Exoskeleton
An exoskeleton is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton which is enclosed underneath other soft tissues. Some large, hard and non-flexible protective exoskeletons are known as shell or armour. Examples of exoskeletons in animals include the cuticle skeletons shared by arthropods and tardigrades, as well as the skeletal cups formed by hardened secretion of stony corals and the prominent mollusc shell shared by snails, clams, tusk shells, chitons and nautilus. Some vertebrate animals, such as the turtle, have both an endoskeleton and a protective exoskeleton.
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