Shells are useful, but they make it hard to breathe.
Since the animal’s body is enclosed in the shell, they don’t breathe like other animals do by expanding and contracting their ribs. The ribs, as we now know, are part of the rigid shell. So turtles breathe by drawing air into their lungs by moving their legs or by contracting one set of abdominal muscles. When they want to exhale, turtles contract an opposite set of abdominal muscles.
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