Opalinida (ōˌpəlĭnˈĭdə) [key], phylum of unicellular heterotrophic organisms of the kingdom Protista. The opalinids are all intestinal parasites of small vertebrates, such as frogs, toads, and fish. Opalinid cells are covered with either short, hairlike structures called cilia or with longer flagella. Each organism has at least two nuclei. Their evolutionary lineage is unknown.