Derma (Latin dermis, from Greek δέρμα - skin), corium (Latin corium, from Greek κόριον - skin.

Fish skin consists of two layers: the upper one is epidermis of ectodermal origin, and the lower one is derma (cutis, corium) of mesodermal origin. The boundary between these layers is the basal membrane. The skin is underlain by subcutaneous connective tissue with fat cells.

Derma is the inner fibrous layer of the skin, which develops from the mesodermal rudiment, the dermal leaflet of the somite. The corium layer is thick and constitutes the main part of the skin, which has great strength. It develops a variety of ossifications in the form of fish scales, covering bones that form the so-called skin skeleton (in contrast to the chondral skeleton).