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environmental osmolarities, indicates involvement of this organellar complex in both the elimination of excess water and in the excretion of metabolic wastes. The cytoproct of the oligohymenophoreans Paramecium and Tetrahymena is a \u201csomatic\u201d cor- tical structure that is the \u201cterminal\u201d component of the \u201cdigestive system\u201d of these cells. Like the contractile vacuole complex , microtubules extend into the cytoplasm from dense material support- ing the cytoproct . These microtubules guide food vacuoles to the cell cortex where their contents can be egested, and where the membranes can be recycled back to the oral region to form new food vacuoles (see Oral Structures ) (Allen & Wolf, 1974, 1979). Extrusomes of oligohymenophoreans are either mucocysts , which can be quite rod-like in some scuticociliates, or spindle trichocysts (Hausmann, 1978). Their similarity to secretory granules in other eukaryotes has made Tetrahymena and Paramecium model systems to deepen our understanding of cel- lular secretion processes in general. Models for this process, involving a variety of molecules, such as rosette particles, Ca 2+ -ATPase, parafusin , and annexins , have been presented for both Tetrahymena (e.g., Satir, 1989; Satir, Schooley, & Satir, 1973; Turkewitz, 2004) and Paramecium (e.g., Froissard et al., 2002; Gilligan & Satir, 1983; Knochel et al., 1996; Plattner et al., 1980; Satir). These extrusomes develop from Golgi-ike membranous systems in the endoplasm and are then transported out to the somatic cortex (Ehret & de Haller, 1963; Hausmann, 1978). Trichocyts are composed of as many as 100 polypeptides that are processed and arranged as elementary units into a crystal lattice (Hausmann,; Vayssie, Garreau de Loubresse, & Sperling, 2001). While the shape changes of wild-type trichocysts depend on Ca 2+ (Adoutte, 1988; Adoutte, Garreau de Loubresse, & Beisson, 1984; Sperling, Tardieu, & Gulik-Krzywicki, 1987), several non-discharge trichocyst mutations have now been described in Paramecium (Beisson, Cohen, Lefort-Tran, Pouphile, & Rossignol, 1980; Pollack, 1974). The function of trichocysts has long been debated (Haacke-Bell, Hohenberger-Bregger, & Plattner, 1990), and trichocyst mutants have permitted the first test of the defensive function hypothesis of these organelles ( see Life History and Ecology ). Trichocyst non-discharge mutants of Paramecium are up to 45 X more susceptible to predation by the litostomes Dileptus and Monodinium , and by the heterotrich Climacostomum than wild-type cells (Harumoto & Miyake, 1991; Miyake & Harumoto, 1996; Sugibayashi & Harumoto, 2000). Backward swimming, which often accompanies an attack by these predators, does not enable a more effective escape than forward swimming, as mutants unable to swim backwards are caught as frequently as wild-type cells (Harumoto, 1994; Sugibayashi & Harumoto). Intriguingly, trichocysts do not protect Paramecium against predation by Didinium , sug- gesting that this predator is currently ahead in the arms race between predator and prey (Miyake & Harumoto). Mucocysts are the other major extrusome type in oligohymenophoreans . Mucocysts provide for a variety of cell functions: they are involved in the formation of cyst walls (e.g., Ewing et al., 1983; McArdle et al., 1980) and loricas (e.g., González, 1979; Wilbert & Foissner, 1980). In the apostome Hyalophysa , Landers (1991a) has observed that the rod-shaped mucocysts of this ciliate are digested in autophagic vesicles during the phoretic stage, perhaps serving as a nutrient source. Mitochondria in the OLIGOHYMENOPHOREA are typical of those of the phylum \u2013 primarily cortical organelles with tubular cristae. They are anchored to the somatic cortex through fibrous connections between the outer mitochondrial membrane and cortical microtubules and the epiplasm (Aufderheide, 1983). The mitochondria grow primarily by elonga- tion and divide when their length is doubled. This growth and division maintains the population of mitochondria in the cytoplasm, but it is not tightly coupled to the cell cyle in Paramecium (Perasso & Beisson, 1978). In scuticociliates , perhaps all taxa have exceedingly large mitochondria, often extend- ing the entire length of the ciliate beneath the cortical ridges, and perhaps are even connected between kine- ties (Antipa, 1972; Kaneshiro & Holz, 1976; Peck, 1977a; de Puytorac et al., 1974a). In rare instances, the mitochondria have transformed into hydrogeno- somes in anaerobic species, such as the scuticociliates Cristigera and Cyclidium (Clarke, Finlay, Esteban, Guhl, & Embley, 1993; Fenchel & Finlay, 1991a). A variety of other organelles typical of eukaryotes have been described in oligohymenophoreans . Golgi complexes , composed of a few flattened cisternae, have been reported in representatives of all the major subclasses (Estève, 1972; Kurz & Tiedtke, 1993; Lobo-da-Cunha & Azevedo, 1994). In Tetrahymena , they are often localized in the cortex adjacent to mitochondria (Kurz & Tiedtke, 1993). Peroxisomes have also been reported in hymenostomes (Fok & Allen, 1975; Lobo-da-Cunha & Azevedo, 1993) and peniculines (Stelly, Balmefrezol, & Adoutte, 1975). Finally, there may be crystals, excretory in function, whose abundance depends on the physiological state of the cell, and which may contain calcium (Nilsson & Coleman, 1977) and/or the purines guanine and hypoxanthine (Creutz, Mohanty, Defalco, & Kretsinger, 2002; Soldo, Godoy, & Larin, 1978). 15.4 Oral Structures The oral region of the oligohymenophoreans , quite similar in four of its six included subclasses, typi- cally includes, on the right side of the oral region, a ciliated paroral and, on the left side, three oral polykinetids of from 3\u20138 rows of kinetosomes (Figs. 15.2\u201315.5). This general pattern applies well to the peniculines , scuticociliates , hymenostomes , and peritrichs , but it does not to the apostomes and astomes . The latter two groups are undoubt- edly derived from within this radiation, based on SSUrRNA gene sequences (Affa\u2019a et al., 2004; Lynn et al., 2004): astomes lack an oral region altogether while apostomes have a highly modified oral region (see below). The oral structures of oligohymenophoreans are also influenced by the polymorphic life histories typical of many of the included species, especially scuticociliates , hymenostomes , and apostomes . As the ciliate transforms from one life history stage to the next, its morphology, both somatic and oral, changes as an adaptation to the new mode of living. A typical change is in the size and shape of the oral organelles, which are adapted to feed on different prey species: Ichthyophthirius has a diminutive oral cavity as the dispersive theront and a larger, seemingly undifferentiated cavity as the feeding trophont (Fig. 15.3) (Canella & Rocchi-Canella, 1976). Hymenostomes , such as some species of Tetrahymena and Glaucoma , and scuticociliates may have microstome forms that feed on bacte- ria and macrostome forms, sometimes cannibals , which feed on their smaller conspecifics (Fig. 15.1) (Corliss, 1973; Njiné, 1972; de Puytorac, Savoie, & Roque, 1973b; Small et al., 1986; Williams, 1960, 1961). The macrostome-microstome transfor- mation in some Tetrahymena species is induced by a \u201cstomatin\u201d preparation derived from the prey (Buhse, 1967; Méténier, 1977). The cell biology of ingestion , digestion , and egestion of ciliates has relied heavily on research 15.4 Oral Structures 309 310 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA on Tetrahymena (Nilsson, 1979) and Paramecium (Allen, 1984; Allen & Fok, 2000; Plattner & Kissmehl, 2003), both ciliates serving as model systems for phagotrophy by other eukaryotic