Structure and Evolution of Invertebrate Nervous Systems

Abstract For many biologists the nervous system is a particularly fascinating organ system. The nervous system is involved in or is even responsible for many features that are regarded as being characteristic of animals in general. Since the last comprehensive work was published about 50 years ago, the time has probably come to provide a new review on recent, newly gathered knowledge on the structure of invertebrate nervous systems, especially since new methods have come into use. These advances now enable us to demonstrate neuronal architecture down to the level of the genes and the cell types involved, allowing a new view on nervous systems and comparisons among different taxa. These new findings may help in development of new hypotheses, or support of existing hypotheses on phylogenetic relationships and evolutionary pathways in the nervous system. In spite of many open questions we already have a good knowledge of how nervous systems work, how they are constructed, and how they may have evolved. There is certain evidence that the first nervous systems are represented by rather simple sensory–motor circuits, followed by nerve nets (plexus) located within the epithelia, and finally neuronal circuits composed of sensory cells, interneurons, and brains. Therefore, this book concentrates on invertebrates and structure and these new evolutionary aspects, here discussed as ‘research highlights’ or ‘perspectives’. Sensory structures are only touched on peripherally. Whereas some taxa have been studied extensively and well-written reviews may already exist, in other taxa our knowledge on the nervous system is still scant and scarce.