Abstract This class includes the two model ciliates – Tetrahymena and Paramecium – laboratory genetic models of protists and the first two ciliates to have completed genome projects. Along with these representatives of the Subclasses Hymenostomatia and Peniculia, there is a diverse variety of species distributed in four other subclasses: Scuticociliatia, Peritrichia, Apostomatia, and Astomatia. While the archetype of the class has a paroral and three left oral polykinetids, there is really no strong morphological or ultrastructural synapomorphy for the class. Molecular phylogenetics generally recov- ers an oligohymenophorean clade with moderate bootstrap support. The somatic kinetid is moderate to that of the last three classes: a generally robust, anteriorly-directed kinetodesmal fibril, a divergent postciliary ribbon, and a radial transverse ribbon, except in the peniculine where it is tangential. Oral structures depart from the archetype in the apostomes with their rosette and complex cytopha- ryngeal apparatus and are absent in the astomes. Oligohymenophoreans are as broadly distributed across habitats as any class, but they include obligate symbionts: the apostomes typically with crustaceans and the astomes with annelids. Ichthyophthirius , the parasite of freshwater fishes, is an hymenostome that can plague aquaculture operations. Division morphogenesis is characterized as buccokinetal and parakinetal. There is a rich literature on the genetics of these ciliates, particularly Paramecium and Tetrahymena , which can be easily induced to conjugate in the laboratory, enabling a deeper understanding of their molecular biology. Keywords Autogamy The Class OLIGOHYMENOPHOREA includes two ciliate genera, Tetrahymena and Paramecium , which one might describe respectively as the “white mouse” and the “white rat” of the Phylum Ciliophora, based both on their relative sizes and on their being the most “highly tamed” labora- tory models in the phylum. Lwoff (1923) suc- cessfully cultured Tetrahymena axenically , that is in a sterile nutrient broth without bacteria, and so initiated the exploration of its physiology and biochemistry as a model organism. It was to be many years later that a partly defined medium was devised to axenically culture Paramecium (Lilly & Klosek, 1961). Now there are genome projects well underway for representative species of both these genera (Eisen et al., 2006; Tetrahymena – www. lifesci.ucsb.edu/ ˜ genome/ Tetrahymena /; and Aury et al., 2006; Paramecium – carroll.vjf.cnrs.fr/pt/), bringing these two ciliates and molecular genetic approaches to understanding their biology into a competitive position with other model organisms. The Class OLIGOHYMENOPHOREA is argu- ably the most diverse assemblage within the phy- lum. While there are fewer included subclasses in it compared to the Class SPIROTRICHEA ( see Chapter 7 ), there have been many more species described. Oligohymenophoreans are commonly encountered in freshwater and marine habitats with peniculines being most conspicuous in the former habitats and scuticociliates being most conspicu- ous in the latter habitats. Some members of the class show a preference for moribund and dead metazoans (e.g., Corliss, 1972c; Fauré-Fremiet & Mugard, 1946), and this capacity has lead several times independently in the class to symbiotic and Chapter 15 Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA – Once a Pivotal Group, Now a Terminal Radiation 279 parasitic modes of life in diverse hosts: the apos- tomes in crustaceans (Bradbury, 1966a; Chatton & Lwoff, 1935a); the thigmotrich scuticociliates in bivalve molluscs (Chatton & Lwoff, 1949, 1950); hymenostomes as parasites of gastropods (Kozloff, 1956) and fishes (Hoffman, 1988; Hoffman et al., 1975); and peritrichs , both on invertebrates (Matthes, 1974) and vertebrates (Lom, 1995), and in invertebrates (Lom, 1994) and vertebrates (Lom, 1958, 1995). These infections on rare occa- sions can cause mass mortalities of their host, such as Ichthyophthirius and fishes (Wurtsbaugh & Tapia, 1988). In other instances, infections may significantly impact fecundity , such as the scutico- ciliate Orchitophrya infecting the testes of starfish (Stickle, Weidner, & Kozloff, 2001). The morpho- logical polymorphism and complex life cycles often characteristic of these symbiotic ciliates is pres- aged by similar dramatic changes in form in free- living species, which may undergo form change in response to feeding conditions, such as starvation (Fauré-Fremiet & Mugard, 1949a; Nelsen & De Bault, 1978) or the presence of different kinds of foods (Small, Heisler, Sniezek, & Iliffe, 1986). The absence of bacteria may stimulate some species to transform into macrostome cannibals , consuming their conspecifics and other smaller species (Fig. 15.1) (Corliss, 1973; Njiné, 1972). Despite these dramatic variations in body mor- phology, the ciliates in this class demonstrate their name, having few oral polykinetids or mem- branelles (oligo, Gr. – few; hymen, Gr. – membrane; phoros, Gr. – bearing). The typical arrangement is three, inconspicuous oral polykinetids on the left side of the oral cavity and a paroral of dikinetids or a stichodyad on the right side. Nevertheless, the oral structures of some pleuronematine scu- ticociliates , such as Pleuronema , and peritrichs can be quite prominent. Corliss (1979) argued that oligohymenophoreans were a linking assem- blage between the “lower” kinetofragminopho- ran groups and the “higher” polyhymenophorans . Molecular phylogenetic analyses have now refuted this vision and demonstrate the oligohymenopho- reans to be a “terminal” branch in the Subphylum Intramacronucleata (Greenwood, Sogin, & Lynn, 1991a; Strüder-Kypke et al., 2000b). With the nota- ble exception of the peritrichs , the vast majority of oligohymenophoreans are holotrichously ciliated. They are medium-sized, typically 65–150 µm in length; some scuticociliate species can be less than 10 µm in length, the large carnivorous peniculine Neobursaridium can reach lengths of 600 µm (Dragesco & Tuffrau, 1967), and some astomes can reach 3 mm in length (de Puytorac, 1994g). As with the tintinnids , the loricate forms – fossilized peritrichs – can be placed in contemporary families even though they date from the Lower Triassic , some 200 million years ago (Weitschat & Guhl, 1994). Even more remarkably, a Paramecium spe- cies has been described from southern Germany in amber -bearing sandstone deposits (Schönborn, Dörfelt, Foissner, Krientiz, & Schäfer, 1999), which have been assigned to the Late Cretaceous , some 90 million years ago (Schmidt, von Eynatten, & Wagreich, 2001). Thus, it is very likely that the class originated over 250 million years ago, and even perhaps in the Paleoproterozoic , if the molec- ular clock can be trusted (Wright & Lynn, 1997c). As noted above, the ease of cultivating spe- cies of Tetrahymena and Paramecium has made them two of the most widely studied genera in the phylum, and therefore in this class. These earlier approaches to their cultivation lead to the establishment of chemically defined media for both Tetrahymena (see Kidder & Dewey, 1951; Orias, Hamilton, & Orias, 2000) and Paramecium (Bihn, Lilly, & Napolitano, 1974; Fok & Allen, 1979; Soldo, Godoy, & van Wagtendonk, 1966). This tractability, together with tractable genetic techniques, has given rise to an enormous lit- erature on these two genera, summarized in books, which can only be listed here: on Tetrahymena – Hill (1972), Elliott (1973a), Asai and Forney (2000); and on Paramecium – Beale (1954), Görtz (1988a), Jurand and Selman (1969), Sonneborn (1970), van Wagtendonk (1974), and Wichterman (1986). Both axenic (Finley & McLaughlin, 1965; Finley,