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McLaughlin, & Harrison, 1959; 
McLaughlin, Johnson & Bradley, 1974) and non-
axenic (Finley et al., 1959; Vacchiano, Kut, Wyatt, 
& Buhse, 1991) techniques have been developed 
for cell biological research on peritrichs . Soldo and 
Merlin (1972, 1977) have successfully developed 
an axenic culture medium for marine scuticocili-
ates , and this has enabled the cultivation of some 
species, reported as parasitic on shellfish and fish 
(Cawthorn et al., 1996; Crosbie & Munday, 1999; 
Iglesias et al., 2003; Messick & Small, 1996). 
There has not yet been a successful axenic cultiva-
tion of the fish parasite Ichthyophthirius , although 
Nielsen and Buchmann (2000) succeeded in induc-
ing trophont transformation using a fish tissue cul-
ture cell line. Mizobuchi, Yokoigawa, Harumoto, 
Fujisawa, and Takagi (2003) have recently dem-
onstrated that hydrogen peroxide is the toxic sub-
stance in the infusion of wheat grass powder used 
to grow Paramecium . This substance, which may 
also inhibit the growth of other ciliates, is detoxi-
fied by catalase excreted by bacteria , explaining 
why “conditioned” media may be more successful 
in cultivating some ciliate species. 
 Molecular phylogenies generally recover an oli-
gohymenophorean clade with strong bootstrap sup-
port (Baroin-Tourancheau, Villalobo, Tsao, Torres, 
& Pearlman, 1998; Miao et al., 2001; Sánchez-
Silva, Villalobo, Morin, & Torres, 2003; Strüder-
Kypke et al., 2000b). Sánchez-Silva et al. (2003) 
Fig. 15.1. Life cycles of oligohymenophoreans . A A hymenostome , like Tetrahymena paravorax , can transform 
between a microstome bacterivore and a macrostome carnivore depending upon food availability. (after Corliss, 
1973.) B The theronts of the ophryoglenine Ichthyophthirius multifiliis seek out the epithelium of a freshwater fish 
host, burrow underneath as a phoront , and then begin to grow as a trophont . Trophonts later drop off the fish and 
undergo palintomy to produce sometimes more than 1,000 theronts . (after Lynn & Small, 2002.) C The ophryogle-
nine Ophryoglena typically feeds on dead or moribund invertebrates. After feeding, the trophont becomes a proto-
mont , encysts as a tomont to undergo palintomy and produce theronts , the dispersal stage that seeks out other prey. 
(after Canella & Rocchi-Canella, 1976.)
argued that contemporary oligohymenophoreans 
may all have arisen from an ancestor with a devi-
ant nuclear genetic code for glutamine/glutamate. 
Nevertheless, there are no strong morphological 
synapomorphies for the class. The somatic kinetid 
has been typified as a monokinetid with a radial 
transverse microtubular ribbon and a typically 
well-developed kinetodesmal fibril (Lynn, 1981, 
1991). However, inclusion of the peniculines in the 
class presents an exception to this rule. As noted 
above, the oral structures typically include three 
or four oral polykinetids on the left side of the oral 
cavity and a paroral of dikinetids on the right. Yet, 
representatives of the Subclass Astomatia entirely 
lack an oral apparatus while those in the Subclass 
 Apostomatia have highly modified oral structures, 
albeit with presumed homologies to their less 
modified kin (Bradbury, 1989). We have relied on 
the molecular phylogenies to support the class and 
presume that modifications in morphology away 
from the type are manifestations of evolutionary 
divergence. The class includes six subclasses: 
Subclass Peniculia , Subclass Scuticociliatia , 
Subclass Hymenostomatia , Subclass Peritrichia , 
Subclass Apostomatia , and Subclass Astomatia . 
 15.1 Taxonomic Structure 
 Corliss (1979) divided the Class OLIGOHY–
MENOPHOREA into two subclasses – Subclass 
 Hymenostomata and Subclass Peritrichia . 
He excluded the apostomes from this class, 
placing them as an order within the Class 
has demonstrated a paroral-like kinetid arrange-
ment in the apostomes . On this basis, and 
together with similarities in the somatic kinetid 
(see below ), we have transferred the apos-
tomes to the Class OLIGOHYMENOPHOREA . 
We now have sequences of the small subunit 
(SSU) rRNA gene of several genera of apos-
tomes to confirm this transfer (J.C. Clamp et al., 
unpublished data 2008). Our system includes all 
of the subclasses that de Puytorac (1994a) rec-
ognized, except for the Subclass Hysterocinetia 
(see below), which we conservatively retain 
as a group within the Subclass Scuticociliatia , 
awaiting molecular genetic evidence to dem-
onstrate that this group is clearly so differ-
ent. Thus, we outline briefly the six subclasses: 
(1)Subclass Peniculia ; (2) Subclass Scuticociliatia ; 
(3) Subclass Astomatia ; (4) Subclass Peritrichia ; 
(5) Subclass Hymenostomatia ; and (6) Subclass 
 Apostomatia – and their major included groups. 
 Small and Lynn (1981, 1985) placed the peni-
culine ciliates, such as Paramecium and Frontonia , 
as an order in their Class NASSOPHOREA , based 
on similarities in the somatic kinetids, pellicular 
ultrastructure, and extrusomes. Molecular genetic 
studies on both SSUrRNA (Strüder-Kypke et al., 
2000a, 2000b) and proteins (Sánchez-Silva et al., 
2003) have refuted this association and confirmed 
the classical view that peniculines derived from the 
same ancestral lineage as the other oligohymeno-
phorean groups. Nevertheless, these molecular 
data clearly confirm the distant relationship of the 
 peniculines to other members of the class, a dis-
tance supported by their morphological features, 
seemingly homologous to those of the nassopho-
reans . The prime synapomorphies for peniculines 
are as follows: three oral polykinetids, called 
 peniculi , that are aligned longitudinally in the 
oral cavity (Fauré-Fremiet, 1950a, 1950b); the 
typical, although not universal, presence of fibrous 
 trichocysts (Didier, 1971; Jurand & Selman, 1969); 
and a stomatogenesis in which the parental paroral 
and its accompanying anarchic field produce the 
new oral structures (Beran, 1990; Foissner, 1996b; 
Yusa, 1957). We have included two orders within 
the subclass. The Order Peniculida , characterized 
by holotrichous somatic ciliation and the presence 
of fibrous trichocysts includes six families: the 
 Frontoniidae , the Lembadionidae , the Maritujidae , 
the Neobursaridiidae , the Parameciidae , and the 
 Stokesiidae . Since there is clear evidence, both 
from its distinctive girdle of somatic cilia, its lack of 
fibrous trichocysts (Didier; Didier & de Puytorac, 
1969), and its divergent SSUrRNA gene sequence 
(Strüder-Kypke et al., 2000b) that Urocentrum is 
very divergent from other peniculines , we support 
the monotypic Order Urocentrida proposed by de 
Puytorac, Grain, and Mignot (1987) to include the 
Family Urocentridae . 
 Species diversity within the genus Paramecium
continues to be exhaustively analyzed (Maciejewska, 
2007). Corliss and Daggett (1983) particularly 
focused on taxonomic and nomenclatural issues 
surrounding those species, previously assigned to 
the aurelia species complex of Paramecium . They 
emphasized that there is no longer a species named 
Paramecium aurelia in the genus Paramecium . 
This taxonomic arrangement had been formalized 
by Sonneborn (1975) after research by Tait (1970) 
and Allen, Farrow, and Golembiewski (1973) 
had demonstrated categorical differences among 
 isoenzymes , such as esterases and dehydrogenases , 
in this sibling species complex . The same year, 
morphologically-oriented systematists had used 
 multivariate techniques to demonstrate the separa-
bility of several of these sibling species or syngens 
(Gates, Powelson, & Berger, 1975;