A snake is an elongate reptile. Snakes are covered in scales. All snakes are carnivorous and can be distinguished from legless lizards by their lack of eyelids, limbs, external ears, and vestiges of forelimbs. The 2,700+ species of snakes spread across every continent except Antarctica ranging in size from the tiny, 10 cm long thread snake to pythons and anacondas at 9 m (30 ft) long. Paired organs (such as kidneys) appear one in front of the other instead of side by side. Many species of snake can be dangerous to humans if mistreated. While venomous snakes comprise a minority of the species, some possess potent venom capable of causing painful injury or death to humans. However, venom in snakes is primarily for killing and subduing prey rather than for self-defense.   Snakes may have evolved from a lizard which adapted to burrowing during the Cretaceous period (c 150 Ma), though some scientists have postulated an aquatic origin. The diversity of modern snakes appeared during the Paleocene period (c 66 to 56 Ma).   Description derived from one accessed from WikiPedia, 3rd August 2008.