Editing Endoskeleton (section)

{{Short description|Internal support structure of an animal}}[[File:Swordfish skeleton.jpg|thumb|upright=1.2|Endoskeleton of a [[swordfish]]]]
{{biomineralization sidebar|endoskeletons}}

An '''endoskeleton''' (From [[Ancient Greek]] ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is a [[structural frame]] ([[skeleton]]) — usually composed of [[mineralized tissues|mineralized tissue]] — on the inside of an [[animal]], overlaid by [[soft tissue]]s.<ref>{{Cite book |last=Hyman |first=Libbie Henrietta |url=https://books.google.com/books?id=VKlWjdOkiMwC |title=Hyman's Comparative Vertebrate Anatomy |date=1992-09-15 |publisher=University of Chicago Press |isbn=978-0-226-87013-7 |pages=192–236 |language=en}}</ref><ref>{{Citation |last=Gillis |first=J. Andrew |title=The Development and Evolution of Cartilage |date=2019 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128096338907702 |work=Reference Module in Life Sciences |access-date=2023-10-03 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-809633-8.90770-2 |isbn=978-0-12-809633-8|url-access=subscription }}</ref> Endoskeletons serve as [[structural support]] against [[gravity]] and [[mechanical load]]s, and provide anchoring attachment sites for [[skeletal muscle]]s to transmit force and allow movements and [[animal locomotion|locomotion]].
 
[[Vertebrate]]s and the closely related [[cephalochordate]]s are the predominant animal [[clade]] with endoskeletons (made of mostly [[bone]] and sometimes [[cartilage]], as well as [[notochord]]al [[glycoprotein]] and [[collagen fiber]]s), although [[invertebrate]]s such as [[sponge]]s also have evolved a form of "[[rebar]]" endoskeletons made of diffuse meshworks of [[calcite]]/[[silica]] [[structural element]]s called [[sponge spicule|spicule]]s, and [[echinoderm]]s have a [[dermal]] calcite endoskeleton known as [[ossicle (echinoderm)|ossicle]]s. Some [[coleoid]] [[cephalopod]]s ([[squid]]s and [[cuttlefish]]) have an internalized [[vestigial]] [[aragonite]]/calcite-[[chitin]] [[mollusc shell|shell]] known as [[gladius (cephalopod)|gladius]] or [[cuttlebone]], which can serve as muscle attachments but the main function is often to maintain [[buoyancy]] rather than to give structural support, and their body shape is largely maintained by [[hydroskeleton]].

Compared to the [[exoskeleton]]s of many invertebrates, endoskeletons allow much larger overall body sizes for the same skeletal [[mass]], as most soft tissues and [[organ (biology)|organ]]s are positioned ''outside'' the skeleton rather than within it, thus unrestricted by the volume and internal capacity of the skeleton itself. Being more centralized in structure also means more compact [[volume]], making it easier for the [[circulatory system]] to [[perfusion|perfuse]] and [[oxygenation (medicine)|oxygenate]], as well as higher [[tissue (biology)|tissue]] [[density]] against [[stress (mechanics)|stress]]. The external nature of muscle attachments also allows [[physiological cross-sectional area|thicker]] and more diverse [[muscle architecture]]s, as well as more versatile [[range of motion]]s.