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bis, meaning 'two', and valvae, meaning 'leaves of a door' ("leaf" is an older word for the main, movable part of a door; we normally consider this the door itself). Paired shells have evolved independently several times among animals that are not bivalves; other animals with paired valves include certain gastropods (small sea snails in the family Juliidae), members of the phylum Brachiopoda and

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