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Age unknown
Germany
Gastropods, which include snails, limpets, and slugs, have a spiral shell and a foot that spreads out broadly when the animal is moving forward but which can be retracted within the shell when the animal is frightened. They also have two tentacles, each ending in an eye, and a mantle that secretes the shell. The gastropods have played an important role in geologic history, voraciously eating cyanobacteria, one of the oldest of life forms, and nearly causing their extinction. But, in keeping the cyanobacteria under control, gastropods may have saved the world! Cyanobacteria produce oxygen, and our atmosphere already has twenty percent. If there had ever been appreciably more oxygen, the earth might have spontaneously combusted millions of years ago.