What is operculus?

The operculus is a structure found in certain animals, particularly in mollusks and fish. In mollusks, it refers to a hard, protective lid that covers the opening of the shell, helping to seal the animal inside. In fish, the operculum is a bony flap that covers and protects the gills, facilitating respiration by allowing water to flow over the gills without the need for the fish to swim continuously.