Oxygen uptake is essential for animals, and many animals take up oxygen through their external surface. In these cases particular respiratory structures are not formed; in water these are generally called gills and on land these are called lungs. Structurally, all respiratory organs correspond to create an extended surface with a minimized distance between external environment and transport systems within the body. Their occurrence and structural composition is so diverse that multiple evolution must be assumed. The transport of oxygen in the body is facilitated by carrier molecules, the respiratory pigments. There are three groups of such pigments: hemoglobins, hemerythrin, and hemocyanin. These groups appear to have originated independently, but phylogenetic information can be derived, especially in the case of hemoglobin and hemocyanin.
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