operculum
/oʊˈpɜːrkjələm/
noun
- A lid or cover that opens and closes, such as on a snail shell, a fish's gill, or a plant capsule.
- The snail's operculum seals the shell opening when it is inside.
- A fish uses its operculum to protect its gills and help pump water over them.
- The operculum of the moss capsule pops off to release spores into the air.