pacemaker

Finally, pacemaker systems are present in animals with nerve nets. In the sea anemone Metridium these systems are expressed in a series of spontaneous rhythmic movements that occur in the absence of any detectable stimulus. It is not known whether the movements originate from a “command”…

… hearts have two noninnervated, myogenic pacemakers, one at each end of the peristaltic pulsating vessel. Separately, each pacemaker causes a series of normal beats followed by a sequence of abnormal ones; together, they provide periodic reversals of blood flow.