In the eyes of the fishes there is no lachrymal gland. True eyelids no fishes possess; the integuments of the head pass over the eye, becoming transparent as they cross the orbit. In some fishes part of this integument is thickened, covering the eye fully although still transparent. This forms the adipose eyelid characteristic of the mullet, mackerel, and ladyfish. Many of the sharks possess a distinct nictitating membrane or special eyelid, moved by a set of muscles. The iris in most fishes surrounds a round pupil without much power of contraction. It is frequently brightly colored, red, orange, black, blue, or green. In fishes, like rays or flounders, which lie on the bottom, a dark lobe covers the upper part of the pupil--a curtain to shut out light from above. The cornea is little convex, leaving small space for aqueous humor. In two genera of fishes, _Anableps_, _Dialommus_, the cornea is divided by a horizontal partition into two parts. This arrangement permits these fishes, which swim at the surface of the water, to see both in and out of the medium. _Anableps_, the four-eyed fish, is a fresh-water fish of tropical America, which swims at the surface like a top-minnow, feeding on insects. _Dialommus_ is a marine blenny from the Panama region, apparently of similar habit. [Illustration: FIG. 86.--_Ipnops murrayi_ Günther.] In one genus of deep-sea fishes, _Ipnops_, the eyes are spread out to cover the whole upper surface of the head, being modified as luminous areas. Whether these fishes can see at all is not known. [Illustration: FIG. 87.--Pond-skipper, _Boleophthalmus chinensis_ (Osbeck). Bay of Tokyo, Japan; from nature. K. Morita. (Eye-stalks shrunken in preservation.)] The position of the optic nerves is described in a previous chapter. In ordinary fishes there is one eye on each side of the head, but in the flounders, by a distortion of the cranium, both appear on the same side. This side is turned uppermost as the fish swims in the water or when it lies on the bottom. This distortion is a matter of development. The very young flounder swims with its broad axis vertical in the water, and it has one eye on either side. As soon as it rests on the bottom it begins to lean to one side. The lower eye changes its axis and by degrees travels across the face of the fish, part of the bony interorbital moving with it across to the other side. In some soles it is said to pass through the substance of the head, reappearing on the other side. In all species which the writer has examined the cranium is twisted, the eye moving with the bones; and the frontal bone is divided, a new orbit being formed by this division. In most northern flounders the eyes are on the right side in the adult, in tropical forms more frequently on the left, these distinctions corresponding with others in the structure of the fish.