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Fig. 9. Working model for ammonia excretion in the terrestrial crab Geograpsus grayi. (1) Na+/K+-ATPase; (2) NH4+-permeable K+ channels; (3) carbonic anhydrase (CA); (4) Na+/H+/NH4+ exchanger; (5) HCO3–/Cl– exchanger; (6) Rhesus-like protein (RhCM), putative ammonium transporter; (7) cation-permeable channel-like structures of the cuticle. For further details please refer to the text. Model compiled from data of Varley and Greenaway (1994).