Sclerites

A sclerite (Greek , 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 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; 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, 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.

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