Cap 12
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Cap 12

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in colpodids (e.g., Beers, 1946a; 
Burt et al., 1941). These extrusion bodies have 
been considered a method of eliminating defective 
DNA (Burt et al., 1941), but they may also serve to 
enable regulation of the nuclear-cytoplasmic ratio 
(Woodruff, 1941). Extrusion does not seem to regu-
late DNA content in vegetative cells, and so it may 
indeed be a \u201cpurification\u201d mechanism (Frenkel, 
1975). Extrusion also appears to maintain a strong 
 nucleocytoplasmic ratio in cells preparing to encyst 
(Morat et al., 1981). 
 12.7 Other Features 
 Colpodeans have been recorded as dominant ciliate
members of activated sludge plants (Aescht & 
Foissner, 1992) and, as noted above, they are com-
mon and dominant elements in soils and other 
terrestrial habitats all over the world (Foissner, 
1993a, 1994b). This has lead to an interest in the 
impacts of toxicants, especially heavy metals on 
Colpoda species, which show reduced growth 
rates in response to heavy metals in laboratory 
and field situations (Forge, Berrow, Darbyshire,
& Warren, 1993; Janssen, Oosterhoff, Heijmans, & 
Van der Voet, 1995), although colpodeans appear 
to be quite resistant to metal toxicity (Díaz, Martín-
González, & Gutiérrez, 2006). Nevertheless, strains 
of Colpoda from around the world appear not to 
differ significantly in their sensitivity to these 
toxicants (Xu et al., 1997).