Trichodina pediculus (possible conjugation)

Trichodina pediculus -- the so-called "polyp louse" (Roesel, 1760) -- is a small peritrich ciliate that lives parasitically on freshwater Hydra. There were two of them on this polyp (the common Hydra viridissima). At one point, one of them mounted the other, and stayed there until shaken loose by the Hydra's death throes. It is possible that this was an attempted conjugation.