Metabolic diversity refers to the secondary, specific features of animal metabolism that stand in marked contrast to the background of biochemical unity and physiological similarities found throughout the animal kingdom. Much of the newer knowledge of metabolic diversity consists of facts about enzymes and pathways that allow some species to tolerate anoxia. These euryoxic species are now providing clues to one of the oldest and most intractable problems of zoology, the origin and early evolution of the phyla.