Fig. 5. Snail learning `grade distributions'. Snails were given grades based on their individual performance. Grades were calculated as follows: a 50% reduction or greater is an A, a B is a reduction of 35–49.99%, a C is a 20–34.99% reduction, and an F is a reduction of less than 20%. (A) Grade distributions observed to occur under standard hypoxic conditions (N=2301). (B) Distribution of grades for snails presented with the `more intense hypoxic challenge' (N=23). These snails showed a statistically greater number of A grades and fewer F grades than controls (P=0.0007). (C) Grade distributions seen for snails trained in the hypoxia + 50 µmol l^–1 H₂S condition (N=24). (D) Distributions for snails trained in the hypoxia + 75 µmol l^–1 H₂S (N=11). (E) Distributions for training in hypoxia + 100 µmol l^–1 H₂S condition (N=23).