Editing Cnidaria (section)

===Skeletons===
In medusae, the only supporting structure is the [[mesoglea]]. ''[[hydra (genus)|Hydra]]'' and most [[sea anemone]]s close their mouths when they are not feeding, and the [[water]] in the digestive cavity then acts as a [[hydrostatic skeleton]], rather like a water-filled balloon. Other polyps such as ''[[Tubularia]]'' use columns of water-filled cells for support. [[Sea pen]]s stiffen the mesoglea with [[calcium carbonate]] [[wikt:spicule|spicule]]s and tough fibrous [[protein]]s, rather like [[sponge]]s.<ref name="Ruppert" />

In some colonial polyps, a [[chitin]]ous [[Epidermis (zoology)|epidermis]] gives support and some protection to the connecting sections and to the lower parts of individual polyps. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with [[sediment]] particles.<ref name="Ruppert" />

A mineralized [[exoskeleton]] made of calcium carbonate is found in subphylum Anthozoa in the order [[Scleractinia]] (stony corals; class Hexacorallia) and the class [[Octocorallia]],<ref>[https://academic.oup.com/gbe/article/12/9/1623/5882021?login=false Comparative Proteomics of Octocoral and Scleractinian Skeletomes and the Evolution of Coral Calcification]</ref> and in subphylum Medusozoa in three [[hydrozoa]]n families in order [[Anthoathecata]]; [[Fire coral|Milleporidae]], [[Stylasteridae]] and [[Hydractiniidae]] (the latter with a mix of calcified and uncalcified species).<ref>[https://academic.oup.com/icb/article/50/3/428/621249?login=false Evolution of Calcium-carbonate Skeletons in the Hydractiniidae]</ref>