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(1966a, 1966c, 1973c) suggested the division into three major groups, now recognized as orders: the Apostomatida , the Astomatophorida , and the Pilisuctorida . The Order Apostomatida is char- acterized by a highly modified \u201chymenostome\u201d oral ciliature accompanied by a rosette and its associated x , y , and z kineties; there are three fami- lies \u2013 the Colliniidae , the Cyrtocaryidae , and the Foettingeriidae . Description of the complete life cycle of members of the Order Astomatophorida , monotypic for the Family Opalinopsidae , may confirm placement of these curious parasites of the internal organs of cephalopods , characterized by division by catenulation . The Order Pilisuctorida includes species that spend most of their life cycle attached to the cuticular setae of crusta- ceans (Bradbury, 1975; Mayén-Estrada & Aladro- Lubel, 2004). Bradbury (1982) has confirmed Jankowski\u2019s (1966a) hypothesis that the pilis- uctorid Conidophrys has a rosette opening in its tomite stage, and so is legitimately an apostome . Bradbury (1989) has interpreted the features of the fine structure of the exuviotrophic apostome Hyalophysa as homologous to the paroral (i.e., two rows of staggered barren kinetosomes) of hymenostomes , an interpretation corroborated by gene sequence data (J.C. Clamp et al., unpublished data 2008). The peritrichs have long been presumed to have derived from a pleuronematine - or thigmotrich - like scuticociliate (Fauré-Fremiet, 1910, 1950a; Lom, Corliss, & Noirot-Timothée, 1968). The ante- rior thigmotactic region of these putative ances- tors has been presumed to have given rise to the attachment structures \u2013 the scopula and adhesive disk \u2013 of the peritrichs , while the posterior oral ciliature was presumed to have evolved into the prominent peristomial ciliature of the Subclass Peritrichia . This oral ciliature is composed of a paroral, called the haplokinety , accompanied by oral polykinetid 1 \u2013 both encircle the apical end in a counterclockwise sense before entering into the oral cavity, called an infundibulum in this group. These oral organellar complexes are accompanied by two other oral polykinetids that appear peniculus -like in that they are oriented lengthwise in the oral cavity. Sequences of the SSUrRNA genes have confirmed the monophyly of the sessilid peritrichs . Moreover, SSUrRNA gene sequences place hymenostomes and peritrichs as sister lineages, refuting the \u201cclassical\u201d hypothesis that peritrichs share a close common ancestry with the scuticociliates (Itabashi, Mikami, Fang, & Asai, 2002; Miao, Yu, & Shen, 2001; Miao et al., 2004b). However, these same gene sequences suggest that sessilids and mobilids may not be sister taxa (Gong, Yu, Villalobo, Zhu, & Miao, 2006)! Monographic works on the peritrichs focus on the two major orders: (1) those that deal primarily with the sessiline forms, those peritrichs attached to substrates, both living and non-living, by the scopula or scopular products; and (2) those that deal with the mobiline forms, peritrichs that attach temporarily by means of an adhesive disk supported by a skeletal apparatus and surrounded by three ciliated girdles (Lom, 1994). Sessiline peritrichs can be solitary or colonial. Solitary species often aggregate, settling very close to each other to form so-called pseudocolonies . True colonial forms remain attached to the same stalk after cell division, and if zooids differentiate a monomorphic colony becomes polymorphic . The monographs on ses- silines by Kahl (1935), Nenninger (1948), Stiller (1971), and Guhl (1979), for example, still remain useful. Foissner and Schiffmann (1975, 1976) have demonstrated that silver-staining can provide a rich set of characters to supplement those of cell size and shape, which were traditionally used to separate species. These surface structures can also be revealed by SEM when the peritrichs are relaxed by chlorbutol (Carey & Warren, 1983). This focus on surface features has lead to the revision of previ- ous descriptions and the recognition of new genera (e.g., see Foissner & Schiffmann, 1976; Leitner & Foissner, 1997b; Warren, 1986, 1987, 1988). Roberts, Warren, and Curds (1983) have also dem- onstrated that multivariate and Fourier analyses of the outline shape of Vorticella species can resolve taxa to some degree. Sequencing of ITS regions has suggested that river-dwelling populations of Carchesium polypinum may show some vicari- ance biogeography (Miao et al., 2004a) while gene flow among lake-dwelling populations appears to be much higher when assessed using inter-sample sequence repeat (ISSR) fingerprinting (Zhang, Yang, Yu, Shu, & Shen, 2006). Clearly, these two studies only scratch the surface of the population genetics and biogeography of the peritrichs . Some sessiline groups secrete a lorica , which may be directly attached to the substrate or which may surround the zooid that is itself attached to the substrate by the scopula . Features of the lorica , such as its shape, character of the opening, and presence of an operculum , have proved useful in discriminating genera and species within genera (e.g., see Clamp, 1987, 1991; Finley & Bacon, 1965; Jankowski, 1985, 1986). The preliminary phy- logenetic analyses based on SSUrRNA suggest that family assignments of sessilids based on morphology may not be correct. Nevertheless, we have remained conservative in our treatment and recognized the following 14 families in the Order Sessilida : the Astylozoidae , the Ellobiophryidae , the Epistylididae , the Lagenophryidae , the Operculariidae , the Ophrydiidae , the Opisthonectidae , the Rovinjel- lidae , the Scyphidiidae , the Termitophryidae , the Usconophryidae , the Vaginicolidae , the Vorticelli\u2013 dae , and the Zoothamniidae . The Order Mobilida is characterized by a mobile zooid as a \u201c permanent telotroch \u201d or swarmer stage, which has a complex, ring-like, skeletal armature of denticles and fibres that support the adhesive disk on the aboral pole. We include the following five families: the Leiotrochidae , the Polycyclidae , the Trichodinidae , the Trichodinopsidae , and the Urceolariidae . Monographs on mobilines includes the work of Wallengren (1897), Haider (1964), Raabe (1964), and Lom (1994). Others continue to record the morphological variability of trichodinids using numbers and sizes of the skeletal denticles , as well as other denticle characters (Kazubski, 1981, 1988, 1991; Van As & Basson, 1989). Denticle characters have been used to assess the phylogeny with the Family Trichodinidae and demonstrate that Hemitrichodina is a very divergent genus (Gong, Yu, Feng, & Shen, 2005). The astomes , now as the Subclass Astomatia , have always presented a problem to ciliate sys- tematists who have relied on oral characters to determine affinities. These endosymbionts, typically of annelids , are all mouthless, but have evolved elaborate holdfast structures in the form of hooks, spines, spicules, and suckers . The group may be polyphyletic as astomy has arisen inde- pendently within the hymenostomes (Kozloff, 1954), and astomatous mutants of Tetrahymena and Glaucoma have been isolated in the labora- tory (Frankel, 1961; Orias & Pollock, 1975; Rasmussen & Orias, 1975). Nevertheless, the cur- rent phylogenetic hypothesis is that astomes arose from a thigmotrich -like ancestor with a reduced, posterior oral apparatus and an anterior thigmotac- tic zone (de Puytorac, 1954; de Puytorac, Grolière, & Grain, 1979). In our scheme, and different from de Puytorac (1994g), the subclass includes the single Order Astomatida with its nine fami- lies: the Anoplophryidae , the Buetschliellidae , the Clausilocolidae , the Contophryidae , the Hapto- phryidae