ABSTRACT
The barnacle shell is mostly calcitic, but aragonite is present in the calcareous base of some warm water species. The organic content of the shell and the amounts of minor chemical elements vary according to species and environmental factors. The structures of the inorganic and organic components of the shell are described in some detail, and the mechanisms of their formation discussed. The membranes forming the base, the outer surface of the shell plates and opercular hinges have retained some characteristics of the Crustacea in that their production is synchronous with the moulting cycle of the body. The various phases of production of this membrane are presented. The calcified parts are laid down in very thin successive layers of shell material deposited on each tidal immersion. CaCO3 is usually deposited in the form of granular crystallites or crystallites aggregated into prisms, but also, to a lesser extent, in the form of radiating, orientated and lamellar structures. The variations in the structure of the shell are presented for various taxa.
