Conduction pathways in the nervous system of Saccoglossus sp (Enteropneusta)

A species of Saccoglossus from Barkley Sound, British Columbia, was observed in the field and found to exhibit a startle withdrawal response. Optical and electron microscopy of the nerve cords failed to reveal giant axons. The dorsal collar cord and ventral trunk cord consist of small axons with a mean diameter of 0.4 mu m. The majority of the axons run longitudinally and there is no indication of a specialized integrative centre. Electrical recordings from the nerve cords show events interpreted as compound action potentials. The potentials are through-conducted from proboscis to trunk. Such propagated events probably mediate startle withdrawal. Conduction velocities did not exceed 40 cm . s(-1) in any part of the nervous system.