132 



The Animal Mind 



but von Uexkull thinks this impossible, as the light reactions 

 occur before the pigment changes do. This migratory pig- 

 ment, he believes, acts merely as a screen; the source of 

 excitation for the optic fibres may lie in another pigment 

 which he has extracted and found very sensitive to light (410). 



47. Vision in Crustacea 



The spatial aspect of vision assumes great importance in 

 the arthropods, both because of the precision of their orien- 

 tation to light in 

 many cases, and 

 because of the 

 peculiar func- 

 tions of the com- 

 pound eye so 

 common in this 

 group. This or- 

 gan appears to be 

 specially adapted 

 to the vision of 

 moving objects 

 (Fig. 10). It con- 



FiG. 10. Diagrammatic representation of the compound sists essentially 



eye of a dragon-fly. C, cornea; K, crystalline cone; Q & number of 

 P, pigment; R, nerve rods of retina; Fb, layer of 



fibres; G, layer of ganglion cells; Rf, retinal fibres; simple CVCS SO 



Fk, crossing of fibres. After Glaus. Crowded together 



that the common cornea is, as it were, faceted, each facet 

 belonging to an eye. These facets are lens shaped, and back 

 of each lies a refractile crystalline cone. Behind these, in 

 turn, are nervous structures, the rods or retinulae, each sepa- 

 rated from its neighbors by a pigment sheath. Light rays 

 passing through each corneal facet probably produce a single 

 spot of light on the retinula, and the total image is thus a 



