The Reactions of Planarians to Light 127 



is, however, by no means settled in the foregoing observations. 

 This matter should be finally tested by comparisons in the behav- 

 ior of different individuals of the same species showing variation 

 in pigmentation or of identical individuals at different times when 

 their phases of pigmentation are unlike, rather than upon indi- 

 viduals of different species. 



It has proven impossible to include such a consideration in the 

 present paper, but the preliminary steps toward attempting an 

 analysis of the function of pigment with reference to light reactions 

 have already been made and it is expected that a discussion of 

 this phase of planarian behavior will be presented later. It may 

 be stated here that when Planaria maculata is fed with a drop 

 of human blood, a decided increase in pigmentation makes its 

 appearance within a few days, due probably to the oxidation of 

 the haemoglobin in the blood corpuscles with which the planarians 

 have become gorged. This single observation suggests an experi- 

 mental means for controlling the amount of pigment in a single 

 individual and it may reasonably be supposed that tests of behav- 

 ior before and after excessive pigmentation will contribute direct 

 evidence upon the part played by pigment in reactions to light. 



Summary. Mutilated planarians in general respond to light 

 with less accuracy than normal individuals. 



Small worms move more slowly than large ones in the case of 

 those species whose photoreceptive apparatus is not solely confined 

 to the anterior end of the body. In the case of Dendrocoelum 

 lacteum, whose photoreceptive apparatus is relatively greater in 

 small individuals than in large, the rate of locomotion is faster 

 among the smaller than among the larger. 



Small worms orient with less accuracy than large ones. Pla- 

 narians in the relaxed, resting position are less responsive to light 

 than when they are stretched out in the act of gliding, a result 

 probably of the greater exposure of the photoreceptors to light in 

 the latter instance. 



The anterior end of the body is the chief photoreceptive region 

 and in certain worms, such as Dendrocoelum lacteum and Bdel- 

 loura Candida, the anterior end is apparently the exclusive seat of 

 this function. 



