Only one of the twenty-odd varieties of armadillos is able to roll up in order to escape it's predators.
Picture from Save The Armadillo
Contrary to popular belief, not all armadillos are able to roll into a ball. Only the three-banded armadillo is able to do so. The other types are covered with too many bony plates to allow them to curl up. Other armadillos have to rely on their armored shells for defense.