Because, as the eel lives in holes, and pushes its head into mud, and under stones, &c., it needed such a covering to defend the eye.
1131. Why is the whale provided with an eye, having remarkably thick and strong coats?
Because, when he is attacked by the sword-fish and the shark, he is almost helpless against his enemies, as they fix themselves upon his huge carcase. He therefore dives with them down to a depth where the pressure of the water is so great that they cannot bear it. The eye of the whale is expressly organised to bear the immense pressure of extreme ocean depths, without impairing the sight.
1132. Why have fishes no eyelids?
Because the water in which they swim keeps their eyes moist. Eyelids would therefore be useless to them.
1133. Why have fishes the power of giving their eye-balls very sudden motion?
Because, having no eyelids (such organs being unnecessary to keep their eyes moist), they still need the power of freeing their eyes from the contact of foreign matters; and this is secured to them by the power they have of giving the eyeball a very rapid motion, which causes reaction in the fluid surrounding it, and sweeps the surface.
This motion may frequently be seen in the eyes of fishes, in glass globes.