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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 
 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 “parts” 
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 – 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
– Didier & Detcheva, 1974; Paranophrys – 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 
 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 “watch-glass” 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 
“brush” or “flamme” (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