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Jerka-Dziadosz, 1980). In fact, 
the ventral kinetids of some stichotrichs , such as 
Engelmanniella , may complete development as 
dikinetids, which are characterized as follows: an 
anterior ciliated kinetosome with a single post-
ciliary microtubule and a tangential transverse 
ribbon at triplets 4, 5; and a posterior ciliated 
kinetosome with a divergent postciliary ribbon 
and a kinetodesmal fibril near triplets 6-8 (Fig. 
7.5) (Wirnsberger-Aescht, Foissner, & Foissner, 
1989). The ultrastructure of hypotrich cirri is quite 
variable, but typically kinetosomes are hexagonally 
packed and joined by a basal plate and a distal plate 
of dense or filamentous material. Microtubules 
originate from the lateral edges of the distal plate 
and extend out into the cortex. These microtubules 
are joined by those of the transverse and postciliary 
ribbons, which originate from the basal plate adja-
cent to kinetosomal bases. Kinetodesmal fibrils 
may be associated with kinetosomes along the 
right edge of the polykinetid (Lynn, 1991; Tuffrau 
& Fleury, 1994). The ultrastructure of stichotrich 
cirri is quite consistent: kinetosomes are typically 
hexagonally packed and joined at the basal level 
and at mid-height. Microtubules originate from the 
mid-height connective material and extend out into 
the cortex. As in the hypotrichs , these microtubules 
are joined by the microtubules of transverse and 
postciliary ribbons, which arise from the basal 
plate adjacent to kinetosome bases. Kinetodesmal 
fibrils may be associated with kinetosomes along 
the right edge of the polykinetid (Fig. 7.6) (Fleury 
et al., 1985a; Grain, 1984; Lynn; Matsusaka et al., 
1984; Tuffrau & Fleury, 1994). However, kineto-
desmal fibrils or ciliary rootlets may be resorbed 
in a domain-specific fashion in some stichotrichs 
(Jerka-Dziadosz, 1990). Therefore, it is difficult to 
generalize about the presence or absence of these 
structures unless a thorough analysis has been 
done of the entire infraciliature. Even the somatic 
polykinetids of Plagiotoma , which was assigned by 
Corliss (1979) to the heterotrichs , demonstrate fea-
tures of the stichotrich cirrus (Fig. 7.6) (Albaret & 
Grain, 1973). This supports transfer of Plagiotoma
to the Subclass Stichotrichia (see also Tuffrau 
& Fleury), a fact that is also corroborated by 
SSUrRNA gene sequences (Affa\u2019a et al., 2004). 
 Members of the Family Halteriidae are now 
placed in the Subclass Stichotrichia (see above 
and Chapter 17 ) (Fig. 7.4). Meseres has somatic 
dikinetids while Halteria and Pelagohalteria have 
fused, bristle-like cilia arising from dikinetids 
(Petz & Foissner, 1992; Song, 1993). The somatic 
dikinetids of Halteria are highly unusual. They 
apparently lack the classical fibrillar associates 
of the somatic kinetid, but are surrounded by 
dense material from which cortical microtubules 
originate, a feature shared with the cirri of other 
stichotrichs (Grain, 1972). 
 We have placed the Family Reichenowellidae 
incertae sedis in the Subclass Hypotrichia . The 
ultrastructural study ofBalantidioides (= Transitella ?) 
Fig. 7.6. Schematics of the somatic polykinetids or cirri of representatives of the Class SPIROTRICHEA . ( a ) Somatic 
polykinetid or cirrus of the stichotrich Plagiotoma (based on an electron micrograph of Albaret & Grain, 1973). ( b ) 
 Somatic polykinetid or cirrus of the stichotrich Histriculus . (Based on an electron micrograph of Matsusaka et al., 1984.)
7.3 Somatic Structures 159
160 7. Subphylum 2. INTRAMACRONUCLEATA: Class 1. SPIROTRICHEA
shows its somatic kinetids to be assembled into 
groups of typically 2-6 dikinetids whose structure 
is characterized as follows: a ciliated anterior kine-
tosome with a tangential transverse ribbon near 
triplets 3-5, a single postciliary microtubule, and 
possibly two microtubules near triplet 1, which 
could be transverse microtubules for the posterior 
kinetosome; and a ciliated post erior kinetosome 
with a large divergent postciliary ribbon and a pos-
teriorly-directed kinetodesmal fibril . The postciliary 
microtubules originate in a large dense structure that 
is the base of an interiorly directed nematodesma 
(Fig. 7.5) (Iftode et al., 1983; Lynn, 1991). 
 The structural variation among spirotrich somatic 
structures contradicts the hypothesis of \u201c structural 
conservatism \u201d of the somatic cortex (Lynn, 1976a, 
1981) (see Chapter 4 ), and begs the question 
\u201cWhy?\u201d. In his discussion of the unusual nature of 
the somatic kinetids of choreotrichs , Grim (1987) 
suggested one explanation: when somatic kinetids 
are no longer used in locomotion, selection may 
be relaxed on the structure since it no longer 
performs a critical locomotor function. Although 
this is helpful in explaining variations in somatic 
kinetids of oligotrichs and choreotrichs and the 
dorsal dikinetids of hypotrichs and stichotrichs , it 
is not helpful in explaining the structural diversity 
of locomotory somatic kinetids among spirotrichs 
(Fig. 7.5). We currently have no explanation for 
this deviation from structural conservatism, except 
to suggest that these lineages could be extremely 
ancient, as suggested by the branch lengths in 
molecular phylogenetic analyses (e.g., Bernhard et 
al., 2001; Snoeyenbos-West et al., 2002; Strüder-
Kypke et al., 2002). 
 The cirri of hypotrichs and stichotrichs can per-
form complex \u201cwalking\u201d or \u201crunning\u201d movements 
(Erra et al., 2001; Sleigh, 1989). These movements 
in Euplotes and Stylonychia are controlled by 
 membrane hyperpolarizations that produce forward 
movement (i.e., rearward beating) and membrane 
depolarizations that produce rearward movement 
(i.e., forward beating) (Epstein & Eckert, 1973; 
Deitmer, Machemer, & Martinac, 1984). Rhythmic 
depolarizations determine the rhythm of walking in 
Euplotes (Lueken, Ricci, & Krüppel, 1996). Similar 
to the model for Paramecium , Ca 2+ ions probably 
interact with ciliary axonemal components, serv-
ing as the intracellular messengers for membrane 
potential changes. The cirri of Stylonychia also 
respond to slow changes in membrane potential by 
\u201cinclining\u201d or bending at the base without beating, 
which adds a further degree of sophistication to 
their movement (Machemer & Sugino, 1989). 
 Spirotrichs can be quite flexible or contractile. 
However, in choreotrichs , this contractility appar-
ently does not rely on filamentous structures but on 
highly unusual membranous elements, for example 
in the tail of Tontonia (Greuet, Gayol, Salvano, & 
Laval-Peuto, 1986) and the posterior end of tintin-
nids (Laval-Peuto, 1994). We do not know how 
these structures work. 
 Contractile vacuoles are common throughout the 
group, especially in freshwater forms. Marine tin-
tinnids do not have contractile vacuoles while their 
freshwater relatives do. 
 A wide variety of extrusomes has been described 
in the group. Mucocyst -like extrusomes are found 
in this class ( Phacodinium , Didier & Dragesco, 
1979; Stylonychia , Görtz, 1982b; Transitella , 
Iftode et al., 1983). Cortical ampules found around 
the dorsal dikinetids of hypotrichs may be a special 
type of mucocyst ( Aspidisca , Rosati et al., 1987; 
Certesia , Wicklow, 1983; Euplotes , Görtz). Other 
 extrusomes with a distinctly lamellar nature appear 
to be involved in cyst formation (e.g., Grim & 
Monganaro, 1985; Walker et al., 1980; Verni, 
Rosati, & Ricci, 1990), while the highly unusual 
 lepidosomes , mentioned above, form a surface coat 
on the cysts of Meseres (Foissner et al., 2005a). The 
 trichites of oligotrichs have been demonstrated to 
be extrusomes , but their function remains unknown 
(Modeo, Petroni, Bonaldi, & Rosati, 2001). 
 Finally, to round out this brief treatment of other 
somatic structures, lithosomes or calculi have been 
observed in both hypotrichs (Lenzi & Rosati, 1993; 
Rosati et al., 1987;