The
operculum, meaning little lid, (plural: opercula or operculums) is a
corneous or
calcareous anatomical structure like a
trapdoor which exists in many (but not all) groups of
sea snails and
freshwater snails, and also in a few groups of
land snails. In other words, this structure is found in some marine and freshwater
gastropods, and in a minority of terrestrial gastropods, including the families
Helicinidae,
Cyclophoridae,
Aciculidae,
Maizaniidae,
Pomatiidae, etc.