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A thermodilution technique is used to measure the flow of hemolymph through the heart of crabs. The technique consists of injecting a small volume of cold filtered sea water into the ventricle and monitoring temperature changes with time in an artery immediately downstream from the heart using a surgically implanted thermocouple. An experimental evaluation of the method affirms that it is a direct measure of the bulk flow of hemolymph through the heart. Values of cardiac output determined in this manner fall within the range of Fick estimates of hemolymph flow made on animals treated similarly. Since measurement of cardiac output by thermodilution can be achieved in a matter of seconds, simultaneous monitoring of heart rate permits the calculation of cardiac stroke volume. In two species of crabs examined, much of the variation in cardiac output is accounted for by changes in stroke volume rather than changes in heart rate.

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