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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, hind limbs, external ears, and the presence of only vestigial forelimbs. The 2,700+ species of snakes spread across every continent except Antarctica ranging in size from the tiny, 10&_160;cm long thread snake to pythons and anacondas at 9&_160;m (30&_160;ft) 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, 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). A literary word for snake is serpent (a Middle English word which comes from Old French, and ultimately from *serp-, "to creep"[1], also e?p? in Greek). The serpent is also a symbol of the healing arts. Primitive groups among the modern snakes, pythons and boas, have vestigial hind limbs tiny, clawed digits known as anal spurs which are used to grasp during mating.[7][2] Leptotyphlopidae and Typhlopidae are other examples where remnants of the pelvic girdle are still present, sometimes appearing as horny projections when visible. The frontal limbs in all snakes are non-existent because of the evolution of the Hox genes in this area. The axial skeleton of the snakes' common ancestor had like most other tetrapods the familiar regional specializations consisting of cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal (tail) vertebrae. The Hox gene expression in the axial skeleton responsible for the development of the thorax became dominant early in snake evolution and as a result, the vertebrae anterior to the hindlimb buds (when present) all have the same thoracic-like identity (except from the atlas, axis and one to three neck vertebrae), making most of the snake's skeleton being composed of an extremely extended thorax. Ribs are found exclusively on the thoracic vertebrae. The neck, lumbar and pelvic vertebrae are very reduced in number (only two to ten lumbar and pelvic vertebrae are still present), while only a short tail remains of the caudal vertebrae, although the tail is still long enough to be of good use in many species, and is modified in some aquatic and tree dwelling species.
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