Tackling one of the most difficult and delicate of the evolutionary questions, this challenging book summarizes the more recent results in phylogenetics and developmental biology that address the evolution of key innovations in metazoans. Divided into three sections, the first considers the phylogenetic issues involving this area of the tree of life and the elucidation of those relationships that continue to trouble taxonomists. The second section considers the developmental biology of metazoan evolution including the development of the nervous system, sensory organs, and physiological maturation. Part three focuses on the evolution of pattern and process in the Metazoa.