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Abstract This class includes the two model ciliates 
– Tetrahymena and Paramecium – laboratory genetic 
models of protists and the first two ciliates to have 
completed genome projects. Along with these 
representatives of the Subclasses Hymenostomatia 
and Peniculia, there is a diverse variety of species 
distributed in four other subclasses: Scuticociliatia, 
Peritrichia, Apostomatia, and Astomatia. While 
the archetype of the class has a paroral and three 
left oral polykinetids, there is really no strong 
morphological or ultrastructural synapomorphy for 
the class. Molecular phylogenetics generally recov-
ers an oligohymenophorean clade with moderate 
bootstrap support. The somatic kinetid is moderate 
to that of the last three classes: a generally robust, 
anteriorly-directed kinetodesmal fibril, a divergent 
postciliary ribbon, and a radial transverse ribbon, 
except in the peniculine where it is tangential. 
Oral structures depart from the archetype in the 
apostomes with their rosette and complex cytopha-
ryngeal apparatus and are absent in the astomes. 
Oligohymenophoreans are as broadly distributed 
across habitats as any class, but they include obligate 
symbionts: the apostomes typically with crustaceans 
and the astomes with annelids. Ichthyophthirius , the 
parasite of freshwater fishes, is an hymenostome 
that can plague aquaculture operations. Division 
morphogenesis is characterized as buccokinetal 
and parakinetal. There is a rich literature on the 
genetics of these ciliates, particularly Paramecium
and Tetrahymena , which can be easily induced 
to conjugate in the laboratory, enabling a deeper 
understanding of their molecular biology. 
Keywords Autogamy
 The Class OLIGOHYMENOPHOREA includes 
two ciliate genera, Tetrahymena and Paramecium , 
which one might describe respectively as the 
“white mouse” and the “white rat” of the Phylum 
Ciliophora, based both on their relative sizes and 
on their being the most “highly tamed” labora-
tory models in the phylum. Lwoff (1923) suc-
cessfully cultured Tetrahymena axenically , that 
is in a sterile nutrient broth without bacteria, and 
so initiated the exploration of its physiology and 
biochemistry as a model organism. It was to be 
many years later that a partly defined medium was 
devised to axenically culture Paramecium (Lilly & 
Klosek, 1961). Now there are genome projects well 
underway for representative species of both these 
genera (Eisen et al., 2006; Tetrahymena – www.
genome/ Tetrahymena /; and Aury 
et al., 2006; Paramecium – carroll.vjf.cnrs.fr/pt/), 
bringing these two ciliates and molecular genetic 
approaches to understanding their biology into a 
competitive position with other model organisms. 
ably the most diverse assemblage within the phy-
lum. While there are fewer included subclasses in 
it compared to the Class SPIROTRICHEA ( see
Chapter 7 ), there have been many more species 
described. Oligohymenophoreans are commonly 
encountered in freshwater and marine habitats with 
 peniculines being most conspicuous in the former 
habitats and scuticociliates being most conspicu-
ous in the latter habitats. Some members of the 
class show a preference for moribund and dead 
metazoans (e.g., Corliss, 1972c; Fauré-Fremiet & 
Mugard, 1946), and this capacity has lead several 
times independently in the class to symbiotic and 
 Chapter 15 
 Subphylum 2. 
a Pivotal Group, Now a Terminal Radiation 
 parasitic modes of life in diverse hosts: the apos-
tomes in crustaceans (Bradbury, 1966a; Chatton & 
Lwoff, 1935a); the thigmotrich scuticociliates in 
 bivalve molluscs (Chatton & Lwoff, 1949, 1950); 
 hymenostomes as parasites of gastropods (Kozloff, 
1956) and fishes (Hoffman, 1988; Hoffman 
et al., 1975); and peritrichs , both on invertebrates 
(Matthes, 1974) and vertebrates (Lom, 1995), 
and in invertebrates (Lom, 1994) and vertebrates 
(Lom, 1958, 1995). These infections on rare occa-
sions can cause mass mortalities of their host, 
such as Ichthyophthirius and fishes (Wurtsbaugh 
& Tapia, 1988). In other instances, infections may 
significantly impact fecundity , such as the scutico-
ciliate Orchitophrya infecting the testes of starfish 
(Stickle, Weidner, & Kozloff, 2001). The morpho-
logical polymorphism and complex life cycles often 
characteristic of these symbiotic ciliates is pres-
aged by similar dramatic changes in form in free-
living species, which may undergo form change in 
response to feeding conditions, such as starvation 
(Fauré-Fremiet & Mugard, 1949a; Nelsen & De 
Bault, 1978) or the presence of different kinds of 
foods (Small, Heisler, Sniezek, & Iliffe, 1986). The 
absence of bacteria may stimulate some species to 
transform into macrostome cannibals , consuming 
their conspecifics and other smaller species (Fig. 
15.1) (Corliss, 1973; Njiné, 1972). 
 Despite these dramatic variations in body mor-
phology, the ciliates in this class demonstrate 
their name, having few oral polykinetids or mem-
branelles (oligo, Gr. – few; hymen, Gr. – membrane; 
phoros, Gr. – bearing). The typical arrangement is 
three, inconspicuous oral polykinetids on the left 
side of the oral cavity and a paroral of dikinetids 
or a stichodyad on the right side. Nevertheless, 
the oral structures of some pleuronematine scu-
ticociliates , such as Pleuronema , and peritrichs 
can be quite prominent. Corliss (1979) argued 
that oligohymenophoreans were a linking assem-
blage between the “lower” kinetofragminopho-
ran groups and the “higher” polyhymenophorans . 
 Molecular phylogenetic analyses have now refuted 
this vision and demonstrate the oligohymenopho-
reans to be a “terminal” branch in the Subphylum 
 Intramacronucleata (Greenwood, Sogin, & Lynn, 
1991a; Strüder-Kypke et al., 2000b). With the nota-
ble exception of the peritrichs , the vast majority of 
 oligohymenophoreans are holotrichously ciliated. 
They are medium-sized, typically 65–150 µm in 
length; some scuticociliate species can be less than 
10 µm in length, the large carnivorous peniculine 
Neobursaridium can reach lengths of 600 µm 
(Dragesco & Tuffrau, 1967), and some astomes 
can reach 3 mm in length (de Puytorac, 1994g). As 
with the tintinnids , the loricate forms – fossilized 
 peritrichs – can be placed in contemporary families 
even though they date from the Lower Triassic , 
some 200 million years ago (Weitschat & Guhl, 
1994). Even more remarkably, a Paramecium spe-
cies has been described from southern Germany 
in amber -bearing sandstone deposits (Schönborn, 
Dörfelt, Foissner, Krientiz, & Schäfer, 1999), 
which have been assigned to the Late Cretaceous , 
some 90 million years ago (Schmidt, von Eynatten, 
& Wagreich, 2001). Thus, it is very likely that the 
class originated over 250 million years ago, and 
even perhaps in the Paleoproterozoic , if the molec-
ular clock can be trusted (Wright & Lynn, 1997c). 
 As noted above, the ease of cultivating spe-
cies of Tetrahymena and Paramecium has made 
them two of the most widely studied genera in 
the phylum, and therefore in this class. These 
earlier approaches to their cultivation lead to the 
establishment of chemically defined media for 
both Tetrahymena (see Kidder & Dewey, 1951; 
Orias, Hamilton, & Orias, 2000) and Paramecium
(Bihn, Lilly, & Napolitano, 1974; Fok & Allen, 
1979; Soldo, Godoy, & van Wagtendonk, 1966). 
This tractability, together with tractable genetic 
techniques, has given rise to an enormous lit-
erature on these two genera, summarized in books, 
which can only be listed here: on Tetrahymena
– Hill (1972), Elliott (1973a), Asai and Forney 
(2000); and on Paramecium – Beale (1954), Görtz 
(1988a), Jurand and Selman (1969), Sonneborn 
(1970), van Wagtendonk (1974), and Wichterman 
(1986). Both axenic (Finley & McLaughlin, 
1965; Finley,