ABSTRACT The heart wall of Anaspides tasmaniae consists of a single-layered epicardium and a single-layered myocardium. The myocardial ultrastructure is advanced. The cardiac sarcomere has a full set of bands, and an M-line is present. The sarcolemma invaginates, mainly at Z-band levels, to form clefts and T-tubules. The abluminal Tz,-tubules penetrate deeply and are partly filled by epicardial processes. These reach far into the tubules and form desmosomes with the sarcolemma at Z-band levels. Longitudinal T-tubules combine with the sarcoplasmic reticulum to form interior couplings at H-band levels. Peripheral couplings are often found at this band level. Contractile elements are absent from the anterior and the posterior aorta and from the lateral arteries.