Fig. 2. The effect on (A,B) the frequency of breathing air (f_air; A) and water (f_water; B), and (C,D) CO₂ excretion into air (air_CO2; C) and water (water_CO2; D), of a 1 h acid (3 mmol kg^–1 NH₄Cl) infusion (marked by the grey bar) in African lungfish Protopterus annectens. Values are means ± s.e.m. for the period prior to acid infusion (`pre'), and for measurement periods 0–1, 2–3, 5–6 and 17–18 h post-infusion; N=9–11 for breathing frequencies and water_CO2, and 8 for air_CO2. An asterisk (^*) indicates a significant increase from the pre-infusion value (one-way RM-ANOVA; P values are indicated on the figure); note that in A, post hoc multiple comparisons tests were unable to detect the origin of the significant difference indicated by the P value.