In phylogenetic terms, sea urchins are members of the echinoderms. Most echinoderms are indirect developers, which means that the fertilized egg gives rise to a planktonic larva that subsequently undergoes metamorphosis resulting in a benthic juvenile. Echinoderms thus form during their life cycle two nervous systems: one in the larva that is completely lost during metamorphosis, and one in the adult that forms independently of the larval one. While the development of the larval nervous system of sea urchins has already been widely studied, that of the adult nervous system has received much less attention. What is known is that the adult nervous system of sea urchins is composed of five radial nerve cords, which are connected by a circumoral nerve ring, and of peripheral nerves innervating the body musculature and the appendages. There is also an enigmatic basiepithelial plexus that covers the entire adult body, but its connection to the radial nerve cords and/or the peripheral nerves remains elusive.