88 EYES OF DIFFERENT ANIMALS. 



animals are two, and they are, generally speaking, 

 moveable ; so that the animal may turn them in various 

 directions without moving its body, or even its head. 

 In the insect tribes the eyes are often compound, con- 

 sisting of a great number of sights or lenses, each of 

 them adapted for receiving and transmitting light, but 

 all of them, even in the most compound eye, communi- 

 cating with one single retina, or organ of perception. 

 Animals that are liable to be chased, have the eyes 

 further back in the head, and so prominent that they 

 can see laterally, or even behind. The eye of the hare 

 is an instance of this, and that of the giraffe is still 

 more remarkable. The eyes of pursuing-animals are 

 more directed to the front ; and those that spring on 

 their prey have them deeply enfonced, so that they 

 may take a more steady view, both in direction and 

 distance. In the eyes of animals that have to seek 

 their ways and their food in the direction of the per- 

 pendicular as in cats that climb trees the eyes have 

 the pupil elongated in that direction, so that they may 

 contract the opening, and exclude light from other 

 objects at the sides of the one principally looked to, 

 and yet have a considerable range in the direction of 

 that. Animals, on the other hand, that have to find 

 their food upon the ground, as those that graze, have 

 the pupil contracted above and below, with the open- 

 ing elongated in the horizontal direction. There is a 

 considerable difference in the eyes of day and night 

 animals, as they are called, as between those of an 

 eagle and those of an owl. The day animal has the 

 interior of the eye lined with a dark membrane or 

 pigment, the surface of which is without gloss; and 



