Flatworms for ID in a freswater tank (thematized about Matano Lake, in Sulawesi, but that's probably irrelevant as the flatworms are almost sure uninvited here). Haus des Meeres/Aqua Terra zoo, Wien, Austria. They look completely white, at least by underside. Any chance to get down to species level please?
This is the only figure from M. Schultze's original description of Prorhynchus stagnalis. He thought that it was an aberrant nemertean; hence the anus (which, I assure you, does not exist in reality).