Frontonia angusta is very similar to Frontonia acuminata. However, both species can be distinguished by a dark pigment spot in the apical area, present only in Frontonia acuminata. Since I could not discover a pigment spot, Frontonia angusta (s. fig. 1-b) must be present here. In Frontonia angusta a rather large micronucleus is attached to the macronucleus and located in an indentation of the macronucleus (s. fig. 2). There is only one contractile vacuole, which is located in the middle of the body on the right side (s. fig. 2). It should have 2 – 4 excretory pores. In my specimens there were always two (s. fig. 7). Since the number is variable in this species, it cannot be used as an identifier, but rather as an indication. However, there are also genera where the number of excretory pores is constant (e.g. Paramecium) and can be used as an identification characteristic. With higher magnification one recognizes the typical features of the mouth organization of Frontonia. The undulating membranelles and two of the three adoral membranelles can be seen (s. fig. 6 a-b). However, there is a special feature in Frontonia angusta, which interestingly is not mentioned by Foissner, but Kahl described it: “…hinter dem Mund, links von der Naht ein kleines dreiseitiges Feld größerer, stark gerahmter Wimpernfeldchen (behind the mouth, to the left of the postoral suture a small three-sided field of framed cilia is visible)”. In fact, this distinctly shaped field is very consoicuous (s. fig. 3 a-b). The purpose of this trigonal field remains uncertain. In Frontonia, there is a so-called preoral suture and a postoral suture. Both can only be seen when carefully focusing on the ventral side (s. figs. 4 and 5). The postoral suture is shaped like a groin and is supposed to be a kind of predetermined breaking point of the pellicle according to Kahl. If very large food is ingested, the mouth opening along the suture can widen like a gap but I have never observed this process myself.