The circulatory system of cephalopods is based on a trio of hearts, with two pairs of associated ganglia linked to the CNS by a pair of visceral nerves. The beat of the hearts was recorded from free-moving octopuses before and after surgical removal or disconnexion of elements of the nervous system. Severing the visceral nerves does not stop the hearts, which continue to beat in a powerful well co-ordinated manner in isolation from the CNS. The nerves seem to be concerned in raising the cardiac output in exercise, and with stopping the hearts when mantle movements cease, but they are not necessary for the initiation of maintenance of the normal rhythm. Removal of the fusiform ganglia severs all nervous connexions between the ywo gill hearts, and deprives the systemic heart of its nerve supply. The trio of hearts continues to beat as strongly as before. Removal or disconnexion of a cardiac ganglion disrupts the beat of the corresponding gill heart which now tends to contract in an ill-coordinated and rather feeble manner, though at much the same frequency as before; with both cardiacs gone the systemic heart, which contracts only when it is filled, tends to drop in frequency and the mean aortic pressure falls. The system remains rhythmic, however, and the beat may recover, to the point where aortic pressures and frequencies approach those found in intact animals at rest; even octopuses with both fusiform and both cardiac ganglia removed can survive for many hours. From the performance of the isolated branchial heart, the existence of a pulsating vesicle in each cardiac ganglion, the effects of cardiac ganglion removal and the remarkable steadiness of heartbeat frequency shown by intact animals under a variety of conditions, it is argued that the heartbeat rhythm is normally controlled by pacemakers in the branchial heart/ cardiac ganglion complexes, and perhaps, in intact animals, from within the cardiac ganglia themselves. The picture of the control of the heartbeat that emerges from the study of free moving essentially intact animals is quite different from that arising from in vitro and acute preparation studies. It suggests that the conventional wisdom about the control of the heartbeat in cephalopods (and perhaps by implication, in other molluscs) may need to be considerably revised.

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