630 



RADIATION BIOLOGY 



Gilbert, however, reported that headless planarians were highly kinetic 

 under ultraviolet stimulation and that their movements lacked direc- 

 tional qualities, but that one ocellus allowed orientation and avoidance; 

 thus they were able to plot the visual fields of Planaria toward ultra- 

 violet. Further experiments with multiple lights and color have con- 

 tributed little new to an understanding of the photoreceptors involved. 



Taliaferro's findings (1920) indicated that the point where the planarian 

 body bent in responding to illumination of the oceUi depended on the 



PLANARIA 



]l))iillll|t ii.iiiiMiiiwiinmYmii 



DO R S A L --"--nillllllLlLl 



EPITHELIUM 



MEDIAL 



PIGMENT 

 CUP 



( IN CROSS SECTION ) 



Fic. 14-4. A pigment cup limits the light rays that can enter, but the resolving power 

 of the system is increased materially if the sensory cells show a diverging pattern and 

 greatest sensitivity when illuminated along their axes. {After Hesse, 1897.) 



intensity — the higher the intensity, the more posterior the bending, 

 although the bend always occurred in front of the pharynx. Further- 

 more the sensory cells in the posterior and ventral portions of the pig- 

 ment cup were involved with responses turning the animal toward the 

 eye of that side, whereas illumination of the other receptor cells was fol- 

 lowed by a turn in the opposite direction. Removal of part of the ocellus 

 provided complex modifications in the response, since the anterior por- 

 tion includes the nervous connections to the central integrating center. 

 Each receptor cell, moreover, responded only in proportion to the amount 

 of light traveling along its longitudinal axis. Hence the pigment cup 

 provides a degree of shading for the entire mechanism, but the directional 

 response is much more precise because of polarity of the receptor cells 

 contained within it (Fig. 14-4). 



Eyespots are found in the parasitic trematodes — in the monogenetic 

 Polystomum, in various cercariae, in the miracidium of Fasciola, and in 



