Editing Skeleton (section)

===Rigidity===

Pliant skeletons are capable of movement; thus, when [[Stress (mechanics)|stress]] is applied to the skeletal structure, it deforms and then regains its original shape. This skeletal structure is used in some invertebrates, for instance in the hinge of [[bivalve shell]]s or the [[mesoglea]] of [[cnidarians]] such as [[jellyfish]]. Pliant skeletons are beneficial because only [[muscle]] contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape. [[Cartilage]] is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of [[proteins]], [[polysaccharides]], and water.<ref name="ruppert102" /> For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton.<ref>{{Harvnb|Pechenik|2015}}.{{page number needed|date=August 2021}}</ref>

Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in [[terrestrial animals]]. Such a skeleton type used by animals that live in water are more for protection (such as [[barnacle]] and [[snail]] shells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Rigid skeletons are formed from materials including [[chitin]] (in arthropods), [[calcium]] compounds such as [[calcium carbonate]] (in [[stony coral]]s and [[mollusk]]s) and [[silicate]] (for [[diatom]]s and [[radiolarian]]s).