222 The Animal Mind 



image which he received was hardly more definite than 

 one of general size and movement. Bauer (28) reports 

 of the mollusk Pecten, which has eyes of peculiar and com- 

 plicated structure, that when a small quickly moving shadow 

 is cast upon it, the tentacles are quickly withdrawn; 

 large or slowly moving shadows have no effect, but a small, 

 slowly moving shadow makes the animal stretch its ten- 

 tacles and eyes towards the shadow. In this way, Bauer 

 thinks, it is enabled to ascertain the nearness of its worst 

 enemy, a starfish : apparently he supposes that the move- 

 ment of the eyes towards the shadow gives an opportunity 

 for visual perception of its form or characteristic move- 

 ments. Wenrich (777) has recently obtained the following 

 evidence of the formation of an image in Pecten. The 

 bivalve normally responds only to a decrease in illumina- 

 tion, not to an increase. If a white card is moved across 

 a black one, the card being not less than fifteen millimeters 

 square and its distance not greater than thirty-five centi- 

 metres, Pecten responds by closing its shell, although the 

 illumination is increased rather than diminished. 



The chief lines of evidence, however, from which the 

 nature of the visual image can be concluded are three : ex- 

 periments on the visual perception of size, experiments 

 on the visual perception of form, and experiments or obser- 

 vations on the recognition of visual landmarks in homing. 



66. The Visual Perception of Size 



Bonn's observations on the mollusk Littorina show that 

 its reactions are influenced by the size of the illuminated 

 or darkened surface, as well as by the intensity of the light. 

 When neither very wet nor very dry, Littorina will react 

 to small objects in its neighborhood, whereas in an extreme 



