The clausilium is a calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 anatomical structure which is found in one group of air-breathing land snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s: terrestrial

Terrestrial animal

Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

 pulmonate gastropod mollusks in the family Clausiliidae

Clausiliidae

Clausiliidae, common name door snails, are a taxonomic family of small, very elongate, mostly left-handed, air-breathing land snails, sinistral terrestrial pulmonate gastropod mollusks....

, the door snails. The clausilium is one part of the clausilial apparatus.

The presence of a clausilium is the reason for the common name

Common name

A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...

 "door snails", because all the snails in this family have a roughly spoon-shaped "door" or clausilium, which can slide down to close the aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 of the shell. However, this structure is emphatically not the same thing as an operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, which is virtually non-existent in pulmonate snails, only occurring in the Amphibolidae

Amphibolidae

Amphibolidae is a family of air-breathing snails with opercula, terrestrial pulmonate gastropod molluscs.The family is considered to be among the most primitive of the pulmonate gastropods, for although amphibolids breathe air, they also have opercula and at least some species go through a...

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The exact shape of the clausilium varies from genus to genus: it can be tongue-shaped, spoon-shaped or spatula-shaped. The wide flat end of the clausilium can close the aperture of the snail shell, and thus protect the soft parts against predation by animals such as carnivorous beetle larvae. The narrow end of the clausilium slides in a groove, which is formed by spiral folds on the inside of the shell around the columella. Because the groove is long, and the muscles that control the clausilium are also long, the whole structure can be retracted into the shell. The mechanism is totally different, but the clausilium is vaguely reminiscent of an automated garage door opener

Garage door opener

A garage door opener is a motorized device that opens and closes garage doors. Most are controlled by switches on the garage wall, as well as by remote controls carried in the garage owner's cars, or more rarely, on keychains.- The electric opener :...

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