622 



RADIATION BIOLOGY 



sity for others. This combined alteration the central nervous system 

 may interpret as a change in the direction from which the light comes. 

 No special organ is required (Fig. 14-1), although such locahzation of 

 these functions is common (Fig. 14-2). 



(S) 



"■■':::■': shadow 



NO DIRECTIONAL MEDIATION 



Fig. 14-1. In a flat animal the direction from which light comes cannot be interpreted 

 by a simple integument, but in a cylindrical organism the source of a light can be 

 located because the body itself casts a shadow that can be identified by dermal sensi- 

 tivity. {Modified from Nagel, 1894.) 



LIMIT 



CUTICULAR LENS 



— PIGMENT CELLS 



Fig. 14-2. Simple modifications allow a local sensitivity to be used in locating a light 

 source. A pigment cup with a narrow opening limits the light rays. A cuticular 

 lens may provide a brighter spot on a sensory layer a short distance below the body 

 surface. (Modified from Nagel, 1894.) 



Structural features that limit the sohd angle of surrounding space from 

 which Ught can stimulate any given unit of a compound photoreceptor, 

 plus an increase in the sensitivity of each unit, may allow an organism to 

 appreciate the distribution of Hght and dark in its surroundings. If the 

 contrast between an object and the general background is sufficient for 

 the photosensory mechanism to detect it, movement of the object from 

 one place to another may be a source of visual stimulation. The mini- 



