SUMMARY
The contractile mechanism of the tunicate heart is situated in the delicate single layer of muscle fibres of the heart wall. The subsequent contraction of these fibres produces a peristaltic wave passing along the tube‐shaped heart, resulting in a true circulation. The mechanism of refilling is not yet understood.
The pacemaker system is of a diffuse nature. This basic automatism is controlled by two more powerful pacemakers, one at each end of the heart.
The peripheral pacemakers are possibly myogenic, and stimulated to contract rhythmically. These local contractions release peristaltic waves. The cause of the rhythmic activity of the pacemakers may be a metabolite which stimulates their sensory mechanism. The active pacemaker adapts itself to this stimulus, which results in a temporary arrest after a period of activity.
The fact that after such a pause the opposite pacemaker becomes active, thus causing regular reversal of pulsation, cannot be understood in terms of increasing back‐pressure. Periodic fatigue (adaptation) and subsequent restoration of the pacemakers offer a more plausible explanation of the phenomena.
Very probably the tunicate heart does not possess extrinsic regulation.
The tunicate heart is not very sensitive to ions or hypotonicity.
There is evidence that the heart possesses a special conductive system.
It has been pointed out that a number of features of the heart mechanism need further investigation.
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