Sclerites

Sclerites are hardened body parts. The term is used in various branches of biology for various structures including hardened portions of sponges[?] and mineralized portions of arthropod shells.

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 Scelrites 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 Halkaria[?]; and another armored worm Microdictyon that is generally considered to be a lobopod/onchyphora/onchyphore[?].

It has been suggested that the tube like sclerites of Cambrian animals are homologous with the bristles of annelid worms.

At least one modern gastropod mollusc living near deep sea hydrothermal vents has similar structures made of iron sulfides.