Fig. 8. Proposed hypothetical model of active ammonia excretion across the gills of the shore crab Carcinus maenas. According to this model, NH₄⁺ is pumped across the basolateral membrane by the Na⁺/K⁺-ATPase (1) or traverses the membrane via Cs⁺-sensitive channels (2). Dissociation of cytosolic NH₄⁺ to H⁺ and NH₃ is accompanied by diffusion of NH₃ into vesicles acidified by a V-type H⁺-ATPase (3). The ammonia-loaded vesicles (4) are then moved via microtubules (5) to the apical membrane, where they fuse with the external membrane, releasing NH₄⁺ into the subcuticular space. The NH₄⁺ is then believed to diffuse across the cuticle via amiloridesensitive structures (6). The possibility of an additional ammonium transporter, probably in the basolateral membrane (7), cannot be discounted. Rates of paracellular ammonia diffusion (8) and non-ionic transcellular diffusion of NH₃ (9) are considered to be low at physiologically meaningful transepithelial ammonia gradients. P_i, inorganic phosphate.