was 

 taken of very small twists, indicating a change of direction of 

 less than 45 °, unless the length of path was over 2 cm. Any 

 error involved in this omission is hardly greater than that in- 

 volved in tracing free-hand the course of a subject often in 

 rapid motion. Measurements were made of the length of path 

 and also of the total number of degrees turned through between 

 successive points of inflexion. 



The symbol "D" is used in this paper to designate throughout 

 any one series that side of the planarian which is away from the 

 light during directive illumination; the symbol "L" is used to 

 designate the side toward the light during directive illumination. 



Table I gives the results of single trials upon eight individ- 

 uals. The horizontal lines of figures for each individual are 

 (1) the initial performance under normal, diffuse, vertical illum- 

 ination, (2) the performance under intense illumination from 

 the right or left side (as indicated in the column for position of 



