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A sclerite (Greek σκληρός, 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 Porifera and soft corals. Used as a universal term of convenience, the word is vague and takes different meanings according to various needs. For example, the trend not to refer to say, vertebrate teeth as sclerites, is arbitrary, and the so-called sclerites of sponges have very little to do with the sclerites of arthropods.
Sclerites in combination
Sclerites may occur practically isolated in an organism, such as the sting of a cone shell or they can occur 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, or the valves of Chitons. When sclerites are organised into such an unarticulated structure, that structure may be referred to as a Scleritome, a term largely used in paleontology.
Some sclerites are articulated into structures such as the beaks of Cephalopoda, or the articulated exoskeletons of Arthropoda.
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.
As a matter of practical convenience the shapes and arrangement of the different sclerites provide most of the morphological features that are used as characters when reconstructing the phylogenetic relationships among different lineages; they provide the foundation for arthropod systematics, primarily through the ability of the systematist to assess the homologies of the different sclerites, as to how they may be lost, gained, fused, divided, or otherwise modified from one lineage to another. For example, the sclerite called the mandible in insects varies dramatically in form between orders, but all the different variations are homologous. However, it is not always equally easy to establish homologies based on such resemblances; they often lead to unreliable and debatable conclusions. Application of molecular biology to compare nucleic acid sequences is seen as the definitive basis for determining homologies and relationships in modern taxonomy.
It should be clearly understood however, that though nucleic acid evidence might be definitive for establishing relationships, it does not follow that morphological description is any the less important for day-to-day purposes of description or identification. One cannot take every field specimen to the genomics laboratory for analysis, especially considering that an appropriately qualified field worker can recognise most relevant organisms on sight on the basis of external appearance, or with the help of a dissecting microscope or similarly undemanding equipment. The two fields are complementary, not in conflict.
Invertebrates other than arthropods
Many other invertebrata grow a few hard parts, largely mineralised, as statoliths, and similar structures, but those are not generally referred to as sclerites. 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.
Clamps, which are are the main attachment structure of the Polyopisthocotylean monogeneans include sclerites and muscles. Clamps are specialized structures attached to the host fish, generally to its gill.
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 fossilise 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. At least one modern gastropod mollusc living near deep sea hydrothermal vents has structures made of iron sulfides similar to some Cambrian sclerites.
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