Page:Natural History, Fishes.djvu/28

14 Fishes, a large number of the rays of light are absorbed and lost in passing through it; hence their eyes are very large, to collect as many of the remaining rays as possible. The cornea is flat, but the crystalline lens is perfectly spherical; the latter is familiar in the form of a white globule in a boiled Fish, the transparency being destroyed by heat. The pupil is large, and the iris is almost motionless. Eyelids are not present; and as the surface of the eye is always bathed by the surrounding water, there is no need for the secretion of tears.

The eyes of Fishes frequently reflect brilliant colours, red, orange, yellow, blue, and black, and not unfrequently display a pearly or metallic lustre, like that of gold or silver. These hues are due to a membrane called the choroid, spread around the back of the eye, composed largely of highly reflecting microscopic crystals. The eyes of some species gleam like those of quadrupeds. In general the eyes are placed opposite each other on the two sides of the head, so as to look laterally, and (owing to the tapering of the head) a little forward. In some, however, especially such species as habitually live at the bottom, they are placed on the top of the head, and look vertically upwards. In one genus of Sharks, called, for this reason. Hammer-heads, the head is enormously widened, or lengthened sidewise, so as to present two long lateral processes, at the extremities of which are placed the eyes;