Yonge (1962) suggests a possible universal occurrence of byssal apparatus in the post-larval spat of bivalves. Subsequent studies have revealed that the byssus thread secreted by the post-larval byssal apparatus of many bivalves is involved in settlement as well as migration by means of bysso-pelagic drifting (Sigurdsson, Titman & Davis, 1976; Blok & Tan-Maas, 1977; Board, 1983). Recent studies on the byssus and other foot glands in the early stages of bivalves have included Ostrea edulis (Cranfield, 1973 a, b, c); Pecten maximus (Gruffydd, Lane & Beaumont, 1975); Mytilus edulis (Lane & Nott, 1975; Lane, Nott & Crisp, 1982); and Chlamys varia L. (Gruffydd, Budiman & Nott, 1979)