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forests (Schnittler, Unterseher, & Tesmer, 2006). Sorogena is a preda- tor of smaller colpodeans . Under an alternating photoperiod of light and dark and when prey abun- dance declines, Sorogena trophonts aggregate as a sorogen on plant fragments or other floating films or objects, typically just before sunrise (Sugimoto & Endoh, 2006). The sorogen secretes a stalk, elevating the secreting cells above the attachment point. These cells then encyst as sorocysts , which are discharged for dispersal as the sorus dries (Olive & Blanton, 1980). Secretory vesicles, probably homo- logues of the mucocysts of other colpodeans, are manufactured in the rough endoplasmic reticulum and their contents released by exocytosis during stalk formation (Blanton & Olive, 1983a). The stalk material expands to a hydrated, fibrillar matrix, pushing the sorogen upwards. The stalk material then solidifies to provide structural support for the sorus (Blanton & Olive, 1983b). Like the cyst wall materials of other colpodeans , the stalk is composed of polysaccharides , which probably include glucose and N -acetyl- d -glucosamine residues, and proteins, which have glycine as a predominant amino acid (Blanton, Warner, & Olive, 1983). 12.3 Somatic Structures As noted above, colpodeans range in size from the very small Nivaliella to the almost macroscopic Bursaria . Somatic kineties of colpodeans are bipolar with a significant number terminating on the oral region, especially in species whose oral apparatus is at the anterior apex. The colpodean body can be slightly twisted, spiralling to the left in forms like Platyophrya and this twisting can become extreme in colpodids (Figs. 12.1, 12.2). Somatic ciliation can be characterized as holotrichous, although in smaller colpodids some regions of the cortex can have very reduced somatic ciliation (Fig. 12.1). The cell surface of colpodeans is covered by the plasma membrane , which rarely demonstrates ele- ments of a superficial glycocalyx (Bardele et al., 1991; Bradbury & Olive, 1980). The plasma membrane is underlain by conspicuous cortical alveoli (Golder & Lynn, 1980; Lynn, 1976a, 1977a, 1980). Fauré- Fremiet and André (1965a) reported an abundance of dense granules filling this space in Colpoda (formerly Tillina ) praestans . The junctions and boundaries between these alveoli may give rise to the complicated silverline system or argyrome of these ciliates, although this is not always the case (Foissner, 1981b; Foissner & Foissner, 1994). As in other ciliates, the alveoli are underlain by an epiplasmic layer whose thickness appears to be correlated with the cell size of the ciliate, at least within the colpodids (Lynn, 1977a). Bursaria does not have a thick epiplasm , but like the larger colpodids , its somatic kinetids give rise to microtubular nema- todesmata that insert upon a cortical microfilamentous network, which presumably supports the form of these larger cells (Lynn, 1977a, 1980). Of all classes of ciliates, the somatic kinetids of colpodeans have caused considerable interest. Golder (1974) was the first to report in Woodruffia the complex overlapping microtubular ribbons, called LKm fibers , that extend along the left side of the somatic kineties. Lynn (1975) demonstrated that Woodruffia also had a typical, striated kine- todesmal fibril and thus was not an exception to the rule of desmodexy as applied to ciliates by Chatton and Lwoff (1935b). A few years later, several simultaneous reports demonstrated that the LKm microtubular ribbons derived from the transverse ribbon of the posterior kinetosome of the somatic dikinetids (Gerassimova, 1976; Lynn, 1976a). The colpodean somatic dikinetid is rotated about 10\u201320° clockwise to the kinety axis, viewed from outside the cell. The posterior ciliated kineto- some bears the following: a divergent postciliary ribbon that sometimes extends far enough to overlap those of more posterior kinetids; a short, laterally-directed kinetodesmal fibril that origi- nates near triplets 5, 6, flattening to a fan-shape as it extends upwards in the cortical ridge; and a very well developed, tangentially-oriented trans- verse ribbon that originates from a medial desmose connecting the two kinetosomes and extends from triplets 3\u20135 posteriorly and often overlaps several other ribbons from more posterior kinetids (Figs. 12.3, 12.4). The anterior ciliated kinetosome bears the following: a single to occasionally several post- ciliary microtubules and a large tangentially-oriented Fig. 12.1. Stylized drawings of representative genera from the orders in the Class COLPODEA . The colpodids Colpoda and Grossglockneria . The sorogenid Sorogena . The cyrtolophosidid Cyrtolophosis . The bursariomorphid Bursaria . Inset is a detail of the adoral polykinetids and adjacent somatic kineties 12.3 Somatic Structures 249 Fig. 12.2. Stylized drawings of representative genera from the orders in the Class COLPODEA . The bryometopid Bryometopus . The bryophryid Bryophrya . The cyrtolophosidids Cyrtolophosis and Woodruffides Fig. 12.3. Schematics of the somatic dikinetids of the Class COLPODEA . ( a ) The bryophryid Bryophrya . ( b ) The col- podid Colpoda . ( c ) The cyrtolophosidid Cyrtolophosis . ( d ) The bursariomorphid Bursaria (from Lynn, 1981, 1991) transverse ribbon that arises near triplets 3\u20135 and extends laterally into the ridge to overlap the compound bundle of posterior kinetosomal trans- verse ribbons. There may be up to four parasomal sacs , two on each side of the kinetid (Fig. 12.3). Nematodesmata may also originate from the base of one or both kinetosomes (Lynn, 1981, 1991). More recent descriptions of colpodean kinetids have confirmed this structural pattern for the class (Aescht et al., 1991; Bardele et al., 1991; Foissner & Foissner, 1994; Platt & Hausmann, 1993). Like some oligohymenophoreans (Antipa, 1972), the cortex of colpodids can undergo structural differentiation . Larger colpodids have a somatic groove that forms the \u201chilum\u201d in the kidney- bean shape. This somatic groove is supported longitudinally by strongly overlapping posterior transverse ribbons and underlain by orthogonally disposed nematodesmata that arise from the bases of the groove somatic dikinetids (Lynn, 1976c; Lynn & Zimmerman, 1981). The groove can be divided into an incurrent and excurrent compo- nent, which facilitates feeding in Colpoda species (Fenchel, 1980a; Lynn, 1976c, 1977a). The larger the Colpoda species, the greater the development of its somatic groove (Lynn, 1978). Colpodeans typically have at least one contractile vacuole , located in the posterior end. Depending upon the species, they can be identified as showing Type A or Type B morphology (Patterson, 1980). Larger cells may have either multiple vacuoles (e.g., Bursaria , Foissner, 1993a) or may have long collecting canals radiating anteriorly in the cell\u2019s cortex (e.g., Colpoda magna (formerly Tillina magna , Tillina canalifera ), Lynn, 1977a; Turner, 1937). As cell size increases in Colpoda species, the relative size of the contractile vacuole and its pore increases as does the output of the contracile vacuole itself (Lynn, 1977a, 1982). Colpodeans have ovoid to rod-shaped mucocysts . These are particularly conspicuous in medium to large cells (Foissner, 1993a; Lynn, 1976c, 1977a), 12.3 Somatic Structures 251 252 12. Subphylum 2. INTRAMACRONUCLEATA: Class 6. COLPODEA but inconspicuous in smaller ones (Foissner, 1993a; Lynn, 1976a, 1976b). The matrix is granular (Lynn, 1977a) or often completely removed upon fixation (Delmonte Corrado, Chessa, & Pelli, 1996; Lynn, 1976c). The characteristic periodic substructure of mucocysts (Hausmann, 1978) is rarely observed (see Perez-Paniagua, Perez-Silva, & de Puytorac,