In this study we compare spermiogenesis and ultrastructure of the mature sperm in two species of acoel Turbellaria from the Philippines. Sperm development is divided into five stages: (1) the early undifferentiated state, with a large nucleus, sparse cytoplasm containing few organelles, and no inclusions; (2) spermiogenesis I, in which Golgi activity is prominent, dense bodies appear in the cyt...
