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(Fig. 15.1; also 
Fig. 4.4). This phenotype is typically characterized 
by a more elongate cell shape than the trophont 
or “feeding” phenotype and by more rapid swim-
ming with fewer tumbles and turns (Fenchel, 
1990; Nelsen & Debault, 1978). Depending upon 
the species, after a certain period of starvation , 
during which autophagy occurs (Nilsson, 1984), 
the second typical response to starvation – encyst-
ment – occurs. Encystment has been reported in 
all major groups of oligohymenophoreans , except 
the astomes . The cyst wall, derived in part from 
extrusomes, is often multilayered, composed of an 
 ectocyst , mesocyst , and endocyst . The cyst wall is 
composed of chitin , other complex carbohydrates, 
and some proteins. Cysts have been analyzed 
in the peniculines Furgasonia (= Cyclogramma ) 
and Pseudomicrothorax (Bussers, 1976), the scu-
ticociliate Pseudocohnilembus (Olendzenski, 
1999), the hymenostomes Tetrahymena (McArdle, 
Bergquist, & Ehret, 1980) and Ichthyophthirius
(Ewing, Kocan, & Ewing, 1983), the peritrichs 
Telotrochidium (Walker, Edwards, & Suchard, 
1989) and Opisthonecta (Calvo, Fernandez-
Aliseda, Garrido, & Torres, 2003), and the apos-
tome Hyalophysa (Bradbury, 1974; Landers, 1991a, 
1991b). Excystment is stimulated by a number of 
factors (for review see Corliss & Esser, 1974). 
Ultimately and in all probability, a small set of 
“signal” molecules may be responsible: for exam-
ple, glycogen is very effective in inducing excyst-
ment of the phoront stage of apostomes (Bradbury 
& Trager, 1967). Paramecium species, which are 
not known to encyst , can survive for more than a 
month without food (Jackson & Berger, 1985b). 
 15.3 Somatic Structures 
 While oligohymenophoreans are an assemblage 
more speciose than the spirotrichs , the range 
of variation in body form is less dramatic. The 
“typical” oligohymenophorean is ovoid in shape, 
ranging in size from about 10 µm in small scutico-
ciliates , to almost 1 mm in the infective stage of the 
 hymenostome Ichthyophthirius , and up to 3 mm in 
some astomes (Figs. 15.2–15.5). As a group, the 
 peritrichs demonstrate considerable diversity of 
body form: zooids can be borne on stalks, attach 
directly to the substrate on a modified scopulary 
15.3 Somatic Structures 297
Fig. 15.2. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass 
 Peniculia – Frontonia , Paramecium , and Lembadion . Members of the Subclass Apostomatia – Hyalophysa and the 
adult of Conidophrys “impaled” on the seta of a crustacean and its ciliated dispersive bud 
Fig. 15.3. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass 
 Hymenostomatia – the tetrahymenid Tetrahymena and the ophryoglenid Ichthyophthirius with its small theront and 
gigantic trophont , which causes “ Ich ”. Members of the Subclass Peritrichia – two sessilids, the stalked and sessile 
Vorticella and its telotroch or swarmer and the permanently mobile and stalkless Opisthonecta ; and the mobilid 
Trichodina , which causes trichodinosis
15.3 Somatic Structures 299
Fig. 15.4. Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass 
 Scuticociliatia – the philasterids Dexiotricha , Anophryoides , Uronema , Philaster , Pseudocohnilembus , and 
region or attach and migrate along substrates with 
a complex adhesive disk , as does the mobiline peri-
trich Trichodina (Fig. 15.3). 
 Oligohymenophoreans are typically holotrichous 
with somatic kinetids as either monokinetids or 
dikinetids (Figs. 15.2–15.5). Dikinetids are often 
Fig. 15.5 Stylized drawings of representatives of the Class OLIGOHYMENOPHOREA . Members of the Subclass 
 Scuticociliatia – the pleuronematid Pleuronema and the thigmotrichids Boveria and Hemispeira . Members of the 
Subclass Astomatia – Anoplophrya , Radiophrya , and Maupasella
15.3 Somatic Structures 301
more common in the anterior half of the body and 
can be organized as paratenes , which are seem-
ingly transverse rows of kinetids (Ehret, 1967). The 
somatic kineties can converge forming a preoral 
suture or anterior secant system at the anterior end 
and a postoral suture or posterior secant system at 
the posterior end. The shape, kind, and arrange-
ment of these convergence regions is system-
atically important, especially in the astomes . The 
posterior end of oligohymenophoreans , especially 
 scuticociliates , often bears kinetids whose cilia are 
significantly longer than the general body ciliature 
(Fig. 15.4). These so-called caudal cilia are typi-
cally not active, but can be used for attachment to 
the substrate. 
 The oligohymenophorean cortex, especially 
those of Tetrahymena and Paramecium , has been 
and continues to be exhaustively studied by cell 
and molecular biologists. The literature on this is 
extensive, and we provide here a few selected ref-
erences as an introduction to the literature and to 
demonstrate the structural attributes of the ciliates in 
this class. The oligohymenophorean cell is covered 
by a plasmalemma , which is underlain by cortical 
 alveoli (Allen, 1967, 1971, 1978). Intramembranous 
particles are distributed over the surface of the 
 plasmalemma , joining this surface membrane with 
the subsurface alveoli and trichocysts and inte-
grating them with the cilia (Allen, 1978; Plattner, 
Miller, & Bachmann, 1973). The lipid content of 
these surface membranes can be varied so that the 
ciliate can maintain a particular membrane fluidity 
over large ranges of environmental temperatures: at 
lower temperatures, membranes have more unsatu-
rated fats and at higher temperatures, membranes 
have more saturated fats (Kitajima & Thompson, 
1977; Nozawa, Iida, Fukushima, Ohki, & Ohnishi, 
1974; Wunderlich, Speth, Batz, & Kleinig, 1973). 
Variations in thermotolerance among species of 
Paramecium may be due to genetic variation in the 
fatty acid profiles of their cell membranes (Sasaki 
et al., 2006). 
 The plasma membrane is covered by a sur-
face coat, which is about 20 nm thick, and com-
posed of glyco- and other proteins (Allen, 1978). 
Prominent among the proteins are the immobili-
zation antigens or i-antigens , so named because 
antibodies to them bind the cilia together and 
prevent ciliary locomotion (Beale, 1954). These 
proteins are GPI-anchored proteins (Capdeville, 
Cardoso de Almeida, & Deregnaucourt, 1987; 
Clark, Gao, Gaertig, Wang, & Cheng, 2001; Ko & 
Thompson, 1992). They show considerable vari-
ation in both laboratory and natural populations 
(Lin et al., 2002; Saad & Doerder, 1995). There 
has long been speculation that these antigens pro-
tect the plasma membrane from the environment. 
Clear seasonal differences in the appearance of 
particular antigens suggests an important, but 
undetermined, ecological role for them (Doerder 
et al., 1996; Gerber, Lopez, Shook, & Doerder, 
2002; Saad & Doerder, 1995). 
 The plasma membrane is underlain, as in other 
ciliates, by cortical alveoli , which are conspicuous 
in the oligohymenophoreans . The alveoli were 
likened to the membrane cisternae of muscle cells 
(Allen, 1971; Satir & Wissig, 1982), a specula-
tion that predicted they would sequester calcium. 
 Calcium , which was later directly visualized in the 
 alveoli of Paramecium using secondary ion mass 
spectrometry, was shown to decrease on extrusion 
of trichocysts – a calcium-induced process. Calcium 
refilled the alveoli over several hours following 
depletion (Mohamed et al., 2003; Stelly, Halpern, 
Nicolas, Fragu, & Adoutte, 1995). In addition to 
calcium, alveoli