Bats are the only mammals that can fly, which has resulted in their anatomy evolving for speed, maneuverability and aerial hunting. The bones of the forelimbs all elongate, becoming greater farther from the body. The bones and muscles of the wing are arranged so that the wing is spread and closed by a single muscle, allowing the arms to be opened and closed very quickly and efficiently with little effort.
There are over 900 species of bat, all belonging to the order Chiroptera. They are second in number only to rodents among mammals. The shape of the skull varies greatly in shape, depending on the diet the particular species of bat has. Nectar-feeding bats have long, thin skulls, while some insect-eating species have short, blunt skulls.
The arms have no hands but do have a clawed thumb. The toes of the hind limbs are all strong and have an automatic locking system that allows it to sleep without falling from its roost. The hind limbs are reversed, so when a bat walks on the ground, its knees bend in the opposite direction of humans. The lower limbs can rotate, allowing a hanging bat to swivel its body almost full circle.