Fig. 6. Phase diagram to illustrate the intersegmental and side-to-side coordination of systole (vertical bars) in heart segments 3–18 for the peristaltic (magenta) and the synchronous (blue) modes. Data are duplicated and shifted by 100% to illustrate the side-to-side coordination over two heartbeat cycles [values for systole from Wenning et al. (Wenning et al., 2004a)]. Complete cardiac cycles (bars) are shown for segments 3, 6, 10 and 16 (labeling as in Fig. 3). Within the period of one heartbeat cycle, the heart segments on both sides completed their individual cardiac cycles. Starting with a nearly simultaneous systole in segments 17–15 (set to 0% phase) in the peristaltic heart, systole traveled rear-to-front within 60% of the heartbeat period. In the synchronous heart, systole traveled front-to-rear to segment 17 within about 30% of the heartbeat cycle. Note that of the 32 heart segments, only a fraction are in systole at any given time; the others, in diastole, may serve as a conduit (see text).