Abstract
1. By studies of D2O exchange the apparent permeability of Nereis diversicolor to water has been shown to vary in response to changes of salinity. 2. The hourly water-exchange fraction drops from over 12 in SW nearly to 6 in fresh water. 3. Calculation of the net flux of water indicates potential urine volumes of 0.3 % of body weight/h in SW, a maximum of 2.8 % in 10 % SW, and a reduction to 2.1 % in fresh water. 4. It is calculated that, if such urine volumes are produced, the urine is probably hypotonic in chloride to the coelomic fluid at salinities of 25 % SW and lower.