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243 Abstract Ciliates now assigned to the Class COLPODEA were scattered throughout the phylum in unrelated classes and orders. However, it was the structural conservatism of the cortex and its somatic kinetids that enabled the iden- tification of this natural assemblage based on cortical ultrastructure, a fact that has been con- firmed by small subunit rRNA gene sequences. Colpodeans are the quintessential cyst-formers in the phylum, and are therefore typical in habi- tats that have a high probability of desiccation: mosses, soils, and leaf litter are typical habitats. However, colpodeans are found in ponds and lakes, although rarely in marine habitats. Their prey varies with their cell size: smaller col- podeans eat bacteria while the largest colpodean Bursaria can ingest Paramecium . The somatic kinetid is a dikinetid with well-developed over- lapping transverse microtubular ribbons derived from the posterior kinetosome and forming what is called the LKm fiber or transversodesma \u2013 the strong synapomorphy for the group. Oral structures range from a paroral that almost encircles a prostomatous oral region to a huge deep oral cavity, almost spirotrich-like, adorned with many oral polykinetids. Stomatogenesis ranges from mero- to pleurotelokinetal, and in the colpodids typically occurs within a cyst after dedifferentiation of all parental oral structures. Conjugation has rarely been observed, and it is often assumed that colpodeans are completely asexual. This needs to be tested by molecular genetic approaches. Keywords Palintomy, cryptobiosis The revolution in our view of what morphological characters are significant to identifying clades of ciliates arose, in part, from investigations of this class of ciliates. Lynn (1976a) proposed the struc- tural conservatism hypothesis based on his study of the cortical ultrastructure of colpodeans and a comparison of his results with those of the litera- ture. This conclusion, independently arrived at by Gerassimova and Seravin (1976), established, as a general rule, the conservative nature of the somatic kinetid within clades of ciliates and provided a \u201crule\u201d for establishing phylogenetic affinity. In the Class COLPODEA , several examples illus- trate the strength of this principle. Ultrastructural study of the cortex of Woodruffia by Golder and Lynn (1980) confirmed its relationship to colpode- ans as suggested by Kahl (1930\u20131935), although von Gelei (1954) had originally considered it a heterotrich -like ciliate. Bursaria , for many years another \u201cheterotrich\u201d (Corliss, 1979), revealed colpodean -like stomatogenetic features on more careful study (Fernández-Galiano, 1979), and its colpodean \u201cnature\u201d was confirmed by ultrastruc- tural studies (Gerassimova, Sergejeva, & Seravin, 1979; Lynn, 1980; de Puytorac & Perez-Paniagua, 1979). McCoy (1974a, 1977) drew attention to the colpodean affinities of the stomatogenesis of Cyrtolophosis , a genus that had been presumed to be a hymenostome (Corliss, 1961), but was confirmed to be a colpodean by ultrastructural features of the somatic kinetid (Didier, de Puytorac, Wilbert, & Detcheva, 1980). As a final example, the unu- sual sorocarp -forming ciliate, discovered by Olive (1978), was named Sorogena (Bradbury & Olive, 1980) and placed with the haptorians , primarily Chapter 12 Subphylum 2. INTRAMACRONUCLEATA: Class 6. COLPODEA \u2013 Somatically Conserved but Orally Diverse 244 12. Subphylum 2. INTRAMACRONUCLEATA: Class 6. COLPODEA because of its oral structures. Lynn (1991) sug- gested that it might be regarded as a colpodean , a conclusion confirmed by the study of its ultrastru- cure and stomatogenesis by Bardele, Foissner, and Blanton (1991). Thus, as beautifully illustrated by Foissner (1985b) (see Fig. 1.4), colpodean oral structures show such broad diversity that the class has really only received broad recognition in the past 20 years. Colpodean ciliates are extremely common, especially in terrestrial habitats where the genus Colpoda is almost ubiquitous. Colpodeans range in size from about 10 µm in length for the genus Nivaliella to over 500 µm in length for Bursaria truncatella , an almost 200,000 times difference in cell volume (Foissner, 1993a). Colpodeans are generally holotrichously ciliated. The kineties are typically bipolar with a number of them ending on the perimeter of the oral region or coursing anteri- orly to abut in a prominent anterior suture or keel in some Colpoda species. The body undergoes torsion to varying degrees, a feature that Stout (1960a) used to imagine a phylogeny for the group based on D\u2019Arcy Thompson\u2019s Cartesian coordinate analysis. This torsion can be lost during cell division as was illustrated years ago by Tuffrau (1952). Colpodeans have interested physiologists with their life cycle that typically includes a resist- ant stage or cyst . Early on, Burt (1940) carefully described species so physiologists would not misi- dentify forms. More recently, multivariate statistical approaches have been used to discriminate among morphospecies (Foissner & Schubert, 1983; Lynn & Malcolm, 1983). Colpoda species have now been grown in chemostat cultures on a variety of bacte- rial species, including Escherichia coli (Drake & Tsuchiya, 1977). Undoubtedly the most intriguing recent examples of bizarre feeding strategies among ciliates have been the discovery of the mycopha- gous colpodeans in the Family Grossglockneriidae . Originally discovered by Foissner (1980d) in the European Alps , they have been described world- wide as predators of fungi , using their minute feeding tube to perforate the cell wall of fungi and remove the contents (e.g., Foissner, 1993a, 1999b; Foissner & Didier, 1983), and they have now been identified from fossilized amber dating from the Lower Cretaceous (Ascaso et al., 2005). De Puytorac et al. (1974b) elevated the colpode- ans to ordinal rank within the Subclass Vestibulifera . Small and Lynn (1981) elevated the group to class rank based on the structure of the somatic dikinetids , which really provide the only synapomorphy for the class. The special feature of the colpodean somatic dikinetid is the presence of a posteriorly-directed transverse ribbon of microtubules associated with the posterior kinetosome. These ribbons can extend for some distance posteriorly, overlapping each other in what has been called the LKm fibre (Golder, 1974; Golder & Lynn, 1980) or the transversodesma (Small & Lynn, 1985). The class name is derived from Colpoda , one of the most common genera of ciliates, and its name, in turn, is derived from the Greek kolpos , meaning breast, referring to the bulging shape of some Colpoda species. 12.1 Taxonomic Structure As noted above, de Puytorac et al. (1974b) were the first to elevate the colpodids to ordinal rank within their Subclass Vestibulifera , a position that was maintained by Corliss (1979). Small and Lynn (1981, 1985) elevated the group to class rank, establishing the Class COLPODEA , based prima- rily on the structure of the somatic kinetid. This distinctness as a class, which has been maintained by Lynn and Small (1997, 2002), is supported both by phylogenetic analyses based on morpho- logical features (de Puytorac, Grain, Legendre, & Devaux, 1984; de Puytorac, Grain, & Legendre, 1994) and by small subunit (SSU) rRNA gene sequences (Lynn, Wright, Schlegel, & Foissner, 1999). However, one recent molecular study sug- gests that the colpodeans may be paraphyletic (Lasek-Nesselquist & Katz, 2001). The colpodeans have been related to the litostomes and the nassophoreans based on kinetid structures (Aescht, Foissner, & Mulisch, 1991). Phylogenetic trees derived from SSUrRNA gene sequences support an affinity with prostomateans (Lynn et al., 1999) or nassophoreans (Lasek-Nesselquist