ABSTRACT The anuran conus arteriosus was studied by direct observation of its structure and function and by observing the effects of artificially induced contraction on the pressure pulse in the arterial arches. The beat of the anuran conus was found to close the outlet to the pulmocutaneous arches. The timing of the beat of the conus in the ventricular cycle is variable; early in the cycle the conus admits less blood to the pulmocutaneous circuit, more blood being admitted when it beats late. The timing of the beat of the conus appears to be adaptively related to beat frequency and to pH of the system. Elementary calculations suggest that these characteristics constitute an important measure of survival value of the anuran conus. The findings of this work are compatible with the theory of selective passage of two streams through the heart, supplementing such passage with additional control of blood volumes entering the pulmocutaneous circuit.