ABSTRACT
Muscles in echinoderms are almost invariably innervated by either a separate hyponeural motor system or indirectly by ectoneural nerves across the basement membrane which separates ectoderm from mesodermally derived tissues. The latter system has been described in detail morpologically and physiologically in the brittle star Ophiura. A worrying exception to these alternatives has been the innervation in the sea-urchin spine (Peters 1985) and this anomaly has beeen re-examined. It has been shown that a population of motor nerve cells associated with the spine muscles exists that are clearly hyponeural and that ectoneural nerves do not cross the basement membrane. Aspects of the hyponeural system in brittlestars are examined which provide some evidence that contrary to previous ideas the hyponeural nervous system may have some integrative function.
