The giant neurone system in Ophiuroids. I. The general morphology of the radial nerve cords and circumoral nerve ring.

The nervous system of Ophiura texturata contains nerve fibres and cell bodies that are an order of magnitude larger than anything previously described in the Asteroidea and Echinoidea. These large nerve cells are designated giant fibres. Giant nerve cells are present in both the ectoneural and hyponeural nervous system. The layout of these nerve cells is described and it is shown that the organization is repeated in each segmental ganglion that makes up the radial nerve cord. The circumoral nerve ring is composed, in the main, of tracts of nerve fibres joining the radial nerves, and it contains only limited areas of neuropil associated with the alimentary canal and muscle of the disc and jaws. Degeneration studies have shown that each segmental ganglion of the radial nerve cords contains a discrete population of neurones separate from adjacent ganglion and that there are not anatomically continuous giant fibres along the whole length of the nerve cord.