Onchidiidae, one of the higher clades of pulmonates, have been understudied for the last 70 years; its systematics is in a state of confusion; and the internal anatomy of most species is still largely unknown. The anatomy of Semperoncis montana, a terrestrial species that lives in high-elevation rain forests of the Philippines Islands, is redescribed based on 91 specimens representing ten populations. New geographic records (Panay Island) and elevation records (up to 1,850 m) are provided. All type material available was re-examined. The intra-specific variation of anatomical characters is analyzed, with special attention to the radula and the male reproductive organs. Male organs (penial stalk and hooks, papillae and spines of accessory glands) are illustrated using scanning electron microscopy for the first time in onchidiids. Ontogenetic changes in the shape of male organs are described and illustrated in detail, indicating that S. montana was described based on immature specimens of the later named Platevindex apoikistes. Therefore, this latter name is regarded as a junior synonym of S. montana. The supra-specific relationships of S. montana are commented upon.