A turtle or tortoise can hide its head for protection in one of two ways. Most turtles pull the head straight backwards so that the neck vertebrae curve into a U-shape inside the shell and the neck skin is inverted. In this position, the opening of the shell can be completely blocked by the front legs. Some turtles, however, withdraw by snaking their necks sideways and tucking their heads under the shell. In this case the legs offer no protection. The extraordinarily long necks of these side-necked turtles contain the normal number of eight vertebrae, but the joints between them are structurally different from those of other turtles.