Cnidaria

AbstractCnidaria are one of the basal groups within the metazoans. They are divided into the Medusozoa (Hydrozoa, Scyphozoa, Staurozoa, and Cubozoa) and the Anthozoa, which are thought to be the basal group within the Cnidaria. Cnidaria together with the Ctenophora are the lowest groups with a nervous system that relies on synaptic conduction and are therefore attractive outgroups to the Bilateria. Investigations of evolutionary changes in nerve cells and neural anatomy and function will shed light on the emergence of the complex nervous systems of the Bilateria. Indeed, Cnidaria use elaborate nervous systems that share with bilaterians common neurophysiological mechanisms. However, the diversity of cnidarian neuroanatomies is high. Therefore, the reconstruction of the urcnidarian nervous system is blurred. The long-thought concept of only one or several ‘diffuse’ nerve nets with little or no centralization has been overcome in recent years by the discovery of centralized structures, particularly in the nervous systems of medusae of Scyphozoa and Cubozoa. This chapter deals at first with some pioneering investigations of the Cnidarian nervous systems, describes the structure of the nervous systems of selected groups (Hydra, Anthozoa) also with respect to the distribution of neurotransmitters and regulators, and highlights the centralized structures in Scyphozoa and Cubozoa, the nerve rings, and especially the rhopalia. These rudimentary ganglia integrate sensory inputs and motor outputs. In cubomedusae, the rhopalia are also the primary sites for photosensitive organs. Photosensitivity is common in the Medusozoa.