The enteric nervous system of echinoderms: Unexpected complexity revealed by neurochemical analysis

Echinoderms are one of the most important groups of metazoans from the poin t of view of evolution, ecology and abundance. Nevertheless, their nervous system has been little studied. Particularly unexplored have been the compo nents of the nervous system that lie outside the ectoneural and hyponeural divisions of the main nerve ring and radial nerve cords. We have gathered i nformation on the nervous components of the digestive tract of echinoderms and demonstrate an unexpected level of complexity in terms of neurons, nerv e plexi, their location and neurochemistry. The nervous elements within the digestive system consist of a distinct component of the echinoderm nervous system, termed the enteric nervous system. However, the association betwee n the enteric nervous system and the ectoneural and hyponeural components o f the nervous system is not well established. Our findings also emphasize t he importance of the large lacunae in the neurobiology of echinoderms, a fe ature that should be addressed in future studies.