Urostomides darwini was first described as Metopus darwini by Kahl in 1927. Later Jankowski (1964) transferred the species to the genus Brachonella. For the genus Brachonella, Jankowski defines the following characteristics:

– anterior end (dome) larger than the posterior end.
– the adoral zone forms a helical line when it runs to the mouth opening

In 2017 Bourland et al. provide a redescription and 18S rRNA gene sequences. Based on these results Brachonella (Metopus) darwini was transferred to the genus Urostomides.

Regarding the occurence of Urostomides darwini Kahl writes: “stets nur vereinzelt, aber an vielen Fundstellen (always only sporadically, but in many localities)”. I also find Urostomides darwini in many localities. The population densities vary a lot. Sometimes I find no specimens for 1–2 years and sometimes up to 10 specimens per milliliter. The reasons for these fluctuations are not known to me.

At high layer thickness Urostomides darwini can be observed quite well even with the coverslip (s. fig. 1 a-c). It swims quite fast and rotates around its own axis. The specimens appears mushroom shaped with a crooked cap (s. fig. 1 a-c). In the freely swimming specimen I could see that the right side is concavely depressed (s. fig. 1 c), which Kahl did not mention and drew. The specimens in my population were mostly yellowish in color, which differs from Kahl’s description, who describes the cytoplasm as colorless. The specimens in my population were also significantly larger than described by Kahl and Bourland et al. with a length of 150–200 µm. Thus, they were about 30% larger than indicated by these authors.

Kahl considered Urostomides darwini as a transitional form to the genus Caenomorpha, which can be easily understood because of the body shape with the caudal spine. The adoral zone crosses the ventral side diagonally (s. fig. 1 d) and continues to the right side where the mouth opening is located (s. fig. 1 a). On the right side the adoral zone descends into the mouth funnel. At the point where it descends, a cytoplasm lip forms the outer edge of the mouth opening (s. fig. 2 a). The macronucleus was always oval in my specimens (Kahl and Bourland et al. describe it as globular). The spherical micronucleus is adjacent to the macronucleus and has a diameter of 5–6 µm  (s. fig. 2 c). The food vacuoles I could find mainly contained rhodobacteria, bacteria, and small algae. Like many other metopid ciliates, Urostomides darwini also bears an aggregation of refractive, brownish granules of unknown function in the apical dome (s. fig. 5).