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cells, like macrophages and leukocytes . Briefly, the model, which can probably be applied generally to all ciliates, involves formation at the cytopharynx of the nascent food vacuole by fusion of membrane vesicles. The food vacuole or phagosome separates from the oral region. In Paramecium , some of these vesicles are called acidosomes because of the acidic nature of their contents. As it circulates through the cytoplasm it fuses with lysosomes , the ingested prey are killed and digested, and nutrients are absorbed. The spent vacuoles make their way to the cytoproct where the contents are egested. Much of the vacuolar membrane is recycled to the cytopharyngeal region via microtubular tracts and in readiness for formation of the next food vacu- ole. While this model has not been demonstrated completely in any oligohymenophorean except Paramecium (Allen, 1984; Allen & Fok, 2000), components of the model have been observed in representatives of most subclasses in this class, and indeed even in other classes of ciliates. The now classic work of Didier (1971) provided working definitions for the oral polykinetids of the oligohymenophorean subclasses. These definitions have stood the test of time (Grain, 1984; de Puytorac & Grain, 1976), although they may only have a loose application (Peck, 1977a, 1978). Among the ciliates obviously exhibiting oral polykinetids, four major kinds have been identified, primarily identi- fied with a subclass: the peniculus and quadrulus of the peniculines ; the membranoid of the scuti- cociliates ; the membranelle of the hymenostomes ; and the polykinety of the peritrichs . These will be briefly described below with some reference to the primary literature. This section will close with a discussion of the apostome oral region. Peniculines typically have three longitudinally oriented oral polykinetids on the left side of the oral cavity. The subclass is named for the peniculus , a term originally proposed by von Gelei (1934a) for the two, left-most oral polykinetids of Paramecium whose kinetosomes are closely packed. Lund (1941) provided the term quadrulus for the peniculine polykinetid whose kinetosomes are more loosely packed. It is now agreed that the quadrulus is only a developmentally differentiated peniculus , peculiar to some, but not all, peniculines (Didier, 1971; Roque, 1961). The peniculus is typically longitudinally oriented in the oral cav- ity, has postciliary microtubular ribbons associ- ated with the right-most row of kinetosomes, has alveoli between the rows, and has parasomal sacs restricted to the outsides of the kinetosomal group- ing. The kinetosomes may be linked by distal and proximal connectives and each polykinetid may be linked to its neighbor by a deeper network of fibrils (Grain, 1984; Lynn, 1981; de Puytorac & Grain, 1976; but see Peck, 1977a, 1978). This structure has been recorded, for example, in Paramecium (Didier, 1971), Urocentrum (Didier; Guinea, Gil, & Fernández-Galiano, 1987), and Frontonia (Didier; Gil, 1984). Minor components of these filament macromolecules include actin (Cohen, Garreau de Loubresse, & Beisson, 1984) and tetrin -related elements (Clerot et al., 2000). Paramecium is a filter feeder, using its oral cilia to not only con- centrate particles from the environment, but also to propel them into the nascent digestive vacuole (Fenchel, 1980a; Ishida, Allen, & Fok, 2001). Actin , as in Tetrahymena (see below), is involved in food vacuole formation (Cohen et al., 1984; Zackroff & Hufnagel, 1998). Acid hydrolases , at least, are found in lysosomal vesicles that fuse with the digestive vacuole (Allen, 1984; Estève, 1970), along with other vesicles that may derive from endocytosis at the parasomal sacs (Ramoino et al., 2001). Actin paralogs may also function in vesicle fusion and phagosome movement through the cytoplasm (Sehring, Reiner, Mansfeld, Plattner, & Kissmehl, 2007). In the frontoniid peniculines , the elongated cytostomal region is bounded by robust nematodesmata (Didier, 1971; Gil, 1984), which probably aid these ciliates in the ingestion of large diatoms and filamentous cyanobacteria . While Small and Lynn (1981, 1985) argued that this was another feature relating nassophorean and peniculine ciliates, SSUrRNA gene sequences now strongly suggest that peniculines are related to other oligohymenophoreans (Strüder-Kypke et al., 2000b). Thus, the frontoniid nematodesmata very probably evolved by convergent evolution as an adaptation for their feeding on these larger prey particles. The scuticociliates , like the peniculines , typi- cally have three oral polykinetids on the left side of the oral cavity. These function to provide a filter- feeding current that directs particles towards the cilia of the paroral , which then filters out these par- ticles (Figs. 15.4, 15.5) (Fenchel, 1980a, 1980b). Paraxonemal bodies bounded by the ciliary mem- brane of some oral polykinetid cilia may provide strength and resilience to these current-creating cilia (Didier, 1976). However, the oral polykinetids can become highly modified, fragmenting during stomatogenesis into structurally independent com- ponents numbering more than three: oral polyki- netid 1 of Porpostoma can have up to 20 \u201cparts\u201d while oral polykinetid 2 of Pleuronema is typically divided into two widely separated parts (Fig. 15.5) (Small, 1967; Song, 2000). At the ultrastructural level, these oral polykinetids demonstrate some diversity \u2013 some being called membranoids and others membranelles . Membranoids might be con- sidered the archetypical scuticociliate oral polyki- netid. Membranoid kinetosomes are irregularly arranged and linked at distal and proximal levels, and only irregularly do the kinetosomes on the right side bear a postciliary ribbon (e.g., Cohnilembus \u2013 Didier & Detcheva, 1974; Paranophrys \u2013 Didier & Wilbert, 1976). Nevertheless, the scuticociliates Cinetochilum and Dexiotricha (Peck, 1977a; de Puytorac et al., 1974a) have oral polykinetids with the basic features of a membranelle (see below). Alveoli have been observed between the cilia of oral polykinetids of thigmotrich scuticociliates (Da Silva Neto, 1992; de Puytorac et al., 1978), and this has suggested to de Puytorac et al. (1978) their closer affinities to the peniculines . Peck (1977a, 1978) carefully reviewed this early literature and came to the conclusion, which still is justified, that it is very difficult to make broad categorical characterizations of peniculi , membranoids , and membranelles . At most, the terms can only be applied loosely to general configurations of oral kinetosomes. The hymenostomes have served as the arche- type of the class, exhibiting an oral structure with a paroral on the right and three oral polyki- netids on the left (Fig. 15.3) (Corliss, 1979; Lynn & Small, 2002). The ophryoglenines are further distinguished by the presence of the organelle of Lieberkühn , a dense \u201cwatch-glass\u201d shaped struc- ture placed between oral polykinetids 2 and 3. In Ophryoglena , oral polykinetid 2 has an enlarged posterior portion whose cilia form the conspicuous \u201cbrush\u201d or \u201cflamme\u201d (Canella & Rocchi-Canella, 1976; Lynn, Frombach, Ewing, & Kocan, 1991b). Filter-feeding on bacteria and smaller particles is typical of most hymenostomes , which can use either the cilia of the paroral or the cilia of the innermost or third oral polykinetid to filter out particles (Fenchel, 1980a, 1980b; Fenchel & Small, 1980). The oral cilia of some hymenostomes have differ- ing intramembranous particle distributions, have bristles on the outside of the ciliary membrane, and paraxonemal bodies extending their length (Didier, 1976; Sattler & Staehelin, 1974). Didier (1976) drew particular attention to the development