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in the Subclass Gymnostomata and Subclass 
 Vestibulifera . Small and Lynn (1981) were the first 
to suggest unifying this assemblage of ciliates. 
They based this on the structure of the somatic 
kinetids, which at that time were characterized as 
having convergent postciliary ribbons , laterally to 
antero-laterally directed kinetodesmal fibrils, and 
a tangential transverse ribbon, and on the pres-
ence of the lamina corticalis or ecto-endoplasmic 
fibrillar layer . Lynn and Small (1997, 2002) have 
maintained these taxonomic assignments, which 
have been confirmed by sequence analyses of 
both ribosomal (Baroin-Tourancheau et al., 1998; 
Strüder-Kypke, Wright, Foissner, Chatzinotas, & 
Lynn, 2006; Wright & Lynn, 1997a, 1997b; Wright 
et al., 1997) and protein genes (Baroin-Tourancheau 
et al.). De Puytorac (1994a) united these same taxa 
in the Subphylum Filicorticata. Since the major 
feature of this subphylum is the presence of an 
 ecto-endoplasmic fibrillar layer , already character-
ized for the Class LITOSTOMATEA by Small and 
Lynn, we do not believe this new name is warranted 
and retain the Class LITOSTOMATEA . 
 We recognize two subclasses within this class. 
The Subclass Haptoria is distinguished by the pres-
ence of toxicysts , typically in the oral region, and 
typically by a “ring” of circumoral dikinetids that 
surround the oral region. Members of the Subclass 
 Trichostomatia have lost the toxicysts , and have 
only “oralized” somatic monokinetids in the oral 
region (Lipscomb & Riordan, 1992; Lynn & 
Small, 2002). The oral region of trichostomes may 
be invaginated as a vestibulum , a cavity lined by 
specialized extensions of somatic kineties. While 
some haptorians have been described to have 
only oral monokinetids (e.g., Foissner & Foissner, 
1985), we believe gene sequence data will show 
that this is a convergent feature arising independ-
ently in haptorians and the endosymbiotic trichos-
tomes (see also Lipscomb & Riordan, 1990, 1992). 
It is likely the case that the trichostomes evolved 
from a microaerophilic haptorian-like ancestor 
that had oral monokinetids and hydrogenosomes : 
Balantidium has both mitochondria and hydrog-
enosomes (Grain, 1994) while some haptorians 
(e.g., Arcuospathidium , Chaenea , Lacrymaria ) 
appear to be adapted to anaerobic habitats by 
harboring endosymbiotic methanogens (Finlay & 
Maberly, 2000). Strüder-Kypke et al. (2006) found 
the free-living Epispathidium papilliferum grouped 
with the trichostomes using small subunit rRNA 
(SSUrRNA) gene sequences. 
 The Subclass Haptoria is divided into three 
orders: Order Haptorida , Order Pleurostomatida , 
and Order Cyclotrichiida . Grain et al. (1973) 
were among the first to suggest that details of 
oral kinetid structure could be used to distinguish 
clades of “gymnostome” ciliates. Foissner and 
Foissner (1988) described these three orders and 
added two more, the Order Spathidiida and the 
Order Pseudoholophryida . They also included in 
this subclass the Order Archistomatida , which we 
assign to the Order Entodiniomorphida (see below). 
Xu and Foissner (2005) argued that a Dileptus -like 
ancestor gave rise to the spathidiid diversity by 
several allometric models of kinety growth. The 
basal position of Dileptus in some SSUrRNA gene 
trees is consistent with this model (Strüder-Kypke 
et al., 2006). 
 Lipscomb and Riordan (1990) used cladistic 
analyses to assess relationships among haptorians 
and concluded that there should be two orders, the 
Order Haptorida and Order Pleurostomatida . They 
placed within the Order Haptorida trichostome 
 vestibuliferids (e.g., Balantidium , Isotricha ) and 
 archistomatids (e.g., Alloiozona , Didesmis ), which 
we assign to the Order Entodiniomorphida (see 
below). Lipscomb and Riordan (1992) affirmed 
these two major divisions using a successive 
weighting cladistic analysis , but concluded that 
a stable classification needed more data. They 
also concluded that the Subclass Ditransversalia 
proposed by Leipe and Hausmann (1989) was an 
unnecessary proposal. Using only an equal weight-
ing analysis, Wright and Lynn (1997b) reanalyzed 
the Lipscomb and Riordan (1992) dataset, and 
affirmed the two major divisions of haptorians pro-
posed here. Wright and Lynn (1997b) were able to 
separate the haptorians from the trichostomes and 
 vestibuliferids . De Puytorac (1994a) essentially 
followed the system of Foissner and Foissner 
(1988). We are unconvinced of the phylogenetic 
significance of a number of the characters used to 
justify these ordinal taxa. Therefore, we conserva-
tively recognize only three orders and no suborders 
within the Subclass Haptoria , until such time as 
molecular characters consistently confirm or refute 
these divisions (see Strüder-Kypke et al., 2006). 
 The Order Haptorida is characterized by an oral 
region that has somatic dikinetids or oralized somatic 
monokinetids whose transverse ribbons support the 
cytopharynx, which is lined by nematodesmata 
originating from the kinetids and supported also by 
an internal “pallisade” of bulge microtubules . We 
include the following families: Acropisthiidae , 
 Actinobolinidae , Apertospathulidae , Didiniidae , 
 Enchelyidae , Helicoprorodontidae , Homalo-
zoonidae , Lacrymariidae , Pleuroplitidae , Pseudo-
holophryidae , Pseudotrachelocercidae , Spathidiidae , 
9.1 Taxonomic Structure 189
 Tracheliidae , and Trachelophyllidae . We have been 
conservative and retained the genus Myriokaryon
as a spathidiid , rather than recognizing a new family 
as suggested by Foissner (2003). 
 The Order Pleurostomatida is characterized by 
a flattened elongated oral region along the ventral 
margin of a laterally compressed body. Somatic 
ciliation shows a right-left differentiation. We 
include here the Amphileptidae and Litonotidae . 
Foissner and Leipe (1995) established the new 
Family Loxophyllidae to include genera (i.e., 
Loxophyllum and Siroloxophyllum ) that have dorso-
lateral kineties. They affirmed the two suborders 
recognized by Foissner and Foissner (1988). We 
again remain conservative here, preferring to await 
gene sequence data to support the separation of the 
 loxophyllids and eschewing the establishment of 
monotypic suborders. 
 The Order Cyclotrichiida is monotypic, including 
the Family Mesodiniidae . These ciliates are distin-
guished by cirrus-like cilia that typically form two 
girdles around the equator of the cell. Although 
these ciliates (e.g., Myrionecta ) do have toxicyts, 
Foissner and Foissner (1988) speculated that they 
may even belong to another subclass based on the 
details of the ciliature and infraciliature, which 
have been well described in the major genera and 
common species (Krainer & Foissner, 1990; Song, 
1997; Tamar, 1992). Johnson, Tengs, Oldach, 
Delwiche, and Stoecker (2004) have sequenced 
the SSUrRNA genes of Myrionecta rubra and 
Mesodinium pulex and found them to be closely 
related, but widely separated from other lito-
stomes, hence supporting the Order Cyclotrichiida . 
Strüder-Kypke et al. (2006) confirmed these results 
and demonstrated litostome features in the second-
ary structure of the SSUrRNA of cyclotrichids . 
We have chosen to retain the order in the Class 
 LITOSTOMATEA (see Chapter 17 ). 
 The Subclass Trichostomatia includes a diverse 
assemblage of ciliates predominantly endosymbi-
otic in vertebrates. These ciliates have haptorian 
somatic monokinetids and typically a conspicuous 
 ecto-endoplasmic fibrillar layer . Their oral region is 
slightly more complex than that of the haptorians . 
It is typically surrounded by extensions of somatic 
kineties that have a higher kinetosomal density 
than the somatic portions and that may be invagi-
nated into an oral cavity called a