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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 ’s 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 
(Drzewińska & Golińska, 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 
 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 – Lipscomb & Riordan, 
1992; Homalozoon – Leipe & Hausmann, 1989; 
Lagynophrya – Grain, 1970; Litonotus – Bohatier 
& Njiné, 1973; Perispira – Johnson et al., 1995) and 
one layer in some trichostomes (e.g., Balantidium
– Grain, 1966a, Grim, 1993a; buetschliids – Grain, 
1966a; entidiniomorphids – Furness & Butler, 1983, 
9.3 Somatic Structures 197
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 
9.3 Somatic Structures 199
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 58–96 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).