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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 
 The cytoproct of the oligohymenophoreans 
Paramecium and Tetrahymena is a “somatic” cor-
tical structure that is the “terminal” component 
of the “digestive system” 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 & 
 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 – 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 3–8 rows of kinetosomes 
(Figs. 15.2–15.5). 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’a 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 “stomatin” 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
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