Shells

Shells
A sturdy shell is the primary defense for a variety of very slowmoving animals, such as turtles, tortoises, snails, and clams.

Turtles and tortoises are reptiles with bodies enclosed in shells. Turtles spend much or all of their lives in water, while tortoises live on land. Both have shells made of two parts: an upper section called the carapace and a lower section called the plastron.

The shell is basically a sturdy box made of bone. The inside of the carapace is made of bones fused together. These bones include the turtle’s spine and ribs. The plastron is made of bone, too. In most species, the outside of the carapace is covered with plates made of a tough material called keratin—the same substance that forms hooves and fingernails. These plates are called scutes. Some turtles have just a few scutes embedded in a thick skin on the carapace. Some have none at all.

Many turtles can pull their heads, tails, and legs partly or fully into their shells. Box turtles have hinged plastrons, so they can close the openings in their shells. Desert tortoises fold their thick, scaly legs in front of their withdrawn heads to form a shield. A turtle can stay inside its shell for hours, waiting for a predator to give up. It will stay tucked in while a predator sniffs it or rolls it around.

Snails, clams, mussels, and other mollusks also are protected by shells. The soft, boneless body of a mollusk is covered with a kind of skin called a mantle. In the mantle are glands that produce the materials that form the shell. These materials include minerals that the mollusk gets from its food and from the water, sand, or soil in which it lives.

A snail seems to carry its shell on its back, but much of its body is actually inside the shell. If threatened, the snail pulls its head and its muscular foot inside the shell. Many kinds of snails seal the shell’s opening with a hard plate on the end of the foot. Sea snails called limpets have feet that work like suction cups and help them grip rocks firmly so that they are difficult to pry off.

Clams, oysters, and mussels are all bivalves: mollusks with two-part shells. A bivalve has a hinge between the halves of its shell. Depending on the species, it can partly or fully close its shell around its body. Many bivalves burrow deeply in sand or mud to stay hidden from predators, such as seabirds.

Mollusks called chitons have shells made of eight plates. A chiton clings to a rock with its wide, flat foot as it grazes on algae. It hangs on tightly enough to prevent being washed away by waves. If a predator manages to pull it off the rock, the chiton rolls up into an armor-plated ball.

In recent years, researchers discovered a snail living on the deep-sea floor that actually wears metal armor. This snail has not only a protective shell, but also extra-tough skin. Scales made of minerals, including iron, cover its soft body.