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some detailed reinvestigations 
of armophoreans before any generalizations can be 
made about their somatic dikinetids . A further intrigu-
ing physiological observation is that Nyctotherus
ovalis switches swimming direction in response 
to voltage changes rather than showing a ciliary 
reversal. Moreover, this behavior appears to be 
influenced by host-dependent factors (van Hoek 
et al., 1999). 
 Contractile vacuoles are present in armopho-
reans . The cytoproct is often conspicuous, and in 
 clevelandellids may open to the outside by a cilia-
lined channel. 
 Mucocysts appear to be present in the cortex 
of clevelandellids (Paulin, 1967; de Puytorac & 
Grain, 1969) and armophorids (Esteban et al., 
 Finally, mention must be made of the apparent 
absence of mitochondria with tubular cristae in all 
 armophoreans . The mitochondria in these ciliates 
have evolved into hydrogenosomes (van Hoek, 
Akhmanova, Huynen, & Hackstein, 2000a; Boxma 
et al., 2005). These hydrogenosomes have a hydro-
genase that uses electrons derived from pyruvate 
oxidation to reduce protons and generate hydrogen 
(Fenchel & Finlay, 1991a; Müller, 1993; Voncken 
et al., 2002). The hydrogen is typically used in 
 armophoreans by endosymbiotic methanogens (see 
Life History and Ecology ). 
 8.4 Oral Structures 
 The armophoreans were placed until recently 
with the heterotrichs because of their holotrichous 
somatic ciliation and the presence of multiple oral 
polykinetids forming an adoral zone . The two or 
three rows of kinetosomes of the oral polykinetids 
are hexagonally packed. In armophorids , a third or 
fourth row of kinetosomes is added continuing the 
hexagonal packing (Esteban et al., 1995; Foissner & 
Agatha, 1999). Armophorid oral polykinetids have 
been called paramembranelles . Clevelandellids 
typically have three rows of kinetosomes hexago-
nally packed, but a fourth, shorter row lies directly 
opposite to, rather than hexagonally packed with, 
the kinetosomes of the third row, leading to their 
designation as heteromembranelles because of 
the different packing of these kinetosomes of the 
fourth row (de Puytorac & Grain, 1976). This dif-
ferent packing leads to a different orientation and 
beating of the cilia that was nicely revealed in some 
published micrographs (Paulin, 1967; Takahashi & 
Imai, 1989). 
 The adoral zones of armophorids and clevelan-
dellids may be quite extensive, spiralling around 
the body one or more times in some armophorids 
Fig. 8.3. Somatic cortex of Metopus whose postciliary ribbons extend alongside each other into the cortical ridges. 
This schema was constructed based on the brief descriptions provided in reports by Schrenk and Bardele (1991) and 
Esteban et al. (1995)
(Fig. 8.1). The clevelandellids have a deeper 
oral cavity called an infundibulum where the 
 heteromembranelles typically occur (Tuffrau & 
de Puytorac, 1994). Postciliary ribbons are associ-
ated with the kinetosomes of the posterior row in 
both armophorids and clevelandellids (Tuffrau & 
de Puytorac). 
 Paroral structures are quite variable in the class. 
 Armophorids appear to have a single file of cilia, 
which may be derived from linearly arranged 
oral dikinetids (Esteban et al., 1995; Foissner & 
Agatha, 1999; Sola, Serrano, Guinea, & Longás, 
1992). Clevelandellids have a paroral with two 
sets of cilia deriving from two files of kinetosomes 
separated by a ridge (Grim, 1998; Paulin, 1967; 
de Puytorac & Grain, 1969; Takahashi & Imai, 
1989), termed a diplostichomonad by de Puytorac 
and Grain (1976). The oral structures of armopho-
reans are underlain by complex fibrillar structures 
and microtubules. The filamentous components 
are implicated in the movement of vesicles to the 
food vacuole forming region (Eichenlaub-Ritter & 
Ruthmann, 1983). 
 8.5 Division and Morphogenesis 
 There have been only a few papers on cell divi-
sion and division morphogenesis of armophoreans 
since Wichterman (1936) described division in 
Nyctotheroides (= Nyctotherus ). He observed the 
oral primordium to develop subequatorially. Since 
silver-staining was not used, kinetosomal replica-
tion was not detailed. As far as we know, armo-
phoreans divide while swimming freely. Foissner 
(1996b) has characterized stomatogenesis as pleu-
rotelokinetal (i.e., occurring within or at the end of 
several somatic kineties). 
 Two studies on the armophorids , Metopus and 
Caenomorpha , demonstrated pleurotelokinetal 
stomatogenesis . Martín-González, Serrano, and 
Fernández-Galiano (1987) showed that the oral 
primordium in Caenomorpha develops by prolif-
eration from the posterior ends of many perizonal 
somatic kineties . The primordial field splits later 
in development with an anterior portion devel-
oping into the paroral and the posterior portion 
developing into the oral polykinetids . In Metopus , 
a number of posterior dorsolateral somatic kineties
begin to proliferate kinetosomes (Foissner & 
Agatha, 1999). These differentiate as the oral 
polykinetids (Fig. 8.4). The paroral differenti-
ates later. Foissner and Agatha (1999) interpreted 
it to develop from kinetosomes derived from 
 perizonal kineties . However, it is just as pos-
sible from the evidence presented that paroral 
dikinetids could derive from “anterior” or “right-
side” kinetosomes in a fashion very similar 
to that reported for Caenomorpha . If this were 
the case, there would be strong similarities in 
Fig. 8.4. Division morphogenesis of Metopus , a representative of the Class ARMOPHOREA . ( a ) Kinetosomal 
replication begins at the “equatorial ends” of a number of somatic kineties. ( b ) Oral polykinetids assemble through 
side-by-side alignment of dikinetids units. ( c ) The posterior ends of several somatic kineties adjacent to the develop-
ing oral region disassemble, and it may be that the paroral ( d, e ) is assembled from these as division proceeds. (from 
Foissner & Agatha, 1999.)
8.5 Division and Morphogenesis 183
 stomato genesis between these two genera. 
Caenomorpha undergoes a complicated post-sto-
matogenesis morphogenesis, reminiscent of the 
 enantiotropic division of some oligotrichous spiro-
trichs (Martín-González et al., 1987). Considering 
the current evidence, we are not convinced that the 
differences between metopids and caenomorphids 
are sufficient to justify ordinal status for these 
two groups, as suggested by Foissner and Agatha 
 Santos, Guinea, and Fernández-Galiano (1986) 
have provided a preliminary account of stoma-
togenesis in the clevelandellid Nyctotherus . Breaks 
occur in somatic kineties posterior to the oral region 
and kinetosomal proliferation occurs at the anterior 
ends of these breaks. A lateral groove develops as 
proliferation proceeds and primordium elements 
on the posterior wall of the groove differentiate as 
oral polykinetids while those on the anterior wall 
develop as paroral dikinetids, eventually forming 
the two files of the diplostichomonad (Santos et al., 
1986). This is clearly a pleurotelokinetal stomato-
genesis , showing significant similarities to that of 
the armophorids. 
 8.6 Nuclei, Sexuality 
and Life Cycle 
 Armophoreans have the typical complement of 
 macronucleus and one or more micronuclei . The 
macronuclei can also be variable in number in 
 caenomorphids , sometimes numbering more than 
four (Fig. 8.1). In smaller forms, the macronucleus 
is typically globular to ellipsoid, but in larger 
 clevelandellids it can become elongated and quite 
irregular in shape. The macronucleus of some 
 clevelandellids is “suspended” from the cortex by 
microfibrillar strands that collectively are called 
the karyophore (Fig. 8.1). 
 Eichenlaub-Ritter and collaborators have under-
taken some detailed