Operculum (gastropod)
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An operculum is a corneous or calcareous anatomical structure like a trapdoor that exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails, including the Hel
Shell of marine snail Lunella torquata with the calcareous operculum in place
Gastropod shell of the freshwater snail Viviparus contectus with corneous operculum in place
Penion sulcatus near Goat Island, New Zealand, with a damaged operculum
A photo of an individual of the freshwater snail species Bithynia tentaculata showing how the back of the shell rests on the round operculum on top of the foot as the snail moves along
Valve (mollusc)
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A valve is each articulating part of the shell of a mollusc or another multi-shelled animal such as brachiopods and some crustaceans. Each part is known as a valve or in the case of chitons, a "plate"
Loose shell plates or valves of Chiton tuberculatus from the beach drift
Live individual of the lined chiton Tonicella lineata, head end towards the right
Paired valves of the bivalve Abra alba
Drawing of the interior of the left valve of the shell of "Julia borbonica"