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LITOSTOMATEA as orders 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 \u201cring\u201d of circumoral dikinetids that surround the oral region. Members of the Subclass Trichostomatia have lost the toxicysts , and have only \u201coralized\u201d 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 \u201cgymnostome\u201d 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 \u201cpallisade\u201d 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 189 190 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA 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