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vast majority of endosymbiotic trichostomes of rumi- nants and non-ruminants have apparently lost this capacity to encyst . Balantidium is an exceptional trichostome , forming a cyst wall by synthesizing materials in mucocysts that are transported to the cell surface prior to encystment (Grain, 1968). Didinium has been a favored subject for encyst- ment studies since the early work of Beers (1927). When Didinium starves, it modulates its swimming speed , apparently in response to both its own popu- lation density and the period of starvation . It swims fastest when starved over 1 h at the highest densi- ties (Salt, 1979). Survival is very short-lived, in the order of several days if Didinium does not encyst (Jackson & Berger, 1985a). Survival rate is related to cell size : larger Didinium have greater survival rates (Hewett, 1987). Like Balantidium , Didinium synthesizes extrusomes prior to encystment . There is an ordered extrusion of these organelles to form the three layers of Didinium \u2019s cyst wall. The ecto- cyst or outer layer is derived from mucocysts ; the mesocyst or middle layer is derived from clathro- cysts , special extrusomes whose internal matrix has a flattened honeycomb-like structure; and the endocyst , which is amorphous (Holt & Chapman, 1971; Rieder, 1971). Polysaccharides , proteins , and lipids are the major constituents of the cyst wall (Rieder, 1973). The cyst wall in Dileptus is also three-layered (Jones, 1951). There is con- siderable dedifferentiation of somatic and oral kinetosomal structures and the ecto-endoplasmic layer in encysted haptorians (Holt & Chapman, 1971; Kink, 1978). These structures gradually dif- ferentiate again as the excystment process proceeds (Holt, 1972). 9.3 Somatic Structures The litostomes range is size from the small endo- symbiotic buetschliids and blepharocorythids , typically less than 50 µm in length, to the larger free-living haptorians , such as Dileptus and Homalozoon , whose elongate and flexible bodies can sometimes exceed 1 mm in length (Fig. 9.1). Litostomes include some of the most bizarre forms in the phylum with some blepharocorythids and entodiniomorphids possessing elongate cortical processes and spines (Fig. 9.3). The rhinozetids , entodiniomorphids found in the digestive system of the rhinoceros , even have flexible processes that can be extended away from the body, which may decrease their transit time through the gut (Van Hoven et al., 1988). The cell processes of haptorians , on the other hand, tend to be related to food capture: Dileptus has an extensible proboscis preceding the cytostome (Fig. 9.1); Lacrymaria has an extremely active and very extensible neck at the end of which sits the cytostome (Fig. 9.1); and Actinobolina has distributed over the somatic sur- face extensible toxicyst-bearing tentacles that aid in prey capture (Holt, Lynn, & Corliss, 1974). Somatic ciliation is variable (e.g., see Foissner et al., 1999; Grain, 1966a; Ito & Imai, 1998). In haptorians and vestibuliferids , the vast majority of the species have bipolar kineties and holotrichous ciliation (Figs. 9.1, 9.2). However, didiniids and cyclotrichiids , for example, have ciliation restricted to equatorial girdles . Somatic ciliation of entodini- omorphids is typically not holotrichous, but rather restricted to the anterior and posterior ends or to bands that may partially encircle the body in the midregions as well as anteriorly and posteriorly (Figs. 9.2, 9.3). The number of somatic kineties is variable, and at least in Dileptus is related to cell size : larger cells have more somatic kineties (Drzewi\u144ska & Goli\u144ska, 1987). Thus, taxono- mists should be cautious about describing a new haptorian species based only on differences in numbers of somatic kineties. There has been a tremendous amount of research on the cortical ultrastructure of litostomes , which has lead different research groups to varied con- clusions about relationships among these forms (Foissner & Foissner, 1988; Grain, 1994; Lipscomb & Riordan, 1990, 1991, 1992). Our discussion will be limited to a general account of the major fea- tures and variations in these structures within the class. The cell surface is covered by a glycocalyx of variable thickness, typically more conspicuous in endosymbiotic forms. In Dileptus , this surface coat can change in response to temperature, as meas- ured by serotype changes, which have also been observed in oligohymenophoreans (see Chapter 15 ) (Uspenskaya & Yudin, 1992). Beneath the plasma membrane, alveoli are often very incon- spicuous. In haptorians , alveoli are typically small and irregularly distributed beneath the plasma membrane in cortical ridges (Foissner & Foissner, 1985; Grain, 1970; Lipscomb & Riordan, 1990, 1991; Williams et al., 1981). In entodiniomorphids , alveoli may not be visible at all. Rather three cell membrane layers are observed, the inner two pre- sumably being collapsed alveoli (Furness & Butler, 1983, 1985). The epiplasm is variable in thickness: typically not conspicuous in haptorians and vestibuliferids but often quite thick in ophryoscolecids . Fauré-Fremiet and André (1968) noted that litostomes often had a conspicuous layer of filaments at the ecto-endo- plasmic boundary , the so-called lamina corticalis of Bretschneider (1959). There is typically one layer of filaments at the ecto-endoplasmic boundary in haptorians (e.g., Didinium \u2013 Lipscomb & Riordan, 1992; Homalozoon \u2013 Leipe & Hausmann, 1989; Lagynophrya \u2013 Grain, 1970; Litonotus \u2013 Bohatier & Njiné, 1973; Perispira \u2013 Johnson et al., 1995) and one layer in some trichostomes (e.g., Balantidium \u2013 Grain, 1966a, Grim, 1993a; buetschliids \u2013 Grain, 1966a; entidiniomorphids \u2013 Furness & Butler, 1983, 9.3 Somatic Structures 197 198 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA Fig. 9.1. Stylized drawings of representative genera from the Subclass Haptoria of the Class LITOSTOMATEA . The haptorids Didinium , Lacrymaria , and Dileptus . These are three classical encounter feeders: Didinium swims through the water bumping into prey; Lacrymaria probes the water above the substratum on which it crawls using its extremely extensible neck; and Dileptus swims through the water like a swordfish, sweeping it with its toxicyst -laden proboscis , whose extrusomes immobilize and kill prey that are then ingested. Inset shows many small macronuclei Fig. 9.2. Stylized drawings of representative genera from the Subclasses Haptoria and Trichostomatia of the Class LITOSTOMATEA . Subclass Haptoria : the haptorid Spathidium ; the pleurostomatid Loxophyllum ; and the cyclot- richiid Myrionecta . Subclass Trichostomatia : the vestibuliferid Balantidium ; and the entodiniomorphid buetschliid Didesmis 9.3 Somatic Structures 199 200 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA Fig. 9.3. Stylized drawings of representative genera from the Subclass Trichostomatia of the Class LITOSTOMATEA . The vestibuliferid Isotricha . The blepharocorythid Blepharocorys . The entodiniomorphids Entodinium , Epidinium , Ophryoscolex , and Troglodytella 1985). However, Isotricha and Dasytricha have an additional layer, internal to the layer commonly found (Grain, 1966a; Paul, Butler, & Williams, 1989). The proteins of the epiplasm and ecto-endo- plasmic layer of trichostomes have been isolated and found to differ. Filamentous proteins , ranging in molecular weight between 58\u201396 kDa, are the major constituent of the epiplasm of Entodinium and Polyplastron (Vigues & David, 1989; Vigues, Méténier, & Sénaud, 1984b), while the 4 nm fila- ments of the ecto-endoplasmic layer of Isotricha are about 22 kDa in size (Vigues, Méténier, & Grolière, 1984a).