{{Short description|Hardened body part}} [[File:Monaco.Musée océanographique089.jpg|thumb|right|The exoskeleton of a spiny lobster is made of a series of sclerites, connected by flexible joints.]] A '''sclerite''' (Greek {{lang|el|σκληρός}}, ''{{lang|el|sklēros}}'', meaning "hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instead it refers most commonly to the hardened parts of arthropod exoskeletons and the internal spicules of invertebrates such as certain sponges and soft corals. In paleontology, a '''scleritome''' is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.
==Sclerites in combination== Sclerites may occur practically isolated in an organism, such as the sting of a cone shell. Also, they can be more or less scattered, such as tufts of defensive sharp, mineralised bristles as in many marine polychaetes. Or, they can occur as structured, but unconnected or loosely connected arrays, such as the mineral "teeth" in the radula of many Mollusca, the valves of chitons, the beak of a cephalopod, or the articulated exoskeletons of Arthropoda.
When sclerites are organised into an unarticulated structure, that structure may be referred to as a scleritome, a term largely used in paleontology.
==Arthropods== [[File:Parasite170028-fig10 Illustrated guide of Phlebotominae (Diptera, Psychodidae).png|thumb|left|Sclerites of cervix and thorax of Phlebotominae]] In Arthropoda, the hardening that produces sclerites is accomplished either by the cross-linking of protein chains in the exocuticle, a process called sclerotization, or by incorporation of minerals such as calcium carbonate into regions of the exoskeleton, or both. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by less-sclerotized, membranous regions or sutures.
Dorsal sclerites of a body segment, often plate-like, are known as ''tergites''. Similarly the ventral sclerites of a body segment are referred to as ''sternites''. Separate sclerites on the lateral aspects of body segments, the pleura, are called ''pleurites''.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=0-412-61390-5 }}</ref>
==Invertebrates other than arthropods== [[File:Journal.pone.0079155.g003 Clamps in various genera of Protomicrocotylidae.png|thumb|400 px|Sclerites in clamps in polyopisthocotylean monogeneans; various sclerites are in grey and black]]
Wide ranges of sclerites of various kinds occur in various invertebrate phyla, including Polychaeta and Mollusca. Two taxa that routinely have the term applied however, are the soft corals and the Porifera. In both those groups certain of their structures contain mineralised spicules of silica or calcium carbonate that are of importance structurally and in defense.
Many other invertebrates grow a few hard parts, largely mineralised, as statoliths and similar structures, but those are not generally referred to as sclerites.
Clamps, the main attachment structure of the parasitic polyopisthocotylean monogenean flatworms,<ref>Bychowsky, B. E. (1961). Monogenetic Trematodes their systematics and phylogeny. English translation edited by W. J. Hargis Jr. Washington: American Institute of Biological Sciences.</ref><ref>Kearn, G. C. (2004). Leeches, Lice and Lampreys. A natural history of skin and gill parasites of fishes. Dordrecht: Springer.</ref> are composed of various sclerites and associated musculature, located on a posterior organ called the haptor. Clamps are specialized structures attached to the host fish, generally to its gill.
==Prehistory== [[IMage:Wiwaxia Smithsonian.JPG|The fossilized scleritome of ''Wiwaxia corrugata''|thumb]] A '''scleritome''' is a skeleton made of isolated elements, like the scales of organisms such as the halwaxiids, the teeth in a radula,<ref name="Butterfield2008">{{Cite journal| doi = 10.1666/07-066.1| title = An Early Cambrian Radula| year = 2008| last1 = Butterfield | first1 = N. J.| journal = Journal of Paleontology| volume = 82| issue = 3| pages = 543–554| bibcode = 2008JPal...82..543B| s2cid = 86083492}}</ref> spicules in a sponge skeleton, or the elements in a conodont apparatus.<ref name="Bengtson1985">{{Cite journal | last1 = Bengtson | first1 = S. | title = Taxonomy of Disarticulated Fossils | jstor = 1304949 | journal = Journal of Paleontology | volume = 59 | issue = 6 | pages = 1350–1358 | year = 1985 }}</ref> The term was coined by the palaeontologist Stefan Bengtson.<ref name="Bengtson1985" />
Although sclerites are of considerable importance in the study of extant animals, in palaeontology they are of far greater relative importance because they often are the only parts of an animal that fossilize at all, let alone well or clearly. Many extinct groups are known only from sclerites, leaving moot the question of what their gross anatomy might have looked like.
An example of the use of the term in paleontology is to describe hollow calcium carbonate, calcium sulfate or calcium phosphate plates grown as body armor by a number of Early Cambrian animals. Unlike sponge spicules, Early Cambrian sclerites appear to be external armor rather than internal structural elements. Sclerites are found on a curious collection of early animals including a common spongelike animal called ''Chancelloria''; an armored slug-like form ''Wiwaxia''; an armored worm with a pair of brachiopod-like shells ''Halkieria''; and another armored worm ''Microdictyon'' that is generally considered to be a lobopod/onychophore.
It has been suggested that the sclerites of the Cambrian ''Wiwaxia'' are homologous with the bristles of annelid worms.<ref>{{cite journal | author = Butterfield, N. J. | year = 1990 | title = A Reassessment of the Enigmatic Burgess Shale Fossil ''Wiwaxia corrugata'' (Matthew) and Its Relationship to the Polychaete ''Canadia spinosa'' Walcott |journal = Paleobiology | volume = 16 | issue = 3 | pages = 287–303 | doi = 10.1017/S0094837300010009 | jstor=2400789| bibcode = 1990Pbio...16..287B | s2cid = 88100863 }}</ref> At least one modern gastropod mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.<ref>{{Cite journal |author=Anders Warén, Stefan Bengtson, Shana K. Goffredi & Cindy L. Van Dover |year=2003 |title=A hot-vent gastropod with iron sulfide dermal sclerites |journal=Science |volume=302 |issue=5647 |page=1007 |url=http://www.nrm.se/download/18.4e32c81078a8d9249800021562/Waren_etal_2003.pdf |pmid=14605361 |doi=10.1126/science.1087696 |s2cid=38386600 |archive-date=2012-03-09 |access-date=2010-12-05 |archive-url=https://web.archive.org/web/20120309170242/http://www.nrm.se/download/18.4e32c81078a8d9249800021562/Waren_etal_2003.pdf |url-status=dead }}</ref> [[File:Chrysomallon squamiferum 2.png|thumb|The scaly-foot gastropod is the only gastropod with sclerites.]]
==References== {{reflist}}
Category:Arthropod anatomy Category:Sponge anatomy Category:Mollusc anatomy