It is our aim in this paper to answer three questions. All of them relate to the well-known ability of lugworms to act as isosmotic osmo-conformers over a wide range of salinities. When exposed to diluted sea water, the body fluids come into osmotic equilibrium with the lowered external concentration, and considerable amounts of water enter the body of the worm. There is no evidence of weight or volume regulation in dilute media, as assessed by a return of the body weight towards its original value. The processes of osmotic adaptation down to the lowest salinity limits for survival include, therefore, tolerance of a lowered osmotic concentration of the body fluids, and of the retention of increased amounts of water in the body. The evidence on which this view of osmotic adaptation in lugworms is based is reviewed by Oglesby (1969, 1973). Additional information on Arenicola marina is provided by Freeman & Shuttleworth (1977).