Opalina is a genus of protozoa found in the intestines of frogs and toads. They lack mouths and contractile vacuoles, they are covered with nearly equal flagelliform cilia, and they have numerous nuclei, all similar. All the species are obligate endosymbionts, most likely commensal rather than parasitic, in cold-blooded vertebrates. Its body is leaflike in shape. They lack cytostomes. They are saprozoic, consuming dead matter, which suggests their commensal role. It undergoes plasmotomy. An example of a species is Opalina ranarum.