The Reactions of Planarians to Light 155 



narians has been shown by Borelli ('93) and Wilhelmi ('04) to be 

 chiefly dependent upon temperature and almost not at all upon the 

 amount of illumination to which they are subjected. Voigt ('04) 

 noticed that worms when hungry may be seen wandering about 

 even in patches of bright sunlight with apparent disregard of light. 

 This seems to be a case of the light reactions becoming over- 

 balanced by other responses. 



Summary. Light is not an essential factor in planarian 

 activities, since the behavior necessary to the welfare of the 

 individual and the race is mainly referable to other factors. 



A planarian's response to light is of a passive character, which 

 may have an adaptive significance only in so far as its phototaxis 

 tends to conceal the worm from its enemies. The presence of 

 pigment may also be regarded as an adaptive condition induced 

 by the animal's relation to light. 



The evolution of the photoreceptive apparatus of the planarian 

 has not reached the degree of differentiation necessary to enable it 

 to secure for itself such adaptations to the factor of light in its 

 environment as would make aggressive activity possible to it in a 

 manner characteristic of higher animals. 



VII Bibliography 



Arloing, S., '87a — [Letter on Mechanism of Destruction of Microbes by Light.] 



Ann. Inst. Pasteur., vol. i, pp. 594-596. 

 '87b — Les spores du Bacillus anthracis sont reellement tuees par la 



lumiere solaire. Comp. Rend. Acad. Sci., Paris, torn. 104, pp. 



701-703. 

 Bardeen, C. R., 'oia — On the Physiology of the Planaria maculata with especial 



Reference to the Phenomena of Regeneration. Amer. Jour. 



Physiol., vol. v, no. I, pp. 1-55. 

 '01 b — The Function of the Brain in Planaria maculata. Amer. Jour. 



Physiol., vol. v, no. 3, pp. 175-179. 

 Barthelemy, A., '84 — Sur la physiologie d'une Planaire verte (Convoluta 



Schultzii). Comp. Rend. Acad. Sci., Paris, torn. 99, pp. 197-200. 

 Beer, T., '01 — Ueber primitive Sehorgane. Wiener klin. Wochenschr., 1901, 



Nr. n-13. 



