Metamorphosis of a Flounder
Flounder have both eyes situated on one side of the head, they are not born this way. Their life involves metamorphosis. During metamorphosis, one eye migrates to the other side of the body so that both eyes are situated on the upward-facing side of its body. After metamorphosis, flounder lie on one side on the ocean floor; either the left or right side might face upward depending on the species. This is the different stages of the flounder. Note the eyes in the young flounder are arranged one on each side of the head.
Winchell, Alexander Sketches of Creation (New York, NY: Harper & Brothers, 1870)