A snake is an animal with a long, legless body covered by dry scales. To move about on land, a snake usually slides on its belly. Many snakes have such a flexible body that they can coil into a ball. The eyes of a snake are covered by clear scales instead of movable eyelids. As a result, its eyes are always open. Snakes have a narrow, forked tongue, which they repeatedly flick out. They use the tongue to bring odors to a special sense organ in the mouth.

Scientists have evidence that snakes developed from lizards about 100 million years ago. Snakes resemble lizards more than they do other reptiles. But unlike most lizards, snakes lack legs, movable eyelids, and external ear openings. Their scales and skulls also differ from those of lizards. Because of their special eye structure, snakes are thought to have developed from lizards that burrowed underground. Their loss of legs is also thought to have occurred as a result of this burrowing phase.

There are about 2,700 species of snakes. The greatest variety dwell in the tropics. The largest snakes are the anaconda of South America and the reticulate python of Asia. Both may grow up to 30 feet (9 meters) long. One of the smallest snakes is the Braminy blind snake, which lives in the tropics and grows only 6 inches (15 centimeters) long.

Some snakes are poisonous. They have two hollow or grooved fangs in the upper jaw. The snakes inject venom (poison) through their fangs when they bite. About 15 percent of all snake species have venom that is harmful or fatal to human beings. About 25 species cause most of the deaths from snakebites. These include the Indian cobra of southern Asia, the black mamba and the saw-scaled viper of Africa, and the tiger snake of Australia.