PARTS OF ANIMALS, II. xm.-xiv. 



not sharp-sighted, since there is no urgent necessity 

 for it in their kind of hfe. 



Many differences in the eye itself are found among 

 the Fishes, the Insects and the hard-skinned Crus- 

 tacea, thougli not one of them has eyehds. In the 

 hard-skinned Crustacea there cannot be an eyelid at 

 all, for the action of an eyelid depends upon s-wdft 

 working of the skin. To compensate for the lack 

 of this protection, all these creatures have hard 

 eyes : it is as though the eyelid were all of a piece 

 with the eyeball, and the creature looked through 

 the lid as well. But since the vision is bound to be 

 dimmed by this hardness of the eye, Nature has 

 given the Insects (and even more noticeably the 

 Crustacea) movable eyes, just as she has given some 

 quadrupeds movable ears ; this is to enable them 

 to turn toAvards the light and catch its rays and so 

 to quicken their vision. Fish have fluid eyes for the 

 following reason. They move about a good deal and 

 have to use their sight over long distances. Now 

 when land-animals do this, they are looking through 

 air, which is highly transparent ; but fish move about 

 in water, which is inimical to sharpness of vision ; so 

 to counteract its opacity their eyes are fluid in 

 composition. At the same time, water contains far 

 fewer objects to strike against the eyes than the air 

 does ; hence fish need no eyelids, and because 

 Nature never makes anything without a purpose, 

 they have none. 



XIV. Those animals that have hair on their body Eyelashes 

 have eyelashes on their eyelids : the others (birds ^" 

 and the creatures with horny scales) have none. 

 There is one exception to this rule : the Libyan 

 ostrich, which has eyelashes. The cause of this 



G 187 



