Fig. 2. Ultrastructure of the swimming muscle of the medusa of Polyorchis penicillatus. Each muscle cell is separated into a somal region and attached muscle feet containing the myofibrils. The somal region contains the nucleus (Nu), vacuoles, Golgi complex, and mitochondria (Mi). Note the larger number of mitochondria (Mi) lying along the striated myofibrils. The myofibrils are closely applied to the basal mesoglea (Me). Arrows indicate vesicles in the subsarcolemmal region that are thought to be part of a sarcoplasmic reticular system. Scale bar, 0.5 µm.