Fig. 3. Relationship between oxygen consumption rates of Sipunculus nudus isolated body wall musculature and extracellular bicarbonate concentrations ([HCO₃^-]). Bicarbonate levels in hypercapnic media cover a broad range of concentrations while normocapnic levels are all very low (0.27-0.015 mmol l^-1 HCO₃^-). ^*Oxygen consumption rate significantly different from the respective control value at pHe 7.9. Values are means ± S.D., N=5-6.