218 The Animal Mind 



through the eye may be accompanied by spatial perception 

 of movement, although if the eye is compound, the experi- 

 ence must differ from our own visual movement perception. 



64 Class IV: Reaction to an Image 



By an image is meant the perception of simultaneously 

 occurring but differently located stimuli as having certain 

 spatial relations to each other. Through its means, or 

 that of the nervous processes underlying it, there arises 

 the possibility of adapting reaction not merely to the loca- 

 tion of a single stimulus, but to the relative location of 

 several stimuli. Responses may thus be adjusted not 

 only to the direction of an object but to its form. On the 

 basis of such adjustments a whole new field of possible 

 discriminations is opened up. 



The commonest arrangement for the production of a 

 visual image is the double convex lens, which collects the 

 rays of light diverging in their reflection from an object 

 and brings them together again upon the sensitive retina. 

 The lenses found in many simple invertebrate eyes seem, 

 however, very ill adapted to the image-producing function. 

 It is probable that they serve rather to intensify the effect 

 of the light rays by bringing them together, than to give 

 a clear-cut image (523). In the eye of certain inverte- 

 brates, such as the Nautilus, a cephalopod mollusk, while 

 there is no lens, the opening admitting the light rays is 

 so small that an inverted image might be formed through 

 it, such as may be obtained through a pinhole. It is un- 

 likely, however, that this eye is really an image-producing 

 organ. Hesse includes under image-forming eyes only 

 the camera or convex-lens eye, the mosaic eye, and the 

 superposition eye. The last is a peculiar form of com- 



