Fig. 3. A tentative model for Na⁺ and Cl^– cotransport by Carcinus gill. Note that transcellular Na⁺ influx is only half that of Cl^–. To maintain electrical neutrality, paracellular Na⁺ influx must equal transcellular Na⁺ influx. The model requires higher ambient Na⁺ and K⁺ than is usually found in freshwater in order to operate the cotransporter, but it would be effective in brackish water. The apical membrane potential difference (–80 mV) is an estimate, supposing that it is a K⁺ diffusion potential with [K⁺]2 mmol l^–1 in 20% seawater. The transepithelial potential difference (–8 mV) is based on many published values between –5 mV and –10 mV.