Pleuronectiformes (class Osteichthyes, subclass Actinopterygii) An order of marine flatfish that have a highly compressed and asymmetrical body. The eyes are on the right or left side of the body. The newly hatched larvae have a normal, symmetrical fish shape, but begin to assume the peculiar flatfish features when they are about 2 cm long. The order comprises about six families, with some 520 species.

Heterosomata An older name for the order Pleuronectiformes (flatfish).