Colored, metal-containing proteins that combine reversibly with oxygen, found in the body fluids or tissues of multicellular invertebrate animals and microorganisms. The role of these pigments is primarily to aid in the transport of molecular oxygen. Thus they are distinguished from respiratory enzymes, which are concerned with the metabolic consumption of oxygen. Four distinctly colored groups of respiratory pigments exist among invertebrates: hemoglobins (purple, become orange-red with oxygen), chlorocruorins (green, become red with oxygen), hemocyanins (colorless, become blue with oxygen), and hemerythrins (colorless, become red with oxygen). Formerly, invertebrate hemoglobins were called erythrocruorins to distinguish them from functionally similar yet structurally distinct pigments of vertebrate bloods. Those hemoglobins confined to muscle cells are called myoglobins.