These tubules bear a tuft of cilia or flagellum. These bacteria are motile by a polar flagellum. Most species are motile with a flagellum located in the polar regions. It is motile by means of a single flagellum. If motile, they have a flagellum. It swims using a single sheathed polar flagellum with a characteristic dampened filament waveform. Its motility is due to a polar flagellum. Other cytoskeletal structures include the specialised attachment between the flagellum and the kinetoplast. From the basal body arises a single flagellum that run towards the anterior end. Thus the model uses a flagellum for propulsion purposes. Analysis showed that "R. pachyptila" symbiont has at least one gene needed for flagellum synthesis. Cavalier-Smith originally proposed that unikonts ancestrally had a single flagellum and single basal body. Multiple processes were involved in the evolution of the flagellum, including horizontal gene transfer. An important diagnostic feature is the lack of a free posterior flagellum. Only the tip of the flagellum is free at the anterior end.