A solid or fluid system which allows muscles to relax after contracting (in general, because there is an opposing muscle which pulls the skeletal part in the opposite direction when it contracts).

The skeletal system may also be a support structure or a form of protection. Types of skeletons include hydroskeletons (consisting of water-filled body cavities, found in many soft-bodied invertebrates) and exoskeletons (supports the organism's body from the outside, found in all arthropods).

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