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The clausilium is a
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 ''Calcareous'' is used as an ad ...
anatomical structure which is found in one group of air-breathing
land snail A land snail is any of the numerous species of snail that live on land, as opposed to the sea snails and freshwater snails. ''Land snail'' is the common name for terrestrial gastropod mollusks that have shells (those without shells are known ...
s: terrestrial
pulmonate Pulmonata or pulmonates, is an informal group (previously an Order (biology), order, and before that a Class (biology), subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a Respiratory system of gastro ...
gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is e ...
s in the family
Clausiliidae Clausiliidae, also known by their common name the door snails, are a taxonomic family of small, very elongate, mostly left-handed, air-breathing land snails, sinistral terrestrial pulmonate gastropod mollusks.MolluscaBase eds. (2020). Mollusca ...
, the door snails. The clausilium is one part of the clausilial apparatus. The presence of a clausilium is the reason for the
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...
"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 In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...
of the shell. However, this structure is emphatically not the same thing as an operculum, which is virtually non-existent in pulmonate snails, only occurring in the Amphibolidae. 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
larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...
e. 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 A garage door opener is a motorized device that opens and closes a garage door controlled by switches on the garage wall. Most also include a handheld radio remote control carried by the owner, which can be used to open and close the door from a ...
.


Gallery

Image:Alinda biplicata anatomy.jpg, Drawing of the inner shell anatomy of '' Alinda biplicata'' shown through a partly broken-open shell. The spoon-shaped end of the clausilium is labelled number 5. Other numbers depicts various plicae and lamellae. Image:Clausilia dubia clausilium.jpg, The clausilium of '' Clausilia dubia'', as it articulates with the
columella Lucius Junius Moderatus Columella (; Arabic: , 4 – ) was a prominent writer on agriculture in the Roman Empire. His ' in twelve volumes has been completely preserved and forms an important source on Roman agriculture, together with the wo ...
of the shell (upper right). Image:Cochlodina laminata clausilium.jpg, Clausilium of '' Cochlodina laminata''. Image:Macrogastra plicatula clausilium.jpg, Clausilium of '' Macrogastra plicatula''.


References

* Iľja Michajlovič Licharev: Klauziliidy (Clausiliidae). Fauna SSSR. Molljuski. Vol. 3, Fasc. 4., 320 pp., Izdat. Akad. n) Gastropod anatomy Clausiliidae {{Clausiliidae-stub