. . are usually top shells, like these from Vietnam which were packaged frozen in a coconut milk and mint sauce. These are sea snails and nowhere near the ecological problem the apple snails are. You wouldn't want to suck apple snails because the stuff up beyond the foot doesn't taste very good. With the top shells you have to suck them out because the foot retreats out of reach when cooked.
And that door that closes the snail shells is call an operculum.
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