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 in 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- 
Fic. 369.—Pineal eye in Chick (P. E.). 
