Sclerite

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 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. 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 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 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 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. 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. 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, 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. A scleritome is a skeleton made of isolated elements, like the scales of organisms such as the halwaxiids, the teeth in a radula, spicules in a sponge skeleton, or the elements in a conodont apparatus. The term was coined by the palaeontologist Stefan Bengtson.