An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. 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.
In-growths of the arthropod exoskeleton known as antipodes serve as attachment sites for muscles. These structures are composed of chitin, and are approximately 6 times as strong and twice as stiff as vertebrate tendons. Similar to tendons, antipodes can stretch to store elastic energy for jumping, notably in locusts.
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