OUR FACE FROM FISH TO MAN 



Indeed, Plate (1924, pp. 738-742) cites much 

 evidence for his view that the paired eyes of 

 vertebrates originated as directional organs, guiding 

 the animal toward the light and that later by acquiring 

 a lens they became true visual organs. 



Fig. 93. How the Eye Capsules of a Flatworm Serve as Direc- 

 tional Organs (from Plate, after Hesse). 



The arrows show the varying directions of the light. In each 

 case only a particular part of each retina is stimulated, the rest being 

 in shadow. 



(From AUgem. ZooL, Gustav Fischer.) 



The higher invertebrates exhibit eyes in all 

 grades of evolution, from the simple types described 

 above to the compound eyes of crustaceans and 

 insects and to the elaborately constructed paired 

 eyes of the higher molluscs. Eyes occur in various 

 parts of the body and sometimes in great numbers, 

 as in certain deep-sea cephalopods. The common 

 scallop (Pecten) has numerous eyes along the 

 scalloped edge of the mantle. Thus in typical 

 invertebrates the eyes are essentially derivatives 



of the skin and may occur almost anywhere on the 



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