Exoskeleton

An exoskeleton is a skeleton on the outside of the body, in contrast to the internal endoskeleton of vertebrates. In arthropods, the group that includes insects, spiders, centipedes and crustaceans, it is a cuticle secreted by the layer of epidermal cells beneath it. Its main component is chitin, a tough, flexible polysaccharide built from linked sugar units, combined with proteins. A thin waxy outer layer makes it waterproof, and in crabs and lobsters calcium salts such as calcium carbonate make it harder.

The exoskeleton protects against injury, predators and drying out, which is especially valuable on land. It also provides the levers for movement: muscles attach to its inner surface and pull across flexible joints between rigid plates. Because the cuticle is not living tissue, it cannot grow with the animal, so arthropods must molt. Under hormonal control, the epidermis builds a new soft cuticle beneath the old one, enzymes digest the inner part of the old layer, and the animal sheds the rest, a step called ecdysis. Until the new cuticle hardens, the animal is soft and vulnerable.

Other animals have external skeletons of a sort, such as the shells of mollusks and the limestone skeletons of reef-building corals.