Fig. 6. (A) Breathing frequency (breaths min^-1), (B) integrated breath area (Vxs) and (C) total ventilation index (Vxs min^-1) as a function of inspired CO₂ levels recorded in vivo from toads exposed to CHC (circles) or CHH (triangles). The data are plotted as the mean ± 1 s.e.m. Letters (a-c) represent significant differences at the different inspired CO₂ levels within any group. A number sign (#) represents a difference between the CHC and CHH groups.