130 The Animal Mind 



become indifferent to green and yellow. These stages last for 

 several hours, but corresponding ones have not been observed 

 in the passage from the violet phase back to the red phase ; 

 perhaps they occurred too rapidly to be noted (274). 



45. Vision in Mollusks 



In the phylum Mollusca we find eyes of all grades of de- 

 velopment, from mere pigment spots in certain Acephala to 

 the elaborate eye of the squid, with its lens, iris, and contrac- 

 tile pupil. Such an eye is fully capable of forming an image. 

 Among the Acephala there are many instances of reaction to 

 light in the absence of all visual organs. The sensitive parts 

 are commonly the siphons, which are projected from the 

 shell to take in currents of water containing nourishment, and 

 withdrawn in response to sudden darkening in some cases, to 

 sudden illumination in others, and in still other instances to 

 either (102, 290, 373). In Pecten varius, which has eyes 

 on the border of its " mantle," Rawitz found that a shadow 

 would cause reaction provided that it fell simultaneously upon 

 a considerable number of the eyes, from which he concludes 

 that they may cooperate in a kind of mosaic vision (360). 



In snails, although the eyes are undoubtedly concerned in 

 light reactions, a certain amount of skin sensitiveness has 

 been shown. Helix aspersa, a negatively phototropic ani- 

 mal, when blinded, reacted one-half as many times to light 

 as when normal; H. nemoralis, positively phototropic, only 

 one-eighth as many times; from which the suggestion was 

 derived that the " dermal light-sense" may be more effective 

 in negative than in positive animals (441). Very interesting 

 observations on periodic changes in the responses of marine 

 gasteropods to light have been made by Bohn (55), but these 

 will be more fully considered in a later chapter. The 

 cephalopods, with their highly developed eyes, offer an inter- 



