1. This paper describes a neurophysiological investigation of the initiation and co-ordination of the peristaltic waves in Urechis.
2. Peristaltic waves are (a) initiated in the proboscis and propagated posteriorly, and (b) initiated in the hind gut and propagated anteriorly, provided that nerve cord, body-wall and peripheral nerves are intact.
3. There is a hierarchy of pacemaker regions in the nerve cord, effective over lengths as small as 30 mm. This hierarchy is normally dominated by the pacemaker in the proboscis.
4. The pacemakers respond with increased frequency to oxygen, epinephrine and acetylcholine in concentrations of 10-6M. Eserine potentiates the acetylcholine response. The system is insensitive to 5-hydroxytryptamine.
5. The giant fibres conduct at 1.5 m./sec. and mediate a startle response; they are not involved in the conduction of the peristaltic wave.