Fig. 2. (A) A recording of falling [O₂] in the closed respirometer. (B) The graph is derived from A and shows the O₂ consumption at different levels of ambient [O₂]. Variables derived from the measurements of O₂ consumption in G. histrio are indicated in A and B. _O2 is the normoxic O₂ consumption, and [O₂]_crit is the critical O₂ concentration (below which the animal loses the ability to regulate its O₂ consumption). (C) The ventilatory frequency at different O₂ concentrations measured during respirometry (means ± S.E.M. of 11 individuals of both forms of G. histrio). A and B are from a typical animal, and the variables measured from several animals are given in Table 1. ^*P<0.05 compared with any of the 70%, 80% and 90% values (Dunn's test).