124 



Herbert Eugene Walter 



such a degree that it is practically impossible to make a worm 

 remain with its dorsal surface in contact with any surface, while 

 its ventral surface is just as strongly positive in its thigmotaxis. 

 In contrasting the receptivity of these two surfaces to light 

 stimulation a field of two adjacent intensities, similar to that used 

 in the experiments on abrupt spacial changes in light intensity 

 (Fig. 3, p. 79), was arranged in such a way that, in the first instance 

 the source of the two lights was below, and in the second above, 

 the field in which the worms were placed. The intensities of the 

 light in each case were approximately 66 and 33 cm. By this 

 means the responses of the worms could be tabulated as they 

 glided from one intensity of light to another and those given when 

 the light impinged on the dorsal surface compared with similar 

 responses made when the light struck directly on the ventral sur- 

 face. It will be seen in Table XXXIII that the results do not indi- 

 cate any particular difference for the dorsal and ventral surfaces 

 with respect to the distribution of the photoreceptors. This con- 

 dition of affairs, however, may be largely due to the translucency 

 of the planarian's body, which would render light-receiving organs 

 accessible from whatever direction the light primarily comes. 



TABLE XXXIII 



A comparison of responses made by Planaria gonocephala to a change in light intensity, tabulated with 

 reference to the source of the light and its relative degree of stimulation upon the dorsal and ventral 

 surfaces of the worm respectively 



Position of light 



Number of 

 observations 



Percentage of 

 total responses 



Percentage of 



wigwag 



movements 



Above. 

 Below. 



101 

 156 



5° 

 53 



42 

 36 



An exact determination of photoreceptors other than the eyes 

 was not made. Both Iijima ('84, p. 438) and Carriere ('82, p. 167) 

 in their histological researches upon planarians found "Neben- 

 augen" frequent and these occasional accessory eyes have also 

 been described by Janinchen ('96, p. 259). Such structures may 



