There's an interesting exception to this statement in one group of fishes. The flatfish (e.g., flounders) have two eyes on the same side of the head. But not initially: the eyes begin on opposite sides in the embryonic and fry stages, and as the fish matures, one eye moves, actually migrating from one side of the head to the other.

The peculiar arrangement of eyes in these fishes results from their natural history: they are predators who lie on the sandy bottom, waiting for a small prey species to come by. By having both eyes on one side of the head, a degree of depth perception is generated that enables them to swim up and capture the prey. The upper side of the fish, on which the eyes are found, is dark-colored, to match the bottom, but the underside is whitish-pink. And there are "right-eyed" and "left-eyed" flounders, depending on which side of the head the eyes migrate to!

The sight of a fish with two eyes very close together on one side of its head seems to be upsetting to some people, and fish vendors selling whole flounders always place them with the white side up, and the eyes hidden from the buyer's view.

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