Practice question

Molluscan shell is external calcareous exoskeleton secreted by specialized epithelium of mantle, particularly outer fold and periostracal groove. Mantle edge contains shell glands sequentially depositing organic periostracum of conchiolin, middle prismatic calcite columns, inner nacreous aragonite tablets. Outer epithelium actively transports calcium from hemolymph and seawater into extrapallial fluid where crystallization controlled by matrix proteins. Growth occurs by marginal accretion allowing concentric expansion, while inner mantle deposits nacre for thickening and pearl formation. Foot provides locomotion, radula rasping feeding, visceral mass houses organs, not shell secretion mechanism in any mollusc class.