Snakes (Serpentes) are a diverse group of squamates that consists of about 2900 species. Snakes have no limbs but their legless nature doesn't stop them from being among the world's most formidable reptilian predators. Limbs are not the only structure snakes lack, they also have no eyelids, no external ears, and most species have only one functional lung. Snakes have long, slender bodies and a long tail. Snakes range in size from the tiny Barbados Threadsnake—which measures a mere 10 cm in length—to the gargantuan anacondas, pythons and boas—which can grow to lengths in excess of 30 feet.
The internal organs of most snakes have been modified over the course of their evolution to accommodate their slender form. As noted above, most snakes only have one functional lung (the right lung) instead of two—this saves space within their slender body. Their stomachs are elongated, their intestines are less coiled than those of other squamates and their kidneys are staggered and elongated.
Like all reptiles, snakes' skin is covered in scales. The color and arrangement of these scales is often helpful in identifying the various snake species. Snakes have many vertebrae in their backbones—in some species, their vertebrae number in the hundreds. The vertebrae of a snake's backbone are loosely jointed together to enable them great flexibility. This enables them to move with great agility and to wind their bodies into compact coils.
Snakes are among the most recently evolved of the main reptile lineages alive today. Their evolutionary history remains somewhat murky though—their delicate skeletons do not preserve well and as a result few fossil remains of ancient snakes have been recovered. The earliest known snake is Lapparentophis defrenni which is estimated to have lived about 130 million years ago, during the early Cretaceous.
Snakes are classified within the following taxonomic hierarchy: