Do you know why? The flounder's eyes are on the same side, which is formed during development. When flounders just hatch from eggs, they have eyes on both sides of their heads, just like other fish. At this time, they are very active and can play on the sea. About 20 days later, when the body grows to 1 cm, due to the uneven development of various parts of the body, the body gradually turns over when swimming, so it begins to lie on its side and live at the bottom of the sea. At this time their eyes began to swim. The growth of bilateral brain bones is unbalanced, especially in the anterior jaw. The eyes under the body, because of the growing soft belt under the eyes, make the eyes swim up, pass through the back and reach the top, juxtaposed with the original eyes above. At this time, the orbital bone of the flounder was also formed, and the positions of the two eyes no longer changed but grew on the same side.
The life habits of flounder are very interesting. When swimming in the water, it doesn't have its back up like other fish, but its side with eyes is up and it swims sideways. It often lies flat on the bottom of the sea, covered with a layer of sand, and only two eyes are exposed to wait for prey and avoid predation. In this way, the advantages of two eyes on one side are displayed, which is of course the result of animal evolution and natural selection.