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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 Halkeria; and another
armored worm Microdictyon that is generally considered to be a
lobopod/onychophore.
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.