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1992); the hymenos- tomes Colpidium , Glaucoma , Tetrahymena , and Turaniella (Iftode et al., 1984; Lynn & Didier, 1978; Nilsson, 1976; Peck, 1978; Williams & Bakowska, 1982); and the peritrichs Termitophrya , Trichodina , and Thuricola (Eperon & Grain, 1983; Maslin-Leny & Bohatier, 1984; Noirot-Timothée & Lom, 1965). In turaniellid hymenostomes , paro- ral kinetosomes may not be ciliated, along only part of or the whole length of the paroral (Iftode et al., 1984; Lynn & Didier, 1978). For some time, it was conjectured that the oral rib microtubules arose from the postciliary microtubules of the paroral dikinetids . If so, these microtubules must break during stomatogenesis, because there is firm evidence now that the oral rib microtubules arise in dense material, often taking the form of a spur or papilla, which may extend over top of the non- ciliated kinetosomes of the paroral (Hausmann & Hausmann, 1981; Iftode et al., 1984; Lynn & Didier, 1978). This origin is reminiscent of the origin of the cytopharyngeal microtubules of the prostomes , such as Coleps (Lynn, 1985). The oral ribs are separated by alveoli and are typically supported by microtubules arranged as 4 + 2 in hymenostomes and scuticociliates (Lynn & Didier, 1978; Nilsson, 1976; Peck, 1978; de Puytorac et al., 1974a; Sattler & Staehelin, 1979), and 3 + 1, 3 + 2, and 4 + 2 in peritrichs (Hausmann & Hausmann; McKanna, 1973b). Bradbury (1989) finally clinched the phylogenetic affinities of the apostomes by discovering a paroral during the development of the free- swimming tomite of Hyalophysa . Molecular genetic evidence now supports this conclusion (J.C. Clamp et al., 2008; Lynn, Strüder-Kypke, & Bradbury, 2005). The apostomes show a bewilderingly bizarre set of cortical kinetosomal assemblages, and all have had various authors suggest that they are homologues of the oral structures of other ciliates. The rosette , although not always present, can also clinch membership of a ciliate to this subclass of oligohymenophoreans . The rosette is a tube-like invagination, lined by ridges or septa that are cov- ered by cortical alveoli and provided with some cilia and an aggregation of dense vesicles. Its func- tion is not known, but its ultrastructure has been described in typical apostomes like Hyalophysa (Bradbury, 1966b) and atypical ones like Collinia (de Puytorac & Grain, 1975) and Conidophrys (Bradbury & Tyson, 1982). The x , y , and z kineties of foettingeriid apostomes have been characterized as perioral (Bradbury, 1966a) or as oral (Lynn & Small, 2002). Given that kinetodesmal fibrils are associated with the kinetosomes of the falciform kinetosomal fields of apostomes (Bradbury, 1966b, 1989; de Puytorac & Grain, 1975), it is more likely that these are highly modified arrays of somatic kinetosomes. Apostomes feed on the exuvial flu- ids of the moulted exoskeleton of their crustacean host (Bradbury, 1973, 1975b), on the blood of their crustacean host (de Puytorac & Grain), by dis- solving the host’s exoskeleton (Bradbury, 1975b; Bradbury & Goyal, 1976; Bradbury, Deroux, & Campillo, 1987), and by penetrating setae on the crustacean exoskeleton to presumably feed on tis- sue fluid (Fig. 15.2) (Bradbury & Tyson, 1982). The ingestatory region in exuviotrophic forms extends between somatic kinety 1 and the x , y , and z kineties. Food may be ingested via a cytophar- ynx lined with microtubular ribbons and to which diskoidal vesicles are directed for phagosome for- mation, reminiscent of other ciliates (Bradbury, 1973, 1975b; Bradbury et al., 1987). Deviants from this typical pattern occur in Terebrospira , which ingests the solubilized components of the exoskeleton by pinocytosis over its entire cell sur- face (Bradbury, 1975b; Bradbury & Goyal, 1976) and by Conidophrys , which ingests, presumably setal tissue fluids , through a broad cytostome char- acterized by delicate tubules (Bradbury & Tyson, 1982). Digestive enzymes, such as phosphatases , 15.4 Oral Structures 313 314 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA may be secreted outside the cell in some apostomes (Bradbury & Goyal, 1976). Phagosomes are acidi- fied during the digestive process and lysosomes are likely involved (Bradbury & Goyal, 1976; Landers, Treadaway, Johnson, & Luckie, 2001). In comparison to the above, we are essentially ignorant about the feeding biology of the astomes . A reasonable conjecture is that they may feed like the astomatous mutants of Tetrahymena and apostomes , presumably by pinocytosis via the parasomal sacs , but in the case of the astomes , over the entire cell surface (Nilsson & Van Deurs, 1983; Rasmussen & Orias, 1975). 15.5 Division and Morphogenesis Division by oligohymenophoreans is typically as free-swimming cells, which divide equally or isot- omically. In rare cases, division in peritrichs , thig- motrichs , and some astomes can be anisotomic or unequal. Multiple division, catenulation , or linear palintomy can occur in which chains of cells are formed in apostomes , like Polyspira , and astomes , like Hoplitophyra and Radiophrya (Fig. 15.5) (de Puytorac, 1994g, 1994h). The isolation of cell divi- sion arrest mutants in Tetrahymena suggests that chain formation may require a modification of per- haps only one gene product (Frankel, 1989; Frankel, Nelsen, & Jenkins, 1977). Palintomy can also occur in a reproductive cyst : the tomont stages of the ophryoglenines Ophryoglena and Ichthyophthirius divide within a reproductive cyst , the latter yielding over 4,000 tomites; and the apostome tomonts and in some special cases, like T. vorax and Porpostoma , divide within a cyst (Fig. 15.1). Foissner (1996b) remarked that the oligohy- menophoreans include “the pets of the ciliatolo- gists” – the genera Paramecium and Tetrahymena . Species in these two genera have provided models for advancing our understanding at two cellular levels: (1) the replication of organelles and organel- lar complexes; and (2) the replication of the pattern of the entire cell. This developmental biological literature has been extensively treated by Frankel (1989), who provides a thorough review of the literature and a thoughtful treatment of a variety of issues. As we have noted previously, we still have much to learn about the cellular and molecular proc- esses underlying division morphogenesis . Much of the systematic literature attributes a particular role to a particular kinetosomal structure, and yet there are unambiguous refutations of this role attribution. For example, somatic kinety 1 of Tetrahymena is the rightmost postoral kinety, and it has been called the “stomatogenic kinety” because the oral primor- dium typically develops in association with it (Fig. 15.11). However, Nanney (1967) demonstrated that the primordium can develop along other kineties, a phenomenon that he called “cortical slippage” . If there were a consistent directional bias in this process, ultimately every somatic kinety would become a stomatogenic kinety . The inevitable con- clusion is that the relative position on the somatic cortex is the important causal determinant and not an association with a particular kinety or kineto- somal structure (Frankel, 1989; Nanney, 1967). It is worth remembering this important point in the subsequent discussion. Our reference to the literature on developmen- tal biology will be very selective, highlighting a few papers that have relevance to the systemat- ics. From this perspective, Foissner (1996b) has thoroughly reviewed the systematic literature on ontogenesis of oligohymenophoreans . Our cita- tions to this literature prior to Foissner will mainly highlight some exemplary studies as we proceed below to characterize each subclass.