A snake is an elongate reptile of the suborder Serpentes. Like all reptiles, snakes are ectothermic and 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 2,900 species of snakes spread across every continent except Antarctica ranging in size from the tiny, 10 cm long thread snake to pythons and anacondas over 7 meters long. In order to accommodate snakes' narrow bodies, paired organs (such as kidneys) appear one in front of the other instead of side by side.
While venomous snakes comprise a minority of the species of snakes and are typically small innocuous creatures, 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, though some scientists have postulated an aquatic origin. The diversity of modern snakes appeared during the Paleocene period.
A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep"). In modern usage, the term serpent usually refers to a mythic or symbolic snake. In Christianity, the serpent is sometimes identified with the devil, as in the Biblical account of Adam and Eve, but also with healing, as in the Biblical account of the brass serpent of Moses. The serpent is also the symbol of the healing arts.
Snakes are categorized in the order Squamata within the entire suborder Serpentes in Linnean taxonomy. There are two infraorders of Serpentes: Alethinophidia and Scolecophidia. This separation is based primarily on morphological characteristics between family groups and mitochondrial DNA.
As with most taxonomic classifications, there are different interpretations of the evolutionary relationships. These include moving of families to different infraorders, merging or splitting of the infraorders and merging and splitting of the families. For instance, many sources classify Boidae and Pythonidae as the same family, or keep others, such as Elapidae and Hydrophiidae, separate for practical reasons despite their extremely close relation.
