Conduction along the Ventral Nerve Cord of a Hemichordate Worm

Although there is general agreement that hemichordates are more closely related to echinoderms and chordates than to any other groups of animals, their nervous systems appear to be no more highly organized than those of the coelenterates. Nerve fibres in enteropneust hemichordates form an intra-epithelial net, spreading extensively over the surface of the body and along the anterior part of the digestive tract. Hess (1937) and Silen (1950) found an internal nervous system, but the claim has been disputed by Bullock (1945) and Knight-Jones (1952). A few fibres have been seen to cross the basal lamina as individual strands, presumably to innervate muscle fibres. The only modification of this diffuse plexus is a concentration of nerve fibres to form dorsal, ventral, collar and, in some species, proboscis nerve cords. Bullock (1945) states that the cords, particularly the internal collar cord, appear to be primarily conduction tracts with little neuropile and cannot be considered to be homologous with chordate central nervous systems. Further evidence for the relatively unsophisticated nervous organization in these animals is the absence of sense organs, the presence of more sense cells than interneurones, and the lack of complexity in behaviour (Bullock, 1940). Studies of the functional organization of the nervous system are few and based on deductions gained from analyses of muscular or ciliary activity (Bullock, 1940; KnightJones, 1952) or of luminescent responses (Baxter & Pickens, 1964). This paper presents the first recordings of muscle and nerve potentials from enteropneusts and decribes conduction and integration along the ventral cord.