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The origin(s), distribution and fate(s) of exosomatic water in the semi-terrestrial amphipod, Orchestia gammarellus (Pallas 1766) have been investigated. These observations were coupled with a study of the gross morphology of the ventral groove and associated structures and a physiological investigation of gill function, allowing an appraisal of the metabolic significance of exosomatic water. The key to the success of amphipods invading land lies, we suggest, in the retention of exosomatic water by capillary action within a water-conducting system (analogous to that found in isopods) located within the ventral groove. This exosomatic water allows ‘aquatic’ methods of gas, water and ion exchange to continue unimpeded as long as the ventral groove can be recharged either from urine or standing water.