Several models for the analysis of data relating the rate of oxygen uptake to environmental oxygen level have been evaluated. We conclude that the quadratic (or second-degree) polynomial, though hardly perfect, is the best. Data from 31 species of aquatic invertebrates are described by constants of the quadratic (or second-degree) polynomial equation. The results suggest a phylogenetic trend of increasing regulation of aerobic metabolism in response to declining environmental levels as animals acquire structures that effectively insulate their respiring tissue from the habitat. Many of these species apparently cease withdrawing oxygen from their external environment long before they have exhausted its supply. Presumably, those species with long lasting internal oxygen reservoirs, such as gas bubbles or pools of high oxygen affinity hemoglobin, continue to operate aerobic pathways, but those without substantial oxygen storage devices must switch over to anaerobic pathways, despite the availability of small residual volumes of external oxygen.