1. A brief description of the proboscis apparatus and body-wall of Acanthocephalus ranae (Schrank, 1788; Lühe, 1911) has been given.
2. Two hydrostatic systems are involved in the movements of the animal. That of the trunk controls the position of the proboscis apparatus as a whole. The proboscis is operated by the separate hydraulic system of the proboscis apparatus.
3. Contraction of the neck retractor muscles forces fluid from the lemnisci to the proboscis wall, which becomes distended in consequence.
4. It has been shown that the lemnisci do not aid in the evagination of the proboscis. Their function may not be solely to act as hydraulic reservoirs.