Periostracum

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As periostracum is called an outer organic layer on the shell at mollusks (molluscs) and Brachiopoda (brachiopods). This consists of conchiolin , a protein mixture tanned and sclerotized by quinones. The periostracum is followed by 2-4 layers of calcium. The periostracum serves as a protective layer for the underlying layers of the shell, for example against the effects of acids or boring predators.

See also: cuticle

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