Fig. 5. Learning, memory and the assignment of marks. (A) The learning and memory curves for snails (N=1490) that receive two 45-min training sessions, with an interval of 1 h between the two training sessions. Memory (MT) was tested 24 h later in 490 of these snails. The other 1000 snails were used in extinction, forgetting, change-of-context and memory-extension experiments and thus could not contribute to the 24 h memory test. An analysis of variance (ANOVA; F_1490,1=1460, P<0.0001) shows that learning occurred. When we tested for memory 24 h later we found that the number of attempted pneumostome openings in the memory test session was not significantly greater (P>0.05) than the number in Session 2 and was significantly less than the number in Session 1 (P<0.001). Thus, the criteria for memory were met. (B) The assignment of marks to individual snails. The vast majority of snails (77%) received either an A or a B mark, while 12% `flunked' (i.e. received an F).

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