The present study employing myogenic hearts of invertebrates (oyster and beetle) was performed to elucidate the mechanism of rhythm formation as an intrinsic control. The two connected half-ventricles in oyster and the two connected fragments of the heart in beetle show characteristics represented by the phase response graphs (Figs. 3, 4) in which the beat interval becomes elongated (inhibited) or shortened (accelerated) due to the application of impulses given by the other segment (neighbouring fibers). Cardic rhythm develops spontaneously from myocardia and is concentrated in a single series of regular beats through self-contrl. The existence of gap junction (cell coupling) assists in the simultaneous excitation of the whole heart in oyster. The beetle heart, however, has scarcely cell coupling to take peristaltic propagation of impulse.