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An exoskeleton (from Greek Î­Î¾Ï, Ã©xÅ outer and ÏÎºÎµÎ»ÎµÏÏÏ, 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.
An exostosis in simple terms is an extra piece of formed bone. Generally this is an outgrowth of an existing bone of the foot or ankle. The exostosis may also break off the existing bone to become a free fragment or a loose body within or around a joint.