Editing Skeleton (section)

==Invertebrate skeletons==
Invertebrates are defined by a lack of vertebral column, and they do not have bone skeletons. Arthropods have exoskeletons and echinoderms have endoskeletons. Some soft-bodied organisms, such as [[jellyfish]] and [[earthworm]]s, have hydrostatic skeletons.<ref>{{cite news |last1=Langley |first1=Liz |title=Why animals developed four types of skeletons |url=https://www.nationalgeographic.com/animals/article/why-animals-developed-four-types-of-skeletons |archive-url=https://web.archive.org/web/20211019163244/https://www.nationalgeographic.com/animals/article/why-animals-developed-four-types-of-skeletons |url-status=dead |archive-date=19 October 2021 |access-date=November 1, 2022 |publisher=National Geographic |date=October 19, 2021}}</ref>

=== Arthropods ===
{{Main|Arthropod exoskeleton}}

The skeletons of [[arthropod]]s, including [[insect]]s, [[crustacean]]s, and [[arachnid]]s, are cuticle exoskeletons. They are composed of [[chitin]] secreted by the [[epidermis]].<ref>{{Cite journal |last=Vincent |first=Julian F. V. |date=2002-10-01 |title=Arthropod cuticle: a natural composite shell system |url=https://www.sciencedirect.com/science/article/pii/S1359835X02001677 |journal=Composites Part A: Applied Science and Manufacturing |language=en |volume=33 |issue=10 |pages=1311–1315 |doi=10.1016/S1359-835X(02)00167-7 |issn=1359-835X}}</ref> The cuticle covers the animal's body and lines several internal organs, including parts of the digestive system. Arthropods molt as they grow through a process of [[ecdysis]], developing a new exoskeleton, digesting part of the previous skeleton, and leaving the [[Exuviae|remainder]] behind. An arthropod's skeleton serves many functions, working as an [[integument]] to provide a barrier and support the body, providing appendages for movement and defense, and assisting in sensory perception. Some arthropods, such as crustaceans, absorb biominerals like calcium carbonate from the environment to strengthen the cuticle.<ref name="Politi-2019">{{Citation |last1=Politi |first1=Yael |title=Mechanics of Arthropod Cuticle-Versatility by Structural and Compositional Variation |date=2019 |url=https://doi.org/10.1007/978-3-030-11942-3_10 |work=Architectured Materials in Nature and Engineering: Archimats |pages=287–327 |editor-last=Estrin |editor-first=Yuri |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-11942-3_10 |isbn=978-3-030-11942-3 |last2=Bar-On |first2=Benny |last3=Fabritius |first3=Helge-Otto |series=Springer Series in Materials Science |volume=282 |s2cid=109418804 |editor2-last=Bréchet |editor2-first=Yves |editor3-last=Dunlop |editor3-first=John |editor4-last=Fratzl |editor4-first=Peter}}</ref>

=== Echinoderms ===
The skeletons of [[echinoderm]]s, such as [[starfish]] and [[sea urchin]]s, are endoskeletons that consist of large, well-developed [[sclerite]] plates that adjoin or overlap to cover the animal's body. The skeletons of [[sea cucumber]]s are an exception, having a reduced size to assist in feeding and movement. Echinoderm skeletons are composed of [[stereom]], made up of [[calcite]] with a [[monocrystal]] structure. They also have a significant [[magnesium]] content, forming up to 15% of the skeleton's composition. The stereome structure is porous, and the pores fill with connective [[Stromal cell|stromal]] tissue as the animal ages. Sea urchins have as many as ten variants of stereome structure. Among extant animals, such skeletons are unique to echinoderms, though similar skeletons were used by some [[Paleozoic]] animals.<ref>{{Cite journal |last1=Kokorin |first1=A. I. |last2=Mirantsev |first2=G. V. |last3=Rozhnov |first3=S. V. |date=2014-12-01 |title=General features of echinoderm skeleton formation |url=https://doi.org/10.1134/S0031030114140056 |journal=Paleontological Journal |language=en |volume=48 |issue=14 |pages=1532–1539 |doi=10.1134/S0031030114140056 |bibcode=2014PalJ...48.1532K |s2cid=84336543 |issn=1555-6174}}</ref> The skeletons of echinoderms are [[mesoderm]]al, as they are mostly encased by soft tissue. Plates of the skeleton may be interlocked or connected through muscles and ligaments. Skeletal elements in echinoderms are highly specialized and take many forms, though they usually retain some form of symmetry. The spines of sea urchins are the largest type of echinoderm skeletal structure.<ref>{{Citation |last1=Nebelsick |first1=James H. |title=Echinoderms: Hierarchically Organized Light Weight Skeletons |date=2015 |url=https://doi.org/10.1007/978-94-017-9398-8_8 |work=Evolution of Lightweight Structures: Analyses and Technical Applications |pages=141–155 |editor-last=Hamm |editor-first=Christian |place=Dordrecht |publisher=Springer Netherlands |language=en |doi=10.1007/978-94-017-9398-8_8 |isbn=978-94-017-9398-8 |access-date=2022-07-31 |last2=Dynowski |first2=Janina F. |last3=Grossmann |first3=Jan Nils |last4=Tötzke |first4=Christian|series=Biologically-Inspired Systems |volume=6 }}</ref>

=== Molluscs ===
Some molluscs, such as conchs, scallops, and snails, have shells that serve as exoskeletons. They are produced by proteins and minerals secreted from the animal's [[Mantle (mollusc)|mantle]].<ref name="National Geographic-2021" />

=== Sponges ===
The skeleton of [[sponge]]s consists of microscopic [[calcareous]] or [[siliceous]] [[Sponge spicule|spicules]]. The [[demosponge]]s include 90% of all species of sponges. Their "skeletons" are made of spicules consisting of fibers of the protein [[spongin]], the mineral [[silica]], or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar [[Hexactinellida|glass sponges]].<ref>{{Harvnb|Barnes|Fox|Barnes|2003|pages=105–106}}.</ref>