A turtle's shell structure consists of two separate pieces—an upper shell and a bottom shell—joined at the sides by a bony bridge. The upper shell, called the carapace, may be highly domed, low and rounded, or flattish on top with steep sides. The bottom shell, or plastron, is flat or slightly concave. In most turtles the two shells are each composed of an inner layer of bone fused to the ribs and other bony structures of the animal's body, and an outer layer of horn. Both layers are made up of sections that fit together like pieces of a mosaic. However, the sections of the two layers do not coincide. One species of sea turtle has a leathery covering of skin instead of horn over the bony layer.

How Does That Shell Work?

Most kinds of turtles can pull their head, legs, and tail into their shell, which serves as a suit of armor. Few other animals with a backbone have such natural protection.

The shell covering the back of a box turtle is round. This makes a box turtle look a little like a stocky lizard carrying an upside-down salad bowl on its back. The box turtle’s shell is covered in horny plates that fit together in a geometric pattern something like a quilt.

Box turtles are different from many other turtles because they have a hinge on the bottom of their shell. They can pull their legs, head, and tail into their shell and then use this hinge to close up and “box” themselves inside.

How Do Box Turtles “Box” Themselves In?

Box turtles are small pond and marsh turtles that live in North America. They’re called box turtles with good reason. If an enemy such as a rat comes near one of these turtles, it can “box” itself up inside its shell.

No other turtles have shells quite like these land-dwellers. A box turtle’s plastron is hinged. This lets the turtle bring the plastron right up against the carapace. Once the box turtle tucks itself in, it can close its shell up tight.

A box turtle lives most of its long life in one area of land. That area is called its home range. A box turtle’s home range is usually not much bigger than two American football fields. There, the turtle can find everything it needs, such as water and soft soil for nesting. A home range also has plenty of worms, insects, and berries for the turtle to eat.

Turtles cannot expand and contract their lungs by rib movements, because the ribs are held rigid by fusion with the shell. Muscles in the turtle's limbs contract and enlarge the lung capacity, drawing in air. Abdominal muscles contract and force the lungs to expel air. In some turtles a pumping action of throat muscles aids in breathing. Some aquatic turtles also respire by getting a little oxygen from water through the mucous membranes.

The head, limbs, and tail of the turtle protrude from the shell, but—in most species—can be drawn inside the shell for protection. Some turtles can move hinged parts of the shell to enclose the softer parts of their bodies completely. Most turtles have strong horn-covered jaws and beaks with which to tear their food into bits. Turtle skin is tough and often covered with scales. Freshwater turtles have webbed feet, and marine species have flippers.

The carapace may be rough or smooth, and the plastron is usually smooth. Carapaces vary in color from greenish brown to almost black. Plastrons are paler and often yellow. Turtles' shells may be marked with dark or light circular lines and with bright markings of red, orange, or yellow. The skin may be brightly marked also.