1. The movements of the scaphognathites of the lobster, Homarus americanus, and the resulting thrust, were resolved in two dimensions accompanied by attack angle changes. Correlation of these scaphognathite and related body movements with branchial hydrostatic pressures during forward and reversed pumping were effected by simultaneously recording on video tape.
2. Forward pumping maintains a negative pressure within the branchial chambers of 2·5-10 mm H2O superimposed on a maintained negative pressure relative to the outside of 0-5 mm H2O. The scaphognathite, by assuming a negative attack angle to the water during the upstroke of the beat, produces the positive pressure of reversals.
3. Contraction of the epimeral attractor muscle appears to contribute to the reestablishment of branchial negativity following each reversal. This muscle inserts on the epimera and by contraction enlarges the branchial chamber.
4. The role of reversals is discussed and it is concluded that their primary function is to clean foreign material from the border of hairs which filter incurrent water. Reversed beating may be a general response to all unusual stimuli to the respiratory system.
5. The pattern of water flow into and through the gill chambers was determined. The relative volume of water entering each incurrent aperture between the legs was calculated and indicated that approximately equal volumes of water entered each aperture. It is concluded that forward pumping adequately irrigates all gill surfaces.