Fig. 3. Sodium uptake rates (nmol g^–1 h^–1) as a function of ambient Na⁺ ion concentration, by the pond snail L. stagnalis. Values are for bled (closed circles) and un-bled (open circles) snails. Regression lines are Michaelis–Menten curves (SigmaPlot 8.0 for Windows). Uptake values at 2 mmol l^–1 [Na⁺] were excluded from the regression analysis because they did not conform to the saturation kinetics observed at and below the rearing concentration of 1 mmol l^–1. N=10 for each data point. Values are means ± s.e.m.