OUR FACE FROM FISH TO MAN 



movements, while the "cones" chiefly detect color 

 differences (Plate, 1924, p. 705). In the jelly- 

 fishes the cavity of the optic pouch is often filled 

 with a transparent jelly-like substance correspond- 

 ing to the "glass body" or vitreous humor of 

 higher eyes, and functionally to the lens. That 

 these organs are really eyes, says Plate (1924, p. 

 428), follows from the fact that if the animal is 

 deprived of them it fails to react in its normal 

 way to light. 



In some of the flatworms the eyes consist of 

 hollow capsules derived from an infolding of the 

 epithelium and deeply lined with pigment. Each 

 capsule has sunk beneath the epithelium, which 

 has grown over it. It is open on one side and 

 into its hollow interior project the flower-like ends 

 of the "light cells," the outer ends of which pass 

 into elongate nerve cells. Hesse (quoted by 

 Plate, p. 433) notes that if two such capsules are 

 symmetrically arranged on either side of the mid- 

 line, then a light in front will give symmetrically 

 placed shadows inside the capsules, a light on the 

 left side will illuminate the left capsule and leave 

 the interior of the right one in shadow, and so 



forth. Thus the nerves inside the capsules on 



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