Shell Mollusca

the mantle edge secretes shell (secondarily absent in number of taxonomic groups, such nudibranchs) consists of chitin , conchiolin (a protein hardened calcium carbonate), except outermost layer in cases conchiolin (see periostracum). molluscs never use phosphate construct hard parts, questionable exception of cobcrephora. while mollusc shells composed of aragonite, gastropods lay eggs hard shell use calcite (sometimes traces of aragonite) construct eggshells.

the shell consists of 3 layers: outer layer (the periostracum) made of organic matter, middle layer made of columnar calcite, , inner layer consisting of laminated calcite, nacreous.