The observations of Schlieper (1929) established the lugworm Arenicola marina (L.) as an osmoconformer which remains virtually isosmotic with the external medium over a wide range of salinities. In a recent comprehensive review of salt and water balance in lugworms, Oglesby (1973) describes ‘the extensive swelling associated with transfer of lugworms to lower salinities’, and ‘when maintained in salinities lower than about 50% s.w. in the laboratory, lugworms are rendered incapable of such vital physiological activities as burrowing and burrow ventilation’. Under these conditions, lugworms exhibit little or no ability to regulate their volume or water content.