Shown at right is a photograph of unidentified gastropods from Germany (age unknown). Gastropods 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 life forms, and nearly causing their extinction. In keeping cyanobacteria under control, gastropods may have saved the world. Cyanobacteria produce oxygen, and our atmosphere already has 20 percent. If appreciably more oxygen had ever existed, the earth might have spontaneously combusted millions of years ago.
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