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).