An electron microscopic study on the formation of the periostracum in Helisoma (Mollusca).

The ultrastructure of the cells involved in the formation of the periostracum in Helisoma duryi duryi is described. Two distinct types of periostracal cells can be recognized: light and dark. The Golgi complexes of the light cell are involved in the biogenesis of periostracal units which are extruded at the base of the periostracal groove. Here they align into a single row so as to form the membrane-like lamellar covering of the periostracum. While the secretions of the dark periostracal cells contribute to the initial component of the dark layer of the periostracum, the mantle edge gland cells are responsible for its thickening. The adult periostracum is about 2--3 mum thick and it consists of a single dark layer covered by a membrane-like lamellar structure.