In vivo cardiac pressure and heart rate, and relative heart mass, have been measured in Busycon canaliculatum. Base pressure was about l.3cmH2O above ambient and effectively the same in the ventricle, atrium, pericardium and veins. Ventricular systolic pressures were very variable. Highest pressure was over 80cmH2O, but sustained levels were lower, up to about 40cmH2O. Heart rate was also very variable, 5–15 beats min−1. Heart mass was about 0.19% of body mass. Cardiac refilling, control and power output are discussed.

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