LIGHT AND MOVEMENT 



55 



turning towards one side and the other in the opposite direction ; 

 although the responses are typically tropotactic in nature, the telotactic 

 response is simulated since each eye exerts a symmetrical control. 



Among worms, these reactions have been most closely studied in Planaria 

 maculata, one of the turbellarian worms. ^ The normal individual orientates 

 photo -negatively, illumination of one side producing a muscular contraction of 

 the opposite side so that the worm proceeds directly away from the light. If, 

 however, one eye is dissected out and the light is accurately directed or if different 

 parts of the remaining eye are removed, it can be shown that stimulation of the 

 elements of the anterior end of the eye makes the animal turn from the illuminated 

 side, while stimulation of the posterior or ventral parts of the eye induces a 

 turning towards the illuminated side. The boundary between these two con- 

 stitutes the " line of fixation " (a functional fovea) stimulation of which evokes 

 no turning movements (Liidtke, 1942). A somewhat similar reaction is seen in 

 the drone-fly, Eristalis, and related insects (Mast, 1923). 



Adams. Amer. J. Physiol., 9, 26 (1903). 

 Boring. J. anim. Behav., 2, 229 (1912). 

 Brandt. Z. vergl. Physiol., 20, 646 



(1934). 

 von Buddenbrock. Zool. Jb., Abt. Zool. 



Physiol, 37, 315 (1919). 

 Clark. J. exp. Zool., 51, 37 (1928) ; 58, 31 



(1931) ; 66, 311 (1933). 

 Garrey. J. gen. Physiol., 1, 101 (1918). 

 Harper. Biol. Bull., 10, 17 (1905). 

 Henke. Z. vergl. Physiol., 13, 534 (1930). 

 Hess, W. N. J. Morph., 39, 515 (1924) ; 



41, 63 (1925). 

 Hesse, R. Z. wiss. Zool., 61, 393 (1896). 

 Holmes. J. comp. Neurol., 15, 98, 305 



(1905). 

 Loeb. Pfliigers Arch. ges. Physiol., 56, 247 



(1894). 

 Liidtke. Z. vergl. Physiol., 22, 67 (1935) ; 



26, 162 (1938). 

 Biol. Zbl., 62, 220 (1942). 



Mast. Yearbook Carnegie Inst., 9, 131 



(1910). 

 Light and the Behavior of Organists, 



X.Y. (1911). 

 J. exp. Zool.,Zi, 109 (1923). 

 Meyer. Z. wiss. Zool., 142, 254 (1932). 

 MuUer. Zool. Jb., Abt. Zool. Physiol., 40, 



399 (1924). 

 Nomura. Tohoku Imp. Univ. Sci. Rep., 



Ser. iv, 1, 294 (1926) ; 2, 1 (1927). 

 Parker and Arkin. Amer. J. Physiol., 5, 



151 (1901). 

 Pearl. Quart. J. micr. Sci., 46, 509 (1903). 

 Prosser. J. Neurol. Psychopath., 59, 61 



(1934). 

 Raymont. Biol. Bull., 77, 354 (1939). 

 Smith. A7ner. J. Physiol., 6, 459 (1902). 

 Steinmann and Bresslau. Die Strudel- 



wiirmer, Leipzig (1913). 

 Taliaferro. J. exp. Zool., 31, 59 (1920). 

 Viaud. Le phototropisme animal, Paris 



(1948). 



\l 



Turbellarian 

 worm 



TELOTAXIS 



In TELOTAXIS orientation is directly towards {or away from) the 

 source of light ; there is no question of bilateral balance, nor, indeed, 

 are two eyes necessary ; but it is essential to have an eye with several 

 receptor elements which are able to ajDpreciate the direction of a single 

 light or each of several sources simultaneously, and a central nervous 

 organization which can inhibit all stimuli except one. It is this factor 

 of inhibition which forms the essential evolutionary advance, for it 

 provides a mechanism much more efficient than is available to the 

 previous types which respond to the summation of all stimuli (Figs. 

 28 to 31)."^ 



1 Pearl (1903), Mast (1910-11), Boring (1912), Steinmann and Bresslau (1913), and 

 particularly Taliaferro (1920). 



