ABSTRACT
Metazoan nervous systems are structurally and functionally diverse and highly complex. Even the simplest nervous systems, the nerve nets of Cnidaria and Ctenophora, showcase a structural complexity on the cellular and sub-cellular level which implies a long evolutionary history. The nervous system in a strict sense, as defi ned by most authors (for overviews see Bullock and Horridge 1969, Hanström 1928, Schmidt-Rhaesa 2007), is an autapomorphy of the Eumetazoa. One of the defi nitions is provided by Schmidt-Rhaesa (2007): “Nervous systems conduct information in a directed way through the body. This is done by electrical and/or chemical signals and by cells specialized for these functions”. The classic neurone doctrine by Ramon y Cayal (1937), however, is strictly morphology-based and does not consider the nature of the signal. Bullock and Horridge (1969) summarized Ramon y Cayals’s doctrine by stating that “all nervous systems consist in essence (whatever other, nonnervous elements may be
Institut für Spezielle Zoologie und Evolutionsbiologie Friedrich-Schiller-Universität Jena, Erbertstr. 1, D-07743 Jena Germany. E-mail: nickel@porifera.net *This chapter is dedicated to Stephen Hillenburg, who greatly directed the interest of a whole future scientist generation towards sponges and the marine life.
