Article: Differentiation of the brachiopod periostracum
The periostracum of living brachiopods is highly variable in microstructure, but the secretory regimes responsible for such differences give rise to either a strictly chronological succession built up on the surface of the vesicular cells at the tip of the outer mantle lobe, or a heterochronous succession secreted simultaneously from both sides of a slot between the vesicular cells and the lobate cells which constitute the junction of the outer and inner epithelium. In heterochronous successions, vesicular cells exude the basal layers of the periostracum while the lobate cells secrete superstructural features varying from the proteinous labyrinths of many terebratellaceans to the horizontally sheeted concentric ridges of Discinisca. Consideration of these secretory processes leads to the assumption that the regimes responsible for chronological successions are the more primitive and that those resulting in heterochronous successions are likely to have evolved repeatedly during the history of the Phylum. Despite the absence of lobate cells in Crania, it seems likely that they were present in the prototypic brachiopod and may well have occupied the very edge of the mantle, thereby separating skeletal-secreting outer epithelium (with vesicular cells) on the outside and ciliated inner epithelium on the inside.