136 The Animal Mind 



But if nothing distinguishes the negative reaction to 

 photic stimuli from the negative reaction to any other 

 stimulus, then nothing shows the existence of a sensation 

 quality peculiar to the effect of light unless a special 

 receptive apparatus can be demonstrated. In a flagellate 

 Protozoon called Euglena, a pigment spot exists near the 

 anterior end. Now although pigment apparently is not, 

 as Hesse (323) has emphasized, a necessary constituent of 

 visual organs, yet its occurrence always suggests some re- 

 lation to light, as it is essentially a kind of matter having 

 the property of absorbing light. Euglena gives the nega- 

 tive reaction on entering a shadow. Is its pigment spot 

 really an "eye spot" and concerned in this response? Ap- 

 parently the reaction occurs before the pigment spot has 

 entered the shadow, and as soon as the transparent tip 

 lying in front of the pigment spot has been pushed into the 

 shaded region (204). It is uncertain, then, what the 

 function of the pigment spot is. But in another organism, 

 which is structurally intermediate between the single- 

 celled and the many-celled forms, pigment spots do play a 

 role in light reactions. This organism is called Volvox, 

 and it is really a colony of globular flagellates, each with its 

 flagellum turned outward, and each with an "eye spot." 

 Very weak light has no effect on the movements of Volvox ; 

 moderate light causes movement toward the source of light, 

 and very strong light causes movement away from the 

 source (332). Accurate observation of these movements 

 indicates that the eye spots are essential to them ; each in- 

 dividual responds to a change of illumination of its eye 

 spot (464). This much evidence, then, we have that if 

 Volvox possesses consciousness, changes of light intensity 

 produce in it a specific sensation. 



Among ccelenterates, response to changes of light in- 



