1. A method is described for obtaining continuous records of the direction of flow of the respiratory current of Carcinus by recording changes of pressure in the gill chamber.
2. Frequent and rhythmic reversals of the normally forward-flowing respiratory current appear to irrigate the upper surfaces of the posterior gills and do not serve primarily to clean the gills.
3. The rate of reversal of this respiratory current increases spontaneously at times of high tide. It decreases under conditions of carbon-dioxide accumulation and increases under conditions of oxygen depletion in the absence of excess carbon dioxide.
4. The pattern of gill ventilation varies according to whether crabs are totally in water, partially buried in sand, or in very shallow water.