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Abstract The ciliates in this class are divided into 
two major assemblages, represented by the Subclass 
Haptoria and Subclass Trichostomatia. The hapto-
rians are predatory ciliates that are commonly found 
in a variety of habitats, feeding on fl agellates and 
other ciliates, which they immobilize and kill with 
extrusomes called toxicysts. An exception to this 
rule is the marine planktonic haptorian Myrionecta
rubra , which harbors a cryptophyte endosymbiont 
and which produce red tides that contribute up to 
70% of the primary production. The trichostomes 
have lost toxicysts and are all endosymbionts in a 
variety of metazoans, ranging from fi sh to humans. 
In fact, the only ciliate known to be pathogenic to 
humankind is the trichostome Balantidium , which 
can be an intestinal parasite. The somatic monokinetid
of litostomes is unique in possessing two transverse 
microtubular ribbons, T1 and T2 – a strong synapo-
morphy for the class. Oral structures are typically 
simple, hence the name litos (Gr.) for simple. Hap-
torians typically have either circumoral dikinetids 
or oralized monokinetids. Trichostomes show more 
diversity: some forms, like Balantidium , have a vesti-
bulum with extensions of densely packed somatic 
kineties lining it, while the entodinio morphids have 
polybrachykineties, more complex assemblages of 
short kinetofragments. Stomato genesis is character-
ized as telokinetal, but there is a range of types from 
holotelokinetal to crypto telokinetal. 
Keywords Karyoklepty, pexicyst, bulge micro-
tubules, conocyst 
 The ciliates included in this class are divided into 
two subclasses, the Subclass Haptoria and the 
Subclass Trichostomatia . The former includes free-
living and the latter endosymbiotic forms. These cili-
ates are extremely variable in size and form. Small 
endosymbiotic trichostomes can be around 50 µm 
in length while some free-swimming haptorians , 
such as Homalozoon species, can be 2,500 µm 
long! Form is no less variable: small forms are 
typically ovoid while elongate forms are ribbon-
like, flexible, and contractile. Endosymbiotic forms 
can have lobes, spines, and unusual cell processes, 
while some free-swimming forms have extensible 
“necks”, flexible proboscises, and toxicyst-bearing 
tentacular processes. Body ciliation is also vari-
able, ranging from isolated tufts and bands in 
 entodiniomorphid trichostomes to holotrichous 
ciliation in most haptorians . Free-living hapto-
rians are distributed world-wide in freshwater and 
marine habitats and are characterized as voracious 
predators of flagellates, other ciliates, and even 
small metazoans. 
 The endosymbiotic trichostomes are found in 
a variety of vertebrates, ranging from fish to rep-
tiles and mammals where they typically consume 
bacteria and plant material. Trichostomes inhabit 
two major groups of mammals : (1) they are found 
in ruminants or foregut fermenters, such as cattle , 
 sheep , hippopotamus , and kangaroos ; and (2) they 
are found in various hindgut fermenters, such as 
 horses , tapirs , and some anthropoid apes . The only 
ciliate known to be pathogenic to human beings is 
the trichostome Balantidium coli , which damages the 
 intestinal mucosa of humans. Balantidium has also 
been found in some invertebrates and other verte-
brates. In the vast majority of human infections, 
there has been a history of human contact with pigs 
 Chapter 9 
 Subphylum 2. 
INTRAMACRONUCLEATA: Class 3. 
LITOSTOMATEA – Simple Ciliates 
but Highly Derived 
188 9. Subphylum 2. INTRAMACRONUCLEATA: Class 3. LITOSTOMATEA
(Ferry et al., 2004; Zaman, 1978), although rare 
human cases occur apparently with no reported 
contact with pigs (Anargyrou et al., 2003). 
 Culture methods have been developed for some of 
the endosymbiotic forms. Entodiniomorphids can be 
cultured short-term or long-term on several different 
media that model the rumen microenvironment 
(Bonhomme, Fonty, & Senaud, 1982; Coleman, 
Laurie, & Bailey, 1977; Hillman, Williams, & Lloyd,
1991; Michalowski, Muszyňski, & Landa, 1991). 
Essential lipids have extended cultivation of ento-
dinia for more than 3 months (Hino, Kametaka, & 
Kandatsu, 1973). These cultivation methods have 
enabled exploration of the biochemistry and physi-
ology of rumen ciliates. Methods for cultivation 
of Balantidium have been in use since the early 
1900s, modeled on media that support the growth 
of Entamoeba histolytica (Zaman, 1978). Klaas 
(1974) has reported on media that enable cultiva-
tion of some isolates for well over 2 years. Cox 
(1963) has argued that initiation of cultures is the 
only reliable method for diagnosis of Balantidium
coli infections. 
 The name of the Class LITOSTOMATEA is 
derived from the Greek litos meaning simple and 
the Greek stoma meaning mouth. Small and Lynn 
(1981) sug gested it as a replacement name for the 
classical Gymnostomata (i.e., Greek gymnos mean-
ing naked) to which a number of the included taxa 
were assigned by Corliss (1979) and others. Electron 
microscopic research on the litostomes had revealed 
that the mouth was not naked, but encircled by a spe-
cialized, but simple, circumoral infraciliature (Grain, 
de Puytorac, & Bohatier, 1973). The litostomes have 
long been regarded as little-modified descendants of 
the most primitive ciliates. However, ultrastructural 
research led several workers (Bardele, 1989; Small 
& Lynn, 1981) to argue that the simplified nature of 
 litostomes was likely secondarily derived. This has 
been confirmed by rRNA gene sequences that show 
 litostomes to be several branches removed from 
the base of the ciliate “tree” (Baroin-Tourancheau, 
Delgado, Perasso, & Adoutte, 1992; Baroin-
Tourancheau, Villalobo, Tsao, Torres, & Pearlman, 
1998; Leipe, Bernhard, Schlegel, & Sogin, 1994; 
Wright, Dehority, & Lynn, 1997). While α-tubulin 
sequences suggested that litostomes were indeed 
basal in the ciliate tree (Baroin-Tourancheau et al., 
1998), it is thought that this may be due to the poor 
resolving power of this gene. 
 Small and Lynn (1981) established the mono-
phyly of the class based on the ultrastructural 
pattern of the somatic kinetids . These are monoki-
netids that were ultimately shown to have a con-
vergent postciliary ribbon, whose microtubules 
are arranged in a double-row configuration, and 
two transverse ribbons (Leipe & Hausmann, 1989; 
Williams, Williams, & Hogan, 1981). This kinetid 
pattern is the primary synapomorphy for the class. 
Three other features unite the ciliates in this class. 
First, the oral kinetids are either monokinetids or 
dikinetids whose transverse ribbons extend to sup-
port the cytopharyngeal apparatus , which is called 
a rhabdos . In all other ciliate classes, cytopharyn-
geal ribbons are derived from postciliary ribbons. 
Second, there are regions of at least several somatic 
kineties in holotrichous species whose ciliature is 
differentiated as clavate cilia , forming a clavate 
field or brush (Foissner, 1996b). Third, conjugation 
in litostomes is often preceded by a preconjuga-
tion cell division during which the first meiotic 
reduction division occurs leaving characteristically 
swollen nuclear division products. Although a 
 preconjugation division has a scattered distribution 
among other classes (Raikov, 1972), we regard it as 
convergently evolved in each group since only lito-
stomes show the micronuclear swelling, although 
there may be exceptions (see Xu & Foissner, 
2004). Finally, McEwan et al. (2000) demonstrated 
a bias against the use of G in the third position of 
the codons for lysine, glutamine, and glutamic acid 
in entodiniomorphid ciliates (e.g., Entodinium , 
Epidinium , Polyplastron ) and one haptorian (i.e., 
Spathidium ). Could this be a molecular synapo-
morphy for the Class LITOSTOMATEA? 
 9.1 Taxonomic Structure 
 Corliss (1979) placed the major groups now 
included in the Class