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Theclausilium is acalcareous anatomical structure which is found in one group of air-breathingland snails:terrestrialpulmonategastropodmollusks in the familyClausiliidae, the door snails. The clausilium is one part of theclausilial apparatus.
The presence of a clausilium is the reason for thecommon name "door snails", because all the snails in this family have a roughly spoon-shaped "door" or clausilium, which can slide down to close theaperture of the shell. However, this structure is emphatically not the same thing as anoperculum, which is virtually non-existent in pulmonate snails, only occurring in theAmphiboloidea.
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 beetlelarvae. 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 automatedgarage door opener.
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