In arthropods, this hardening is accomplished by the cross-linking of the protein chains in the exocuticle, a process called sclerotization. Thus, the arthropod exoskeleton is divided into numerous sclerites, joined by unsclerotized, membranous regions. The precise shapes and arrangement of the different sclerites provide the vast majority of 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 accurately 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.
One specialized usage 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 (Butterfield 1990). At least one modern gastropod mollusc living near deep sea hydrothermal vents has structures made of iron sulfides (Bengtson and Warén, 2003) similar to some Cambrian sclerites, although presumably not homologous in structure.