There exists an extreme variability in the biochemical properties of animal organisms. Each species, each individual, in fact each cell type of a multicellular animal has its own molecular characters. This advanced text- and reference book deals with the molecular structures of body substances and the chemical processes of metabolism for the first time throughout the whole animal kingdom from the protozoans to the higher vertebrates. The biochemical variation is explained as adaptation to particular environmental conditions or as the result of phylogenetic diversification. The fascinating insights into the fundamental mechanisms and the time course of evolution that have been gained by the analysis of molecular data are extensively described. More than 4600 actual references give the reader access to the original literature.