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.