Snakes are elongate legless carnivorous reptiles that can be distinguished from legless lizards by their lack of eyelids and external ears. They range in size from the tiny, 10 cm long thread snake to pythons and anacondas of up to 7.6 metres in length. Most species are non-venomous. Those that have venom use it primarily to kill prey rather than for self-defense. Those which are non-venomous either swallow prey alive or kill it via constriction.