Gastronauta membranaceus belongs to the cyrthophorid ciliates. The genera Chilodonella or Trithigmostoma also belong to this group. The affiliation to this group results from the type of ciliation. On the dorsal side they are (almost) naked, while the ventral side have stripe-like rows of cilia (kineties) in front of and behind the mouth opening. However, while Chilodonella and Trithigmostoma have developed a mouth opening with a pharyngeal basked, that of Gastronauta membranaceus is shaped as a transverse slit (s. fig. 1a-d). This is a useful adaptation to graze on bacteria growing on surfaces. I have found Gastronauta membranaceus once so far (January 2019) and then only a few specimens. I found them on a floating coverslip, the underside of which they grazed. At low magnifications the specimens are very easy to confuse with Chilodonella. Perhaps this is because only a few finds have been reported. On the floating coverslip, the ventral side is always facing the observer. Typical for Gastronauta membranceus is a non-ciliated field in the posterior, ventral half of the body (s. fig. 1c). In the similar species Gastronauta clatratus the entire posterior, ventral half of the body is ciliated. The dorsal side of Gastronauta mambranaceus is naked except for the dorsal brush, which consists of two short rows of cilia that are nearly perpendicular to each other (s. fig. 2a). Two contractile vacuoles are present, which are diagonal to each other. One anteriorly on the right and the other posteriorly on the left (s. fig. 2b). Near the macronucleus, an hourglass-shaped light-refractive structure is visible (s. fig. 2a). This is not phagocytosed food, but an organell of unknown function.