Asclerite (Greekσκληρός,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 ofarthropodexoskeletons and the internalspicules of invertebrates such as certainsponges andsoft corals. Inpaleontology, ascleritome is the complete set of sclerites of an organism, often all that is known from fossil invertebrates.
Sclerites may occur practically isolated in an organism, such as thesting of acone shell. Also, they can be more or less scattered, such as tufts of defensive sharp, mineralised bristles as in many marinepolychaetes. Or, they can occur as structured, but unconnected or loosely connected arrays, such as the mineral "teeth" in theradula of manyMollusca, thevalves ofchitons, the beak of acephalopod, 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 inpaleontology.
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InArthropoda, the hardening that produces sclerites is accomplished either by the cross-linking of protein chains in theexocuticle, a process calledsclerotization, 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 astergites. Similarly theventral sclerites of a body segment are referred to assternites. Separate sclerites on the lateral aspects of body segments, thepleura, are calledpleurites.[1]
Wide ranges of sclerites of various kinds occur in various invertebratephyla, includingPolychaeta andMollusca. Twotaxa that routinely have the term applied however, are thesoft corals and thePorifera. In both those groups certain of their structures contain mineralised spicules ofsilica orcalcium carbonate that are of importance structurally and in defense.
Many other invertebrates grow a few hard parts, largely mineralised, asstatoliths and similar structures, but those are not generally referred to as sclerites.
Clamps, the main attachment structure of the parasiticpolyopisthocotyleanmonogenean flatworms,[2][3] are composed of various sclerites and associated musculature, located on a posterior organ called thehaptor. Clamps are specialized structures attached to the host fish, generally to its gill.
Ascleritome is a skeleton made of isolated elements, like the scales of organisms such as thehalwaxiids, the teeth in a radula,[4]spicules in a sponge skeleton, or the elements in aconodont apparatus.[5] The term was coined by the palaeontologistStefan Bengtson.[5]
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 orcalcium phosphate plates grown as body armor by a number of EarlyCambrian animals. Unlikesponge 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 calledChancelloria; an armored slug-like formWiwaxia; an armored worm with a pair ofbrachiopod-like shellsHalkieria; and another armored wormMicrodictyon that is generally considered to be alobopod/onychophore.
It has been suggested that the sclerites of the CambrianWiwaxia are homologous with the bristles ofannelid worms.[6]At least one moderngastropod mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.[7]
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