Structure
The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous layer. The layers usually incorporate a substance called conchiolin, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone-tanned proteins.
The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle.
Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashells, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrroles and porphyrins.
Shells are almost always composed of polymorphs of calcium carbonate - either calcite or aragonite. In many cases, such as the shells of many of the marine gastropods, different layers of the shell are composed of calcite and aragonite. In a few species which dwell near hydrothermal vents, iron sulfide is used to construct the shell. Phosphate is never utilised by molluscs, with the exception of Cobcrephora, whose molluscan affinity is uncertain.
Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides.) Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, nacre is the most studied type of layer. Shells of the class Polyplacophora are made of aragonite.
Read more about this topic: Mollusc Shell
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