Opalinida

Opalinida is an order of protists. They are flat, unicellular, solitary flagellates which are all intestinal parasites of small aquatic vertebrates, such as frogs, toads, and fish. They do little damage to their hosts, mainly absorbing intestinal fluids commensally. Opalinid cells are covered with short flagella which look and act like cilia and usually a few longer flagella. Each organism has at least two nuclei. Many reproduce sexually Their evolutionary lineage is unknown.