In this volume outstanding specialists review the state of the art in nervous system research for all main invertebrate groups. They provide a comprehensive up-to-date analysis important for everyone working on neuronal aspects of single groups, as well as taking into account the phylogenesis of invertebrates. The articles report on recently gained knowledge about diversification in the invertebrate nervous systems, and demonstrate the analytical power of a comparative approach. Novel techniques in molecular and developmental biology are creating new perspectives that point toward a theoretical foundation for a modern organismic biology. The comparative approach, as documented here, will engage the interest of anyone challenged by the problem of structural diversification in biology.