The membrane systems of the cardiac muscle cell of Tmetonyx cicada O. Fabricius (Crustacea, Amphipoda)

The membrane systems of the cardiac muscle cell of the amphipod Tmetonyx cicada (O. Fabricus) are described. The sarcolemma invaginates and forms a transverse network of tubules at the level of the Z band. Narrow longitudinal tubules branch from the network and connect to another transverse network of tubules at the H band level, where dyadic and triadic junctions are formed with the sarcoplasmic reticulum. Adjacent myofibrils are normally separated by a well developed double layer of the sarcoplasmic reticulum. In areas where the myofibrils closely approach the outer sarcolemma, peripheral couplings have been found at the level of the H band.