Fig. 8. Working model for ammonia excretion in the terrestrial crab Ocypode quadrata. (1) Na⁺/K⁺-ATPase; (2) NH₄⁺-permeable K⁺ channels; (3) carbonic anhydrase (CA); (4) HCO₃^–/Cl^– exchanger; (5) Na⁺/H⁺ exchanger; (6) Rhesus-like protein (RhCM), putative ammonium transporter; (7) cation-permeable channel-like structures of the cuticle; (8) transmembranous NH₃ diffusion. For further details please refer to the text. Model compiled from data of DeVries and Wolcott (1993) and DeVries et al. (1994).