The heart of prawn, Penaeus japonicus Bate, is basically a single chambered globular sac. The cardiac muscle bands form a fine and irregular, three dimensional network. The cytoplasm of the muscle cells is occupied by many glycogen granules, myofibrils and mitochondria. The sarcotubular system of the heart muscle consists of three components; the sarco-plasmic reticulum, transverse tubules formed by invagination of the sarcolemma, and longitudinal tubules connecting adjacent transverse tubules. At the H or A-H zone level, fine side branches arise from the longitudinal tubules to form dyads or triads with the sarcoplasmic reticulum. When adjacent muscle cells attach end-to-end to each other, they form a structure homologous to the intercalated disc in vertebrates. The neuron somas of the local system for pacemaker are concentrated in the medial nerve trunk on the dorsal inner aspect of heart. Axons from these cells extend into the myocardium to terminate in fine branches. At the contact area between muscle cell and axon terminal, the apposed plasma membranes partly increase in electron density. The terminals contain many synaptic vesicles and are surrounded by laminated infoldings of the plasma membrane of muscle cell.