The opalines are a small group of peculiar protists, currently assigned to the family Opalinidae, in the order Slopalinida. Their name is derived from the opalescent appearance of these microscopic organisms when illuminated with full sunlight (p. 247). Most opalines live as endocommensals in the large intestine and cloaca of anurans (frogs and toads), though they are sometimes found in fish, reptiles, molluscs and insects. The unusual features of the opalines, first observed by Antoine van Leeuwenhoek in 1683, has led to much debate regarding their phylogenetic position among the protists.

The relationships opalines and other protists has been a subject of great controversy since the late 19th century, and is not completely resolved at present. Initially, microscopists believed that the thousands of rhythmically beating hair-like structures which cover their surface were cilia, and they placed the opalines in Ciliophora. In the early 20th century other aspects of opaline biology clearly differentiated them from the ciliates and they were placed in Sarcomastigophora, with the amoebae and flagellates. In the 1980s, detailed ultrastructural studies of Opalina ranarum revealed that they share many features with the heterokonts of the family Proteromonadidae. A new order—Slopalinida Patterson 1985—was proposed to include the members of the families Proteromonadidae Grassé 1952 and Opalinidae Claus 1874. In 2004, the first reliable opaline genetic sequence data supported the monophyletic nature of the order Slopalinida. The authors of that study considered the opalines to be a family (Opalinidae) within the order Slopalinida.