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of disc-shaped vesicles 
fuse with the cytopharyngeal plasma membrane. 
Digestion occurs by processes typical of most 
eukaryotes, although in Paramecium an unusual set 
Fig. 4.13. Cross-sections of the paroral dikinetids of genera representative of classes in the Phylum Ciliophora. ( a ) 
Eufolliculina – Class HETEROTRICHEA . ( b ) Lepidotrachelophyllum – Class LITOSTOMATEA . ( c ) Chilodonella
– Class PHYLLOPHARYNGEA . ( d ) Woodruffia – Class COLPODEA . ( e ) Furgasonia – Class NASSOPHOREA . 
(f ) Paramecium – Class OLIGOHYMENOPHOREA . ( g ) Cyclidium – Class OLIGOHYMENOPHOREA . ( h ) 
Colpidium – Class OLIGOHYMENOPHOREA (from Lynn, 1981, 1991) 
4.4 Oral Structures 109
Fig. 4.14. Ultrastructure of the oral polykinetids of ciliates. A A square-packed oral polykinetid of the nassopho-
rean Nassula citrea with the posterior row of kinetosomes bearing postciliary microtubular ribbons ( Pc ) (from Eisler & 
Bardele, 1986). B A hexagonally-packed oral polykinetid of the oligohymenophorean Colpidium campylum . Note the 
 parasomal sacs ( Ps ) lying on either side of the three rows of kinetosomes (from Lynn & Didier, 1978). C Cross-sec-
tion through the oral cavity of C. campylum shows the three oral polykinetids separated by two cortical ridges ( R ) 
underlain by alveoli . The polykinetids are connected by filamentous connectives ( FC ) (from Lynn & Didier, 1978). 
D A rhomboid-packed oral polykinetid of the oligohymenophorean Thuricola folliculata (from Eperon & Grain, 
1983). E A slightly off square-packed oral polykinetid of the colpodean Woodruffia metabolica
of vesicles, called acidosomes , fuse with the phago-
some to first acidify the phagosomal compartment 
prior to fusion of lysosomes (Allen, 1984; Allen & 
Fok, 2000). The old food vacuoles ultimately arrive 
in the region of the cytoproct where their contents 
are expelled to the outside. Excess food vacuole 
membrane is then recycled to the cytopharyngeal 
region as disc-shaped vesicles (Allen; Allen & Fok; 
Allen & Wolf, 1974). 
 More details on the oral region and its function 
can be found in later chapters and in reviews by 
Grain (1984), de Haller (1984c), Lynn (1981), Lynn 
and Corliss (1991), Paulin (1996), de Puytorac 
(1984a), de Puytorac and Grain (1976), and Radek 
and Hausmann (1996). 
 4.5 Division and Morphogenesis 
 Ciliates can be studied as cells, and like all cells 
during the interphase period of the cell cycle, they 
can be expected to faithfully duplicate all their 
component parts (Berger, 2001; Méténier, 1984a). 
This is what is called balanced growth . Following 
this duplication, ciliates as unicellular organisms 
reproduce by cell division . Unlike animals, this 
reproductive process in ciliates is separate from 
sexual processes (see below, Nuclei, Sexuality, and 
Life Cycle ) so that months to years of asexual repro-
duction can take place between sexual events. 
 Ciliates typically divide by binary fission , in 
which the parental cell divides into two filial products, 
offspring, or progeny (Fig. 4.8). The anterior 
“daughter” cell is termed the proter and the poste-
rior “daughter” cell is called the opisthe (Chatton 
& Lwoff, 1935b). This binary fission is usually 
equal or isotomic , that is both filial products are 
the same size, but it can be unequal or anisotomic . 
 Budding is a common type of anisotomy , which is 
found especially in sessile taxa, such as suctorians 
and chonotrichs (see Chapter 10 ). Ciliate fission 
is also termed homothetogenic in the vast majority 
of cases, meaning that the cell axes of proter and 
opisthe have the same orientation or polarity: 
typically the posterior end of the proter is in 
contact with the anterior end of the opisthe. This 
is modified in two main ways. Some spirotrichs , 
especially oligotrichs and choreotrichs , undergo a 
modified division mode called enantiotropic divi-
sion : the axes of proter and opisthe of these plank-
tonic ciliates shift during cell division so that 
they have an almost opposite polarity. The second 
Fig. 4.15. Filter feeding ciliates can use their oral structures to function as a downstream filter feeder , which 
creates a current with the cilia of the oral polykinetids and captures particles in the cilia of the paroral, or as an 
 upstream filter feeder , which both creates the current and captures the particles using the cilia of the oral polykinetids .
(Redrawn after Fenchel, 1980a.)
4.5 Division and Morphogenesis 111
112 4. Phylum CILIOPHORA – Conjugating, Ciliated Protists with Nuclear Dualism
modification is found in peritrichs whose sessile 
life style is accompanied by a seemingly parallel 
type of cell division: the proter and opisthe appear 
to develop “alongside” each other with the 
fission furrow separating them “longitudinally”. 
However, Lom (1994) has argued that this may 
just be a highly modified form of homothetogenic 
fission , easily re-interpreted by assuming that the 
stalk of peritrichs arises out of the dorsal surface, 
and is not the posterior end of the cell. 
 In some ciliates, binary fission may not occur 
when the ciliate doubles all its components. For 
example, the parasitic ciliate Ichthyophthirius
may grow several orders of magnitude as a 
parasite in the epithelium of its fish host before 
dropping off, encysting, and dividing up to eight 
times sequentially to yield over 1,000 offspring. 
Even free-living ciliates, which may undergo a 
period of starvation as they disperse from one 
food patch to another, may undergo a period of 
 “unbalanced” growth , presumably as they replen-
ish and “balance” cell constituents that were dif-
ferentially more exhausted during the starvation 
period. Upon refeeding, these free-living ciliates, 
like Tetrahymena , may grow larger than the typi-
cal size during balanced growth and then, undergo 
several sequential cell divisions without interven-
ing growth (Lynn, 1975; Lynn & Tucker, 1976; 
Lynn, Montagnes, & Riggs, 1987). The process 
of multiple divisions without intervening growth 
is termed palintomy . It can occur sequentially in 
a cyst , as it does in Ichthyophthirius and some 
 colpodean ciliates, or it may occur in a linear 
fashion in highly elongate ciliates, as it does in 
some astomes. In the latter case, it can also be 
called catenulation or strobilation . 
 Cell division can be thought of as being com-
posed of two processes: division of the cytoplasm 
or cytokinesis and division of the nucleus or karyo-
kinesis , often called mitosis . Cytokinesis is most 
obvious in its last stages where a fission furrow 
appears near the equator in ciliates undergoing 
 isotomy . The furrow develops in some ciliates by 
assembly and then contraction of special kinds 
of microfilaments (Yasuda, Numata, Ohnishi, & 
Watanabe, 1980). Prior to furrow formation, special 
microtubules may appear in the cortical ridges, 
above the epiplasm, the so-called cytospindle of 
Paramecium (Sundararaman & Hanson, 1976). As 
the isthmus between the cells narrows further, the 
twisting and pulling movements of the progeny 
achieve the final separation. 
 Karyokinesis is more complicated in ciliates, 
since they have two nuclei. The typically globular 
or ellipsoid micronucleus undergoes a eukaryotic 
cell mitosis except that the nuclear membrane does 
not break down. Spindle microtubules assemble 
within the nuclear envelope and are used to separate
the sister chromatids (LaFountain & Davidson, 1980).
Raikov (1982) categorized the ciliate micro nuclear 
mitosis as a closed intranuclear orthomitosis . The 
 macronuclei of ciliates may take a variety of 
shapes and may be subdivided into apparently 
disconnected nodules. Prior to division, these 
macronuclear nodules often condense so that the 
many nodes, for example, may ultimately comprise 
a single ellipsoid body. The macronucleus then 
divides in two phases –

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