The structure and growth of the polyplacophoran shell, characteristically consisting of eight plates surrounded by a girdle, is examined in the light of current views on the relationships of mantle and shell in the Bivalvia. The periostracum and outer and inner calcareous layers of the shell of the latter group are homologous with the cuticle, tegmentum and articulamentum respectively of the shell of the Polyplacophora. The margin of the mantle consists of a large marginal fold, which secretes the cuticular girdle, and a small accessory fold bearing mucous cells. These are functionally comparable with all three folds of the mantle margin found in other molluscs, although anatomically the marginal fold of the chitons probably represents only the inner surface of the outer fold of the mantle margin.
The cuticle not only forms the girdle, which bears calcified spines or spicules, but also extends between the shell plates. The principal part of the cuticle consists largely of mucopolysaccharide material but there is also a thin discrete inner region which is similar chemically to the periostracum of other molluscs. The cuticle, possibly without spines, probably covered the entire dorsal surface of a primitive placophoran and beneath this, plates developed. As these grew the cuticle became worn away except marginally and between the plates. It is suggested that a covering of mucus over the visceropallium may have been the forerunner of the molluscan shell and the possible evolutionary relationships of the shell throughout the Mollusca are discussed.