INVERTEBRATE EYES 125 



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THE STRUCTURE OF INVERTEBRATE EYES 



We are now in a position to study the various types of photo- 

 receptors found among Invertebrates. In unicelhdar organisms the 

 diffuse sensitivity to hght evolves into the locahzed sensitivity of an 

 " eye-spot " ; in multiceHular organisms the diffuse dermal sensitivity 

 evolves into the specialization of certain epithelial cells as specific 

 photoreceptors. 



EYE-SPOTS : STIGMATA 



The earliest stage in the evolution of an eye is seen in unicellular 

 organisms in which a small area of the protoplasm is differentiated to 

 become specially sensitive to light ; this development is seen in actively 

 motile ciliate and flagellate Protozoa, and since in these organisms light 

 serves not as a visual but as an orientating stimulus, the specialized 

 area is always in close association ^viih the cilia or flagella. Among 

 Ciliates such specialization is primitive ; thus without observable 

 structural differentiation, the oral surface of Stentor coendeus is more 

 photosensitive than the aboral so that, as the organism rotates while 

 swimming, successive exjjosures of these two areas determine a negative 

 klinotaxis. orientating it away from the light (Jennings, 1904 ; Mast, 

 1 906-1 1).^ In Flagellates, however, a degree of structural differentia- 

 tion appears which is sufficient to dignify the organelle thus formed 

 with the name of an eye-spot or stigma {ariyiJix, a spot) ; there is a 

 light-sensitive area, a mass of pigment which serves to make the 

 organelle a directional detector useful in phototaxis. and occasionally 

 a refractile structure wliich serves to concentrate the light, thus acting 

 as a lens.- 



The stigma of tlie common green protozoon of ponds, Eugle?ia 



» p. 49. 



- As occurs in the Alg;p, Cladophora and Gonium (Mast, 1916). 



Stentor 



