Fig. 3. The mean (±s.e.m.) number of elicited shadow responses of snails in the PW and CE conditions. Snails in CE elicited full pneumostome withdrawal more often when presented with a passing shadow than did snails in PW. Specifically, the mean (±s.e.m.) number of shadow withdrawal responses in PW was 1.00±0.09, whereas snails in CE closed their pneumostomes on average 1.60±0.07 times (N=22, ^**P<0.01, repeated measures analysis).