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==Classification== Skeletons can be defined by several attributes. Solid skeletons consist of hard substances, such as [[bone]], [[cartilage]], or [[cuticle]]. These can be further divided by location; internal skeletons are endoskeletons, and external skeletons are exoskeletons. Skeletons may also be defined by rigidity, where pliant skeletons are more elastic than rigid skeletons.<ref name="ruppert102">{{Harvnb|Ruppert|Fox|Barnes|2003|page=102}}.</ref> Fluid or [[hydrostatic skeleton]]s do not have hard structures like solid skeletons, instead functioning via pressurized fluids. Hydrostatic skeletons are always internal.<ref name="National Geographic-2021">{{Cite web |date=2021-10-19 |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=2022-07-31 |website=National Geographic |language=en}}</ref> ===Exoskeletons=== {{Main article|Exoskeleton}} [[Image:Hym-myrmicinae.gif|thumb|[[Exoskeleton]] of an [[ant]]]] An exoskeleton is an external skeleton that covers the body of an animal, serving as armor to protect an animal from predators. Arthropods have exoskeletons that encase their bodies, and have to undergo periodic [[moult]]ing or [[ecdysis]] as the animals grow. The [[Mollusc shell|shells]] of [[Mollusca|molluscs]] are another form of exoskeleton.<ref name="National Geographic-2021" /> Exoskeletons provide surfaces for the attachment of muscles, and specialized appendanges of the exoskeleton can assist with movement and defense. In arthropods, the exoskeleton also assists with [[Perception|sensory perception]].<ref name="Politi-2019" /> An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The [[southern giant clam]], a species of extremely large saltwater clam in the [[Pacific Ocean]], has a shell that is massive in both size and weight. ''[[Syrinx aruanus]]'' is a species of sea snail with a very large shell. ===Endoskeletons=== {{Main article|Endoskeleton}} [[Image:Eptesicus fuscus skeleton.jpg|thumb|[[Endoskeleton]] of a [[bat]]]] Endoskeletons are the internal support structure of an animal, composed of [[mineralized tissues]], such as the bone skeletons found in most vertebrates.<ref name="de Buffrénil et al. 2021">{{cite book |last1=de Buffrénil |first1=Vivian |last2=de Ricqlès |first2=Armand J |last3=Zylberberg |first3=Louise |last4=Padian |first4=Kevin |last5=Laurin |first5=Michel |last6=Quilhac |first6=Alexandra |title=Vertebrate skeletal histology and paleohistology |date=2021 |publisher=CRC Press |location=Boca Raton, FL |isbn=978-1351189576 |pages=xii + 825|url=https://books.google.com/books?id=tJcwEAAAQBAJ&dq=Vertebrate+Skeletal+Histology+and+Paleohistology&pg=PT8}}</ref> Endoskeletons are highly specialized and vary significantly between animals.<ref name="National Geographic-2021" /> They vary in complexity from functioning purely for support (as in the case of [[Porifera|sponges]]), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from [[germ layer#Mesoderm|mesodermal]] tissue. Endoskeletons occur in [[chordate]]s, echinoderms, and sponges. ===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). ===Hydrostatic skeletons=== {{Main article|Hydrostatic skeleton}} Hydrostatic skeletons are flexible cavities within an animal that provide structure through fluid pressure, occurring in some types of [[soft-bodied organism]]s, including jellyfish, [[Flatworm|flatworms]], [[Nematode|nematodes]], and earthworms. The walls of these cavities are made of muscle and connective tissue.<ref name="National Geographic-2021" /> In addition to providing structure for an animal's body, hydrostatic skeletons transmit the forces of muscle contraction, allowing an animal to move by alternating contractions and expansions of muscles along the animal's length.<ref>{{Cite journal |last=Kier |first=William M. |date=2012-04-15 |title=The diversity of hydrostatic skeletons |journal=Journal of Experimental Biology |volume=215 |issue=8 |pages=1247–1257 |doi=10.1242/jeb.056549 |pmid=22442361 |s2cid=1177498 |issn=0022-0949|doi-access=free |bibcode=2012JExpB.215.1247K }}</ref> ===Cytoskeleton=== {{Main article|Cytoskeleton}} The cytoskeleton (''cyto-'' meaning 'cell'<ref>"cyt- ''or'' cyto-". {{Harvnb|Mish|2003|page=312}}.</ref>) is used to stabilize and preserve the form of the cells. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion using structures such as [[flagella]], [[cilia]] and [[lamellipodia]], and transport within cells such as the movement of [[Vesicle (biology)|vesicles]] and [[organelle]]s, and plays a role in cellular division. The cytoskeleton is not a skeleton in the sense that it provides the structural system for the body of an animal; rather, it serves a similar function at the cellular level.<ref>{{Cite journal |last1=Fletcher |first1=Daniel A. |last2=Mullins |first2=R. Dyche |date=2010 |title=Cell mechanics and the cytoskeleton |journal=Nature |language=en |volume=463 |issue=7280 |pages=485–492 |doi=10.1038/nature08908 |pmid=20110992 |pmc=2851742 |bibcode=2010Natur.463..485F |issn=1476-4687}}</ref>
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