LIGHT AND MOVEMENT 43 



certain photo -positive animals travel towards a light even although 

 this movement brings them into a region of lower intensity of illumina- 

 tion,^ or certain photo -negative animals to seek a dark shelter even 

 although this entails moving towards a light. ^ It is a response, 

 however, which requires one or more receptor organs specially designed 

 to appreciate the direction of the incident light rather than merely 

 changes in its intensity, and as the response becomes more and more 

 efficient and therefore more and more complex, the receptor organs 

 become progressively specialized until they eventually achieve the 

 structural differentiation necessary to mediate the faculty of vision. 

 The directional phototropic movements of sessile plants are slow and 

 leisurely, essentially kinetic in nature, quantitative in type and 

 chemical in execution ; but motile organisms require a more efficient 

 mechanism capable of qualitative responses — a shock-reaction eventu- 

 ally mediated by nervous activity. The difference between the two 

 types of response is well exemplified in the mutilation experiments of 

 Viaud and Medioni (1949) on the flat -worm. Planar ia luguhris, an 

 animal in which both reactions are present ; they found that its 

 positive photokinesis was entirely due to the action of light on the skin 

 while positional orientation by phototaxis depended on the eyes. 



As they evolved, these phototactic responses increased in com- 

 plexity and efficiency ; the various stages may be classified as follows 

 (Kiihn, 1919-32 ; Gunn et al, 1937). 



(i) KLEsroTAXis (kXlvo), tum ; rafts-, a precise arrangement), 

 wherein turning movements, normally alternating regularly, are 

 directed towards or away from the light. One receptor organ only is 

 necessary which responds by comparing the intensities of successive 

 stimuli as the organism turns, and the directional path is consequently 

 irregular and wavy. 



(ii) TEOPOTAXis (rpoTT-q, a tum), wherein orientation is effected 

 by the sirrmlianeous comparison of the intensities of the stimulation of 

 two symmetrical receptors and the maintenance of a bilateral balance. 

 The path is thus continuously corrected so that it is practically straight 

 towards or away from the light, and it is obvious that greater accuracy 

 and precision are obtained by a simultaneous comparison than by 

 comparing present experiences with past. 



(iii) TELOTAXis (TeAo?, a goal), a direct orientation towards or 

 away from the light without the necessity of bilateral balance. A 

 single receptor organ which can fixate the source of light is sufficient 

 for its initiation, but it must possess a number of elements spatially 

 distributed so that the stimulus can be localized on the sensory surface 

 and the head and body can be orientated directly in line with the light. 



1 See the experiments of Richard (1948) on termite larvae {Calotermes flavicollis). 

 * See Gousrard (1948-50) experimenting on the cockroach, Blatella ; Bolwig (1954) 

 experimenting on the stomatopod, Gonodactylus. 



