External skeleton

An exoskeleton (from Greek έξω, éxō "outer" and σκελετός, skeletos "skeleton") is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.

Exoskeletons contain rigid and resistant components that fulfill a set of functional roles including protection, excretion, sensing, support, feeding and acting as a barrier against desiccation in terrestrial organisms. Exoskeletons have a role in defense from pests and predators, support, and in providing an attachment framework for musculature.

Exoskeletons contain chitin; the addition of calcium carbonate makes them harder and stronger. Ingrowths of the arthropod exoskeleton known as apodemes serve as attachment sites for muscles. These structures are composed of chitin, and are approximately six times as strong and twice as stiff as vertebrate tendons. Similar to tendons, apodemes can stretch to store elastic energy for jumping, notably in locusts.

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