Operculum

Operculum - bony plate covering the gill chamber and forming most of the side of the head behind the eye.

The back edge of the operculum roughly defines the junction between head and body on most fishes.

Only bony fishes have these bony plates. Cartilaginous fishes (sharks, rays and sturgeons for example) have gill slits - simple flaps of skin that cover the gill opening. Cartilaginous fishes may have more than one pair of gill slits, but bony fishes only have one pair of opercula.

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