Formation and structure of the periostracum inLymnaea stagnalis

Abstract

The periostracum ofLymnaea stagnalis consists of three morphologically distinguishable layers. The outer layer is protected against the surrounding medium by a membrane-like lamella. At points where this lamella lies within the periostracal groove, it is covered by a homogeneous substance. Materials for both—the lamella and its covering—are synthesized by cells deeply sunken into the connective tissue and secreted into the periostracal groove by the slender necks of these cells. The transfer of periostracal material from the endoplasmic reticulum by way of very prominent dictyosomes to the secretion vesicles can be inferred morphologically in these cells. The three layers of the periostracum are each secreted by a well-defined zone of the mantle epithelium. Layer 1 (the exterior), is produced by the proximal wall of the periostracal groove and by Zone 1 of the supramarginal ridge. Zone 2, which is free of microvilli, forms Layer 2. Zone 3, consisting of only 2–3 cells, forms Layer 3. On, or even in, Layer 3 are situated the first nucleation sites at which the aragonite crystals grow. Calcium is secreted by the transitional zone between the supramarginal ridge and the mantle roof, and by the mantle roof itself.