A turtle is the only reptile with a shell. Most kinds of turtles can pull their head, legs, and tail into their shell, which serves as a suit of armor. Few other backboned animals have such excellent natural protection.

There are more than 250 species of turtles, about 50 of which live in North America north of Mexico. Some turtles live only on land, but others spend almost their entire life in the sea. They live almost everywhere else--in deserts, forests, grasslands, lakes, marshes, ponds, rivers, and the sea.

Turtles vary greatly in size. The largest turtle species, the leatherback turtle, grows from 4 to 8 feet (1.2 to 2.4 meters) long. But the common bog turtle measures only about 4 inches (10 centimeters) in length.