The operculum is composed of four fused bones; the opercle, preopercle, interopercle, and subopercle. These appear to be derived from the separate gill-slit covers of an elasmobranch ancestor of the teleost fishes. The posterior rim of the operculum is equipped with a flexible, ribbed structure which acts as a seal to prevent reverse water flow during respiration. The morphology of this anatomical feature varies greatly between species. For example, the bluegill (Lepomis macrochirus) has a posteriorly and dorsally oriented rounded extension with a small black splotch present. In some species, the operculum can push water from the buccal cavity through the gills.
For some fish, opercula are vital in obtaining oxygen. They open as the mouth closes, causing the pressure inside the fish to drop. Water then flows towards the lower pressure across the fish's gill lamellae, allowing some oxygen to be absorbed from the water.
Whereas the cartilaginous ratfishes have soft and flexible opercular flaps, the sharks, rays and relatives (elasmobranch fishes) lack opercula completely and respire instead through a series of gill slits perforating the body wall. Without opercula, other methods of getting water to the gills are required, such as ventilation.
See also 
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