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Abstract This class is truly a riboclass because it 
assembles three groups of ciliates that were never 
suspected of being phylogenetically related, and 
yet there is an extremely strong signal from the 
small subunit rRNA gene sequences that they are. 
The now “classic” plagiopyleans, the sonderiids 
and plagiopylids, are now united with the trimyem-
ids and tentatively also the odontostomatids. These 
ciliates are all considered anaerobic to microaer-
ophilic, and are often found in sapropelic habitats. 
Several species have conspicuous assemblages of 
hydrogenosomes and methanogens, which presum-
ably enable these ciliates to survive in these anoxic 
habitats. There are really no unifying morphologi-
cal features. The somatic kinetids are monokinetids 
in the sonderiids, plagiopylids, and trimyemids and 
highly unusual dikinetids in the odontostomatids. 
Oral structures in the plagiopylids and sonderiids 
are modifi ed extensions of somatic kineties; trimy-
emids apparently have a kind of “circumoral” 
ciliature; and odontostomatids have several small 
oral polykinetids. Stomatogenesis is apparently 
holotelokinetal in all but the odontostomatids, and 
we are ignorant of how this latter group divides. 
There remains much to be learned about their life 
cycle, sexual processes, and nuclear features. 
Keywords Epalxella , Plagiopyla , Trimyema
 The Class PLAGIOPYLEA , like the Class 
 ARMOPHOREA , is in essence a “ riboclass ” – a 
group whose monophyly is based only on the 
evidence of sequences of the small subunit (SSU) 
 rRNA gene . Small and Lynn (1985) established 
the subclass Plagiopylia , including the sonderiids 
and plagiopylids , and transferred these ciliates to 
the Class OLIGOHYMENOPHOREA primarily 
on the basis of the ultrastructure of the somatic 
kinetids. De Puytorac et al. (1993) elevated the 
group to class status, a move supported by Lynn 
and Small (1997). Sequencing of the SSUrRNA 
genes of several species of Trimyema and several 
 plagiopylid genera has now demonstrated these to 
be sister taxa (Baumgartner, Stetter, & Foissner, 
2002; Embley & Finlay, 1994; Lynn & Strüder-
Kypke, 2002). Stoeck, Foissner, and Lynn (2007) 
have evidence that the SSUrRNA gene sequence of 
the odontostomatid Epalxella clusters with strong 
support with these plagiopyleans , and so we have 
made the risky decision to assign the odontosto-
matids to this class as the second order, beside the 
Order Plagiopylida . 
 The plagiopyleans are anaerobic or microaer-
ophilic ciliates that range in size from about 15 µm 
in length but rarely exceed 200 µm in length. They 
are typically ovoid or elongate in body shape and 
not contractile or flexible. In the larger genera, 
such as Lechriopyla and Sonderia , the ciliation is 
holotrichous. In smaller forms, such as trimyem-
ids and odontostomatids , the number of somatic 
kinetids is reduced and much of the body surface 
is non-ciliated. In trimyemids , the kineties even 
appear to spiral, but both light and electron micro-
scopic study of Trimyema refute this interpreta-
tion, and confirm the interpretation proposed by 
Fauré-Fremiet (1962b): the kinetosomes in each 
longitudinally oriented kinety are distributed in 
such a manner that they appear to be spiralling 
(Baumgartner et al., 2002; Detcheva, de Puytorac, 
 Chapter 14 
 Subphylum 2. 
PLAGIOPYLEA – A True Riboclass 
of Uncommon Companions 
& Grolière, 1981). Plagiopyleans are typically 
found in anaerobic freshwater and marine habitats, 
ranging from hydrothermal vents to anoxic marine 
sediments to the intestines of sea urchins and to 
 sewage treatment plants . 
 A notable feature of these ciliates is the pres-
ence of hydrogenosome-methanogen assemblages 
in their cytoplasm in which the methanogens are 
typically sandwiched between hydrogenosomes 
forming groups of up to a dozen units. These 
assemblages have been observed in representatives 
of the order Plagiopylida – in Sonderia (Fenchel, 
Perry, & Thane, 1977), Plagiopyla (Berger & 
Lynn, 1992), and Trimyema (Detcheva et al., 1981). 
However, the “sandwich” pattern can depend upon 
the particular species of methanogen involved: 
Methanocorpusculum parvum is polymorphic – 
ovoid when free in the cytoplasm of Trimyema and 
profusely dentate when associated with its hydrog-
enosomes (Finlay, Embley, & Fenchel, 1993). 
 Biochemical analyses supported the conclusion
that the ciliate organelles are not mitochondria , but 
rather are hydrogenosomes : they exhibit hydroge-
nase activity (Zwart et al., 1988) and do not dem-
onstrate cytochromes , cytochrome oxidase , and 
 catalase activities (Goosen, Wagener, & Stumm, 
1990). There are now techniques for culturing 
both Trimyema (Wagener & Pfennig, 1987) and 
Plagiopyla (Fenchel & Finlay, 1991c), using 
 bacteria isolated from the environment or cultured 
bacterial strains. Furthermore, electromigration 
has been used to concentrate these ciliates from 
environmental sludge samples (Wagener, Stumm, 
& Vogels, 1986) and from mass cultures to enable 
biochemical research (Broers, Molhuizen, Stumm, 
& Vogels, 1992). 
 The name of the class PLAGIOPYLEA is derived 
from the Greek words, plagios meaning oblique 
and pylon meaning gate. This refers to the nature 
of the oral opening in plagiopylids , which is an 
oblique slit whose walls are covered by extensions 
of the somatic kineties. While the somatic kinetid 
of plagiopylids bears some resemblance to that 
of the oligohymenophoreans , the odontostomatid 
dikinetid is quite different (see below Somatic
Structures ). There is no morphological synapo-
morphy for the class, and so it was designated as 
one of the “ riboclasses ” of ciliates by Lynn (2004), 
since SSUrRNA gene sequences appear to be the 
only “strong” characters that support the clade. 
 14.1 Taxonomic Structure 
 Corliss (1979) retained both the plagiopylids and 
 trimyemids in the Order Trichostomatida , following
research by Fauré-Fremiet (1950a, 1962b, 1973) 
among others. Corliss did note that this order 
was a rather heterogeneous taxon with respect 
to the morphological diversity of the families 
placed in it. De Puytorac, Grain, Legendre, and 
Devaux (1984) used a phenetic analysis to place 
the trimyemids in an order Trimyemida in the 
subclass Gymnostomia , while de Puytorac, Grain, 
and Legendre (1994) used parsimony methods 
to provisionally place Trimyema adjacent to the 
 phyllopharyngeans and vestibuliferians , noting that 
stomatogenetic characters might lead to reconsid-
eration of this result. Berger and Lynn (1984) noted 
a peculiar microtubular ribbon associated with 
triplets 2, 3, tentatively unique for the plagiopylids . 
Partly based on this mistaken interpretation (see 
Somatic Structures ), Small and Lynn (1985) 
established the subclass Plagiopylia within the 
Class OLIGOHYMENOPHOREA , based on fea-
tures of the somatic kinetid, which had long anteri-
orly extending kinetodesmal fibrils and a divergent 
postciliary ribbon similar to that of other oligohy-
menophoreans . De Puytorac et al. (1993) elevated 
the subclass to the Class PLAGIOPYLEA . This 
position was maintained by de Puytorac (1994c) 
for the plagiopylids and sonderiids , although he 
placed the trimyemids in the subclass Prostomatia , 
based on the assumption that the oral dikinetids 
were homologues of the prostomatean brosse . 
 Lynn and Small (1997) also recognized the 
Class PLAGIOPYLEA , and included in it both 
 plagiopylids , sonderiids , and trimyemids (Lynn 
& Small, 2002). This was rationalized by simi-
larities in the somatic kinetids with their anteriorly 
directed kinetodesmal fibrils . To these features, we 
can now add the typical sandwich-like arrangement 
of the hydrogenosome-methanogen assemblages in 
 plagiopylids . Finally, SSUrRNA gene

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