The Amphisbaena however is a snake with two heads, one at the top and one in the direction of the tail. When it advances, as need for a forward movement impels it, it leaves one end behind to serve as tail, while the other it uses as a head. Then again if it wants to move backwards, it uses the two heads in exactly the opposite manner from what it did before. — Claudius Aelianus, Characteristics of Animals[