478 



The Bird 



turns to the right (in ourselves, the left-side gill-channel 

 forms this aortic arch), is what is left of the fourth pair 

 of aortic gill-arches, while the two arteries which, in all 

 higher animals, leads to the right and left lungs, are the self- 

 same channels which in the creatures of olden time encircled 

 the sixth pair of gill-bars. 



Although the eye of the bird is far superior to that of 



a fish in seeing ability, 



^ yet m actual structure 



there is not very much 

 difference, except that 

 the bird has gained 

 eyelids, tear-glands, 

 and a few other struc- 

 tures. Fishes, frogs, 

 lizards, birds, and 

 mammals, through all 

 the ages, have depend- 

 ed on these two eyes 

 and have found them 

 all-sufficient ; but there 

 are hints that once, 

 long ago, the ancestors of all the higher animals had a sense- 

 organ, probably of sight, situated, like that of the mythical 

 Polyphemus, in the centre of the head. In lizards this 

 vestigial organ is sometimes quite well developed, having 

 a nerve which leads up from the centre of the brain to a 

 kind of translucent, lens-like scale which lies among the 

 other scales of the skin, upon the centre of the forehead. 

 In the long-extinct Ichthyosaurs this median eye was prob- 



FiG. 369.— Pineal eye in Chick (P. E.). 



