snake

snake (suborder Serpentes), also called serpent ,  any of about 2,900 species of reptiles distinguished by their limbless condition and greatly elongated body and tail. Classified with lizards in the order Squamata, snakes represent a lizard that, over the course of evolution, has undergone structural reduction, simplification, and loss as well as specialization. All snakes lack external limbs, but not all legless reptiles are snakes. Certain burrowing lizards may have only front or hind limbs or be completely legless. Unlike lizards, snakes lack movable eyelids, which results in a continuous and often disconcerting stare. Snakes also lack external ear openings. Internally, they have lost the urinary bladder. The visceral organs are elongated, with reduction of the left member in relation to the right; the left lung is greatly reduced or even lost entirely. However, snakes possess increased numbers of vertebrae and have developed two novelties among vertebrates: a tracheal lung in the neck region and a venom-conducting system for subduing prey.

Snakes are thought to have evolved from terrestrial lizards during the Early Cretaceous Epoch (145.5 million to 99.6 million years ago). The oldest known fossil snake, Coniophis precedens, which lived in North America some 65–70 million years ago, was a limbless species that possessed a lizardlike head.

Aspects of the topic snake are discussed in the following places at Britannica.

Articles from Britannica encyclopedias for elementary and high school students.

snake - Children's Encyclopedia (Ages 8-11)

A snake is a reptile with a long, slender body but no arms or legs. Snakes are closely related to lizards. There are about 2,900 species, or kinds, of snake. The best-known snakes include cobras, vipers, boas, and pythons.

snake - Student Encyclopedia (Ages 11 and up)

Of all the animals, snakes are among the best known but perhaps the most misunderstood. Snakes are characterized by their elongated limbless bodies. They are found in almost every part of the world except the polar regions. Although the greatest species diversity is found in the tropics, snakes occupy a wide range of habitats, from deserts to rainforests, lakes, and oceans. Snakes belong to the class Reptilia, which includes all reptiles. Within Reptilia they are classified with the lizards into the order Squamata. There are roughly 2,500 to 3,000 snake species living today.