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Abstract Ciliates in this class were thought to 
 represent the pinnacle of ciliate evolution, along 
with the spirotrichs. However, small subunit rRNA 
gene sequences and the presence of postciliodes-
mata in the somatic cortex strongly relate members 
of this class to the Class KARYORELICTEA. The 
heterotrichs are typically majestic ciliates of large 
cell size and with a conspicuous adoral zone of 
polykinetids or membranelles (AZM) that extend 
out over the peristomial surface. The ciliates in this 
class are not subdivided, and so there is one order 
– Order Heterotrichida. Heterotrichs are found in a 
diversity of habitats, from the marine benthos and 
hydrothermal vents to the plankton of high altitude 
oligotrophic lakes. They feed on a diversity of prey, 
ranging from bacteria up to small metazoa, like 
rotifers, and sometimes are conspicuous by carry-
ing symbiotic zoochlorellae. Their body is highly 
contractile, elongated by postciliodesmal micro-
tubules and shortened by contractile myonemes. 
The oral structures have a paroral and multiple 
paramembranelles. Stomatogenesis is parakinetal. 
Macronuclei can be nodular, and are divided by 
 extramacronuclear microtubules. Conjugation has 
not been studied in any breadth in the class with the 
gamone-receptor system of Blepharisma being the 
only model. The heterotrich Spirostomum has been 
developed as a bioassay model for heavy metal. 
Keywords Ampliploid, microbiotest 
 Heterotrichs are common and large ciliates, some 
Spirostomum species achieving body lengths of up 
to 4,000 µm. They include some of the best-known 
and most common ciliates in the phylum. Stentor , 
a typical representative, has long attracted attention 
from protozoologists and cell biologists because of 
its size, ubiquity, ease of general laboratory culture, 
contractility, and regenerative powers (Fig. 6.1). 
Typically heterotrichs are free-swimming and holot-
richously ciliated, although members of the Family 
 Folliculinidae secrete attached loricas in which they 
live. The group is at least 100–200 million years 
old as demonstrated by the fossil ized lorica of 
Priscofolliculina (Deflandre & Deunff, 1957). 
 Heterotrichs were so-named because of the 
marked difference between their holotrichous 
somatic ciliation and the conspicuous, typically 
spiralling adoral zone of membranelles or oral 
polykinetids . They were conceived as the pivotal 
group in the evolution of the “higher” or polyhy-
menophorean ciliates (Corliss, 1961, 1979). Doubt 
about this vision began to emerge in the late 1970s. 
Ultrastructural data indicated dramatic differences 
in their somatic kinetids compared to other polyhy-
menophoreans ; and similarities in the heterotrich 
somatic cortex to that of the karyorelicteans sug-
gested a closer relationship between the presuma-
bly most derived and the presumably most ancestral 
groups (Gerassimova & Seravin, 1976; Lynn, 1981, 
1991). The nature of membrane particle arrays in 
different ciliate groups also suggested a stronger 
relationship between heterotrichs and karyorelict-
eans (Bardele, 1981). Finally, sequences of nuclear 
ribosomal RNA genes from heterotrichs and 
 karyorelicteans supported their sister group status 
in an early diverging lineage (Baroin-Tourancheau, 
Delgado, Perasso, & Adoutte, 1992; Greenwood, 
Schlegel, Sogin, & Lynn, 1991b; Hirt et al., 1995). 
 Chapter 6 
 Subphylum 1. 
Once Close to the Top 
130 6. Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top
Fig. 6.1. Representative genera of the Class HETEROTRICHEA . Blepharisma with a somewhat linear arrangement 
of the adoral zone of polykinetids along the left margin of the oral region. In contrast, the oral polykinetids of Stentor
and Climacostomum spiral out of the oral cavity in a counter-clockwise direction, bounding a peristomial field that 
is covered by kineties. The folliculinid Eufolliculina exemplifies this unique family of heterotrichs in being anchored 
in a lorica and in having its oral region drawn out into two extensive peristomial “wings”
Thus, we are now certain that the postciliodesmata , 
shared by both karyorelicteans and heterotrichs , 
demonstrate their shared common ancestry. 
 The ciliates now assigned to this class are 
united by four major features. First, they have 
highly ampliploid and dividing macronuclei. 
Macronuclear karyokinesis is accomplished 
primarily by extramacronuclear microtubules 
(Diener, Burchill, & Burton, 1983; Jenkins, 1973; 
Lynn & Small, 1997) and probably evolved inde-
pendently of macronuclear karyokinesis in the 
Subphylum Intramacronucleata (Lynn, 1996a; 
Orias, 1991a; and see Chapter 4 ). Second, the 
microtubular components of their postciliodes-
mata are more simply organized than those of the 
 karyorelicteans : they appear as ribbons oriented 
perpendicular to the cell surface, only separated 
by a single microtubule. Third, the oral polyki-
netids on the left side of the oral region are char-
acterized as paramembranelles (de Puytorac & 
Grain, 1976), which form a conspicuous adoral 
zone often extending out onto the anterior cell 
surface. Fourth, the differentiation of these oral 
polykinetids during stomatogenesis occurs from 
the center of the oral primordium towards the 
anterior and posterior, a unique pattern within the 
phylum (Aescht & Foissner, 1998). 
 6.1 Taxonomic Structure 
 There has been considerable change in the 
composition of this taxon since Corliss (1961, 
1979). Corliss (1979) recognized six suborders 
within the Order Heterotrichida : (1) Suborder 
 Heterotrichina ; (2) Suborder Clevelandellina ; (3) 
Suborder Armophorina ; (4) Suborder Coliphorina ; 
(5) Suborder Plagiotomina ; and (6) Suborder 
 Licnophorina . Results from the study of ultrastructure 
and molecular sequences now suggest the following. 
The somatic kinetids of clevelandellids (Affa’a, 
unpublished, 2007; de Puytorac & Grain, 1969), 
 armophorids (Schrenk & Bardele, 1991), plagioto-
mids (Albaret & Grain, 1973), and licnophorids (Da 
Silva Neto, 1994a) do not exhibit postciliodesmata 
and also have different patterns of fibrillar associ-
ates (Lynn, 1981, 1991). Furthermore, nuclear small 
subunit rRNA (SSrRNA) gene sequences separate 
the clevelandellids (van Hoek, van Alen, Sprakel, 
Hackstein, & Vogels, 1998, 2000b) and armo-
phorids (Embley, Finlay, Thomas, & Dyal, 1992; 
van Hoek et al., 1998, 2000b) to a new class, the Class 
 ARMOPHOREA (see Chapter 8 ), while plagiotomids 
(Affa’a, Hickey, Strüder-Kypke, & Lynn, 2004) 
and licnophorids (Lynn & Strüder-Kypke, 2002) 
are transferred to the Class SPIROTRICHEA (see 
Chapter 7 for details). 
 The Suborder Coliphorina only included the 
Family Folliculinidae . However, the somatic 
kinetids of Eufolliculina are extremely similar to 
other heterotrichs in their fibrillar associates and 
the character of the postciliodesmata (Mulisch, 
Barthlott, & Hausmann, 1981), while SSrRNA 
sequences indicate this genus falls within the heter-
otrich radiation and is the sister taxon to Maristentor
(Miao, Simpson, Fu, & Lobban, 2005). Thus, this 
group should not be separated at such high rank, 
and we do not now recognize this suborder. 
 Within the Suborder Heterotrichina , Corliss 
(1979) included the following: Family Bursariidae , 
Family Chattonidiidae , Family Climacostomidae , 
Family Condylostomatidae , Family Metopidae , 
Family Peritromidae , Family Phacodiniidae , Family 
 Reichenowellidae , Family Spirostomidae , and Family 
 Stentoridae . The somatic kinetids of Phacodinium
(Didier & Dragesco, 1979; Da Silva Neto, 1993a), 
Transitella , a reichenowellid (Foissner, Adam, & 
Foissner, 1982; Iftode, Fryd-Versavel, Wicklow, 
& Tuffrau, 1983), metopids (Schrenk & Bardele, 
1991), and bursariids (Gerassimova, Sergejeva, &