OUR FACE FROM FISH TO MAN 



octopuses. In these highly elaborate organs there 

 are eyelids in front of the eyes, a contractile iris, 

 muscles of accommodation, a highly complex 

 retina of many layers, a large optic nerve and 

 muscles to move the eyeball. But when we 

 compare the parts of these cephalopod eyes with 

 those of vertebrates we find many striking and 

 profound differences. Thus in the squid (Sepia) 

 the lids serve as a pupil, there are two corneas, 

 the outer one perforated, the inner one dividing 

 the lens into inner and outer parts; the so-called 

 iris lies entirely outside of the retinal layer instead 

 of next to it as in the vertebrates; and there is 

 apparently no true choroid layer. More important 

 still, in the cephalopods the optic nerve lies 

 entirely behind the retina, while in vertebrates it 

 pierces the retina and is then distributed over its 

 front surface; finally, in the cephalopods the rods 

 are on the front layer of the retina, pointing 

 toward the light, while in the vertebrates they are 

 on the back layer of the retina and point in the 

 opposite direction. 



Not all the cephalopods have eyes as compli- 

 cated as the type described above and there is a 



gradation of forms leading back to the very 



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