Echinodermata

Abstract

AbstractEchinoderms are exclusively marine deuterostomes with about 7000 described living and roughly 13,000 extinct species. They are extremely diverse, occurring from the intertidal to the abyssal zones. They are unique among Bilateria for the secondary pentaradial symmetry of their body. Five sets of organs, including nerve cords, constitute radial units that are arranged around the oral–aboral axis. The radial body plan is clearly secondary, since all echinoderm larvae are bilaterally symmetrical. Likewise, some of the extinct echinoderms also had either a bilateral body plan or an asymmetrical body. Until recently the nervous system has been the most mysterious and the least understood part of the echinoderm body. The established clichés about its ‘diffuse’ and ‘decentralized’ organization, or else the ‘enigmatic’ nature of neural structures often precluded careful and unbiased analysis. One of the most serious impediments to studies of neural structures in many adult echinoderms has been heavy calcification of surrounding tissues, which often makes it a non-trivial task to properly fix and process samples for microscopy, immunohistochemistry, or in situ hybridization. This was probably a reason for the long-standing belief that echinoderms lacked classical chemical synapses. However, with time, the fixation protocols have been optimized to produce optimal and consistent results with well-preserved morphology, a growing panel of cell-type specific antibodies have been developed, and, most recently, the molecular mechanisms underlying development and maintenance of the echinoderm nervous system have started to be unravelled. This chapter sets out to summarize the latest progress in our understanding of the echinodermate nervous system.