Fig. 4. The change in the mean (± s.e.m.) interfilament space when isolated gills were exposed to 5-hydroxytryptamine (5-HT). Treatments: 0, initial measurements on untreated, quiescent gills; 10^–5 mol l^–1 and 5x10^–5 mol l^–1, the same pieces of gill after exposure to increasing concentrations of 5-HT. Sample size: N=8 measurements made on each of two pieces of gill. All of the means are significantly different from each other (P<0.05).