Arenicola marina (L.) is one of the most common members of the outer estuarine sandy and muddy shores. A moderately euryhaline osmoconformer, this polychaete cannot survive in water of less than about 10‰ constant salinity and then only in the Baltic; however, in the outer portions of estuaries, it is not uncommon for the overlying water to be practically fresh at some stages of the tide. Wells (1949b) postulated that parts of the complex burrowing behaviour of the animal prevents irrigation of the burrow with noxious water, but the advantages to the lugworm of its periodic inactivity have never been fully investigated.