Fig. 4. Ventilatory response to 2 h exposure periods at various oxygenation levels for all studied podocopids (N=56–106 animals; same data as in Table 2). (A) Respiratory frequencies within bursts, fR, are given as means ± 1 S.E.M. (B) Number of apnoeic animals during each exposure period expressed as percentages of the studied animals. No significant difference was observed as a function of P_O2.