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Opalinidae
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).〔Delvinquier, B.L.J. and D.J. Patterson (1993) The opalines. In: J.P. Kreier and J.R. Baker (eds) Parasitic Protozoa. 2nd Ed., Vol. 3. Academic Press: San Diego. ISBN 0-12-426013-6. pp. 247-325.〕 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,〔Dobell, C. (1932) Antony van Leeuwenhoek and his "little animals". Bale, Sons and Danielson: London〕 has led to much debate regarding their phylogenetic position among the protists.
==Taxonomy and phylogeny==
The relationship between 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〔Corliss, J.O. (1955) The opalinid infusorians: Flagellates or ciliates? ''Journal of Protozoology'' 2:107-114.〕 and they were placed in Sarcomastigophora, with the amoebae and flagellates.〔Corliss, J.O. and W. Balamuth (1963) Consideration of the opalinids as a new superclass in the subphylum Sarcomastigophora. ''Journal of Protozoology'' 10(Suppl):26.〕 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.〔Patterson, D.J. (1985) The fine structure of ''Opalina ranarum'' (family Opalinidae): Opalinid phylogeny and classification. ''Protistologica'' 21(4):413-428.〕 In 2004, the first reliable opaline genetic sequence data supported the monophyletic nature of the order Slopalinida.〔Kostka, M., V. Hampl, I. Cepicka and J. Flegr (2004) Phylogenetic position of ''Protoopalina intestinalis'' based on SSU rRNA gene sequence. ''Molecular Phylogenetics and Evolution'' 33(2):220-224.〕 The authors of that study considered the opalines to be a family (Opalinidae) within the order Slopalinida.
There are currently about 200 recognized species of opalines in 5 genera: ''Opalina'' Purkinje and Valentin 1835, ''Protoopalina'' Metcalf 1918, ''Cepedea'' Metcalf 1920, ''Zelleriella'' Metcalf 1920, and ''Protozelleriella'' Delvinquier et al. 1991. Two additional genera, ''Hegneriella'' Earl 1971 and ''Bezzenbergeria'' Earl 1973, have not been considered as valid by subsequent authors (p. 249)〔 The 5 recognized genera differ in terms of the number of nuclei, the appearance and location of the falx (two short, sickle-shaped rows of flagella), and whether the long rows of flagella (called "kineties") cover the body evenly or if there is a "bald spot". Due to the differences in body shape among the different life cycle stages within a species, the use of overall body shape - whether flat or cylindrical - to differentiate the genera has been de-emphasized.
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