An order of reptiles comprising the lizards and snakes. They are thought to have appeared at the end of the Triassic period, about 200 million years ago, and have invaded a wide variety of habitats. Most lizards have four legs and a long tail, eardrums, and movable eyelids. Snakes are limbless reptiles that lack eardrums; the eyes are covered by transparent immovable eyelids and the articulation of the jaws is very loose, enabling a wide gape to facilitate swallowing prey whole. In both snakes and lizards, males possess paired hemipenes for copulation.