View source for Skeleton

{{Short description|Part of the body that forms the supporting structure}}
{{about|skeletons in general|the human skeleton|Human skeleton|other uses}}
{{pp-vandalism|small=yes}}
{{Use dmy dates|date=October 2018}}
{{Infobox anatomy
|Name = Skeleton
|Greek = σκελετός
|Image = Horse and Man.jpg
|Caption = A [[Skeletal system of the horse|horse]] and [[human skeleton]] placed in a display at  [[Australian Museum]] in Sydney
|Image2 =
}}
A '''skeleton''' is the [[structural frame]] that supports the body of most [[animal]]s. There are several types of skeletons, including the [[exoskeleton]], which is a rigid outer shell that holds up an organism's shape; the [[endoskeleton]], a rigid internal frame to which the [[organ (biology)|organ]]s and [[soft tissue]]s attach; and the [[hydroskeleton]], a flexible internal structure supported by the [[hydrostatic pressure]] of [[body fluid]]s.

[[Vertebrate]]s are animals with an endoskeleton centered around an [[Axial skeleton|axial]] [[vertebral column]], and their skeletons are typically composed of [[bone]]s and [[cartilage]]s. [[Invertebrate]]s are other animals that lack a vertebral column, and their skeletons vary, including hard-shelled exoskeleton ([[arthropod]]s and most [[mollusc]]s), plated internal shells (e.g. [[cuttlebone]]s in some [[cephalopod]]s) or rods (e.g. [[ossicle (echinoderm)|ossicle]]s in [[echinoderm]]s), hydrostatically supported body [[Body cavity|cavities]] (most), and [[sponge spicule|spicule]]s ([[sponge]]s). Cartilage is a rigid [[connective tissue]] that is found in the skeletal systems of vertebrates and invertebrates.

== Etymology ==
The term ''skeleton'' comes {{etymology|grc|''{{wikt-lang|grc|σκελετός}}'' ({{grc-transl|σκελετός}})|dried up}}.<ref>"skeleton". {{Harvnb|Mish|2003|page=1167}}.</ref> ''Sceleton'' is an archaic form of the word.<ref>{{Cite web |title=Definition of SCELETON |url=https://www.merriam-webster.com/dictionary/sceleton |access-date=2022-07-31 |website=www.merriam-webster.com |language=en}}</ref>

==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>

== Vertebrate skeletons ==
[[File:Huxley - Mans Place in Nature.jpg|thumb|'''Pithecometra''': From [[Thomas Huxley]]'s 1863 ''[[Evidence as to Man's Place in Nature]]'', the compared skeletons of apes to humans.]]

Vertebrate skeletons are endoskeletons, and the main skeletal component is bone.<ref name="de Buffrénil et al. 2021" /> Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in [[mammal]]s is found mainly in the joint areas. In other animals, such as the [[Chondrichthyes|cartilaginous fishes]], which include the [[shark]]s, the skeleton is composed entirely of [[cartilage]]. The [[segmentation (biology)|segmental]] pattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage.<ref>{{cite journal |last1=Billet |first1=Guillaume |last2=Bardin |first2=Jérémie |title=Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals |journal=Systematic Biology |date=13 October 2021 |volume=70 |issue=6 |pages=1101–1109 |doi=10.1093/sysbio/syab007 |pmid=33560370 |url=https://doi.org/10.1093/sysbio/syab007}}</ref><ref>{{cite journal |last1=Buffrénil |first1=Vivian de |last2=Quilhac |first2=Alexandra |title=An Overview of the Embryonic Development of the Bony Skeleton |journal=Vertebrate Skeletal Histology and Paleohistology |date=2021 |pages=29–38 |doi=10.1201/9781351189590-2 |url=https://www.taylorfrancis.com/chapters/edit/10.1201/9781351189590-2/overview-embryonic-development-bony-skeleton-vivian-de-buffr%C3%A9nil-alexandra-quilhac |publisher=CRC Press|isbn=978-1351189590 |s2cid=236422314 |url-access=subscription }}</ref>

Bones are rigid [[organ (biology)|organs]] providing structural support for the body, assistance in movement by opposing [[muscle contraction]], and the forming of a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as [[hydroxyapatite]], while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are [[osteoblast]]s, [[osteoclast]]s, or [[osteocyte]]s.<ref>{{Cite journal |last1=Sommerfeldt |first1=D. |last2=Rubin |first2=C. |date=2001-10-01 |title=Biology of bone and how it orchestrates the form and function of the skeleton |url=https://doi.org/10.1007/s005860100283 |journal=European Spine Journal |language=en |volume=10 |issue=2 |pages=S86–S95 |doi=10.1007/s005860100283 |issn=1432-0932 |pmc=3611544 |pmid=11716022}}</ref>

Bone tissue is a type of dense [[connective tissue]], a type of [[mineralized tissue]] that gives rigidity and a [[Honeycomb (geometry)|honeycomb-like]] three-dimensional internal structure. Bones also produce [[red blood cell|red]] and [[white blood cell]]s and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include [[bone marrow]], [[endosteum]] and [[periosteum]], [[nerve]]s, [[blood vessel]]s and cartilage.

During [[embryonic development]], bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone.<ref>{{Citation |last1=Lefebvre |first1=Véronique |title=Chapter Eight - Vertebrate Skeletogenesis |date=2010-01-01 |journal=Current Topics in Developmental Biology |volume=90 |pages=291–317 |editor-last=Koopman |editor-first=Peter |series=Organogenesis in Development |publisher=Academic Press |language=en |last2=Bhattaram |first2=Pallavi|doi=10.1016/S0070-2153(10)90008-2 |pmid=20691853 |pmc=3077680 }}</ref> Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in [[perichondrium]] tissue.<ref name="Gillis-2019">{{Citation |last=Gillis |first=J. Andrew |title=The Development and Evolution of Cartilage |date=2019 |url=https://www.sciencedirect.com/science/article/pii/B9780128096338907702 |work=Reference Module in Life Sciences |publisher=Elsevier |language=en |isbn=978-0-12-809633-8}}</ref> [[Ligament]]s are elastic tissues that connect bones to other bones, and [[tendon]]s are elastic tissues that connect muscles to bones.<ref>{{Cite encyclopedia |title=Tendon vs. Ligament |encyclopedia=A.D.A.M. Medical Encyclopedia |publisher=Ebix |url=https://medlineplus.gov/ency/imagepages/19089.htm |access-date=2021-08-06 |last=Vorvick |first=Linda J. |date=2020-08-13 |via=MedLinePlus}}</ref>

=== Amphibians and reptiles ===
The skeletons of turtles have evolved to develop a [[Turtle shell|shell]] from the ribcage, forming an exoskeleton.<ref>{{Cite journal |last1=Nagashima |first1=Hiroshi |last2=Kuraku |first2=Shigehiro |last3=Uchida |first3=Katsuhisa |last4=Kawashima-Ohya |first4=Yoshie |last5=Narita |first5=Yuichi |last6=Kuratani |first6=Shigeru |date=2012-03-01 |title=Body plan of turtles: an anatomical, developmental and evolutionary perspective |url=https://doi.org/10.1007/s12565-011-0121-y |journal=Anatomical Science International |language=en |volume=87 |issue=1 |pages=1–13 |doi=10.1007/s12565-011-0121-y |pmid=22131042 |s2cid=41803725 |issn=1447-073X|url-access=subscription }}</ref> The skeletons of [[snake]]s and [[caecilian]]s have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.<ref>{{Cite journal |last=M. Woltering |first=Joost |date=2012-06-01 |title=From Lizard to Snake; Behind the Evolution of an Extreme Body Plan |journal=Current Genomics |volume=13 |issue=4 |pages=289–299 |doi=10.2174/138920212800793302|pmid=23204918 |pmc=3394116 }}</ref>

=== Birds ===
{{Further|Bird anatomy#Skeletal system}}The skeletons of birds are adapted for [[Flying and gliding animals|flight]]. The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thin [[humeral shaft]] and the fusion of skeletal elements into single [[ossification]]s.<ref>{{Cite journal |last=Dumont |first=Elizabeth R. |date=2010-07-22 |title=Bone density and the lightweight skeletons of birds |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1691 |pages=2193–2198 |doi=10.1098/rspb.2010.0117 |pmc=2880151 |pmid=20236981}}</ref> Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true [[jaw]], instead having evolved a [[beak]], which is far more lightweight. The beaks of many baby birds have a projection called an [[egg tooth]], which facilitates their exit from the amniotic egg.

=== Fish ===
{{Further|Fish anatomy#Skeleton|Fish bone}}The skeleton, which forms the support structure inside the fish is either made of cartilage as in the [[Chondrichthyes]], or bones as in the [[Osteichthyes]]. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, the [[fish fin|fins]], are composed of either bony or soft spines called rays which, with the exception of the caudal fin (tail fin), have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.

Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming.<ref name="National Geographic-2021" />

=== Mammals ===

====Marine mammals====
[[File:Zalophus californianus 01.JPG|thumb|left|[[Californian sea lion]]]]
To facilitate the movement of [[marine mammal]]s in water, the hind legs were either lost altogether, as in the whales and [[manatee]]s, or united in a single [[fish fin|tail fin]] as in the [[pinnipeds]] (seals). In the whale, the [[cervical vertebrae]] are typically fused, an adaptation trading flexibility for stability during swimming.<ref>{{cite journal |last1=Bebej |first1=Ryan M |last2=Smith |first2=Kathlyn M |title=Lumbar mobility in archaeocetes (Mammalia: Cetacea) and the evolution of aquatic locomotion in the earliest whales |journal=Zoological Journal of the Linnean Society |date=17 March 2018 |volume=182 |issue=3 |pages=695–721 |doi=10.1093/zoolinnean/zlx058 |url=https://academic.oup.com/zoolinnean/article/182/3/695/4554339?login=true |access-date=7 March 2022 |issn=0024-4082|url-access=subscription }}</ref>

====Humans====
{{Main article|Human skeleton}}
[[File:Leonardo Skeleton 1511.jpg|thumb|''Study of Skeletons'', {{Circa|1510}}, by [[Leonardo da Vinci]]]]

The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, [[muscle]]s and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, [[human lung|lungs]], [[human heart|heart]] and [[spinal cord]].<ref>{{Cite news |title=Skeletal System: Facts, Function & Diseases |work=Live Science |url=http://www.livescience.com/22537-skeletal-system.html |url-status=live |access-date=7 March 2017 |archive-url=https://web.archive.org/web/20170307123921/http://www.livescience.com/22537-skeletal-system.html |archive-date=7 March 2017}}</ref> The biggest bone in the body is the [[femur]] in the upper leg, and the smallest is the [[stapes]] bone in the [[middle ear]]. In an adult, the skeleton comprises around 13.1% of the total body weight,<ref>{{Harvnb|Reynolds|Karlotski|1977|page=161}}</ref> and half of this weight is water.

Fused bones include those of the [[Human pelvis|pelvis]] and the [[Human skull|cranium]]. Not all bones are interconnected directly: There are three bones in each [[middle ear]] called the [[ossicles]] that articulate only with each other. The [[hyoid bone]], which is located in the neck and serves as the point of attachment for the [[tongue]], does not articulate with any other bones in the body, being supported by muscles and ligaments.

There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the [[hip bone]]s on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variable [[wormian bone]]s between skull sutures. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together.{{citation needed|date=August 2021}} These bones are organized into a longitudinal axis, the [[axial skeleton]], to which the [[appendicular skeleton]] is attached.<ref name="tozeren6-10">{{Harvnb|Tözeren|2000|pages=6–10}}.</ref>

The human skeleton takes 20 years before it is fully developed, and the bones contain [[Bone marrow|marrow]], which produces blood cells.

There exist several general differences between the male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis.<ref name="Balaban-61">{{Harvnb|Balaban|2008|page=61}}</ref><ref name="stein73">{{Harvnb|Stein|2007|page=73}}.</ref>

==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|url-access=subscription }}</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|url-access=subscription }}</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|url-access=subscription }}</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 |url-access=subscription }}</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>

==Cartilage==
{{Main article|Cartilage}}

Cartilage is a connective skeletal tissue composed of specialized cells called [[chondrocyte]]s that in an [[extracellular matrix]]. This matrix is typically composed of Type II [[collagen]] fibers, [[proteoglycan]]s, and water. There are many types of cartilage, including [[elastic cartilage]], [[hyaline cartilage]], [[fibrocartilage]], and lipohyaline cartilage.<ref name="Gillis-2019" /> Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

==See also==
{{div col}}
* [[Bonesetter]]
* [[Endochondral ossification]]
* [[Intramembranous ossification]]
* [[Exoskeleton]]
* [[Osteoblast]]
* [[Osteometric points]]
* [[Skeletal system of the horse]]
* [[Skeleton (undead)]]
* [[Skeletonization]]
* [[Stolon]] 
{{div col end}}

==References==
{{reflist}}

==Bibliography==
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* {{Cite journal|last1=Forbes|first1=R. M.|last2=Mitchell|first2=H. H.|last3=Cooper|first3=A. R.|url=https://www.jbc.org/action/showPdf?pii=S0021-9258%2818%2965095-1|year=1956|journal=Journal of Biological Chemistry|doi=10.1016/S0021-9258(18)65095-1|publisher=American Society for Biochemistry and Molecular Biology|title=Further studies on the gross composition and mineral elements of the adult human body
|pmid=13385244|pages=969–975|volume=223|number=2|doi-access=free}}
* {{Cite book |editor-last=Mish |editor-first=Frederick C.|title=Merriam-Webster's Collegiate Dictionary |publisher=Merriam-Webster |year=2003 |isbn=978-0-87779-807-1|edition=11th}}
* {{Cite journal |last=Nasoori |first=Alireza |year=2020 |title=Formation, structure, and function of extra-skeletal bones in mammals |url=https://archive.org/details/formation-structure-and-function-of-extra-skeletal-bones-in-mammals |journal=Biological Reviews |publisher=Cambridge Philosophical Society |volume=95 |issue=4 |pages=986–1019 |doi=10.1111/brv.12597 |pmid=32338826 |s2cid=216556342}}
* {{Cite book |last=Pechenik |first=Jan A. |title=Biology of the Invertebrates |publisher=McGraw-Hill Education |year=2015 |isbn=978-0-07-352418-4 |edition=7th}}
* {{Cite journal |author1-last=Reynolds|author1-first=William W. |author2-last=Karlotski|author2-first=William J.|year=1977 |title=The Allometric Relationship of Skeleton Weight to Body Weight in Teleost Fishes: A Preliminary Comparison with Birds and Mammals |journal=Copeia|volume=1977 |number=1 |pages=160–163 |doi=10.2307/1443520 |jstor=1443520 |publisher=American Society of Ichthyologists and Herpetologists}}
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* {{Cite book |last=Stein |first=Lisa |url=https://archive.org/details/bodycompletehuma0000unse |title=Body: The Complete Human |publisher=[[National Geographic Society]] |year=2007 |isbn=978-1-4262-0128-8 |url-access=registration}}
* {{Cite book |last=Tözeren |first=Aydın |title=Human Body Dynamics: Classical Mechanics and Human Movement |publisher=Springer |year=2000 |isbn=0-387-98801-7}}

== External links ==
{{Commons category-inline|Skeletons}}
{{Wikibooks|Anatomy and Physiology of Animals|The Skeleton}}
* [http://umorf.ummp.lsa.umich.edu/wp/wp-content/3d/bonePicker.html?name=Buesching 3-D Viewer] of a male [[American mastodon]] skeleton, with bones labelled, at the [[University of Michigan Museum of Natural History|University of Michigan]] [https://umorf.ummp.lsa.umich.edu/wp/mammutidae2/ Mammutidae digital fossil repository]
* [http://eskeletons.org/ Interactive views] of various [[primate]] skeletons at eSkeletons.org (associated with the [[University of Texas at Austin]])

{{Authority control}}

[[Category:Skeletons| ]]
[[Category:Animal anatomy]]