Published online by Cambridge University Press: 11 May 2009
The glands which secrete the byssus in bivalve molluscs are located in the foot. Some glands discharge into a duct at the proximal end of the foot whilst others discharge into a ventral groove which runs forward from the opening of the duct for about two thirds of the length of the foot. In Mytilus edidis a stem or ‘rachis’ is formed by the gland which surrounds the duct (Brown, 1952). The secretions of this gland are formed into flat sheets or ribbons as a result of being discharged between numerous lamellae which occupy the blind end of the duct. The secretions are derived from two types of secretory cell. One produces large granules, packed with banded filaments, which are secreted into the crypts formed by the lamellae (Tamarin, 1975). The other produces smaller, cylindroid shaped granules which are secreted along the two faces of each lamella. The stem serves as a point of attachment for collagenous threads, produced in the groove (Mercer, 1952; Pujol, 1967 a, b) by the collagen gland which secretes globules displaying regular banding (Pujol, Houvenaghel & Bouillon, 1972). The proximal end of each thread is attached to the stem by means of a collagenous cuff, each one overlapping the previous one to form a concentric arrangement of cuffs around the stem. The distal ends of the threads are attached to the substratum by a holdfast.