are prominent members of the ciliate communities in water treatment plants (Curds, 1969, 1975b). The peritrichs are a particu- larly important group, responsible for clarification of the water by their bacterivory (Fried et al., 2000; Lee, Basu, Tyler, & Wei, 2004; Martin-Cereceda et al., 2001a, 2001b; Rivera et al., 1988). The wide- spread distribution of scuticociliates , hymenos- tomes , and peritrichs , and the ease with which they can be cultivated has also established them as model organisms in the assessment of toxicants . Hymenostomes , in particular Tetrahymena and Colpidium , have often been used in a wide vari- ety of applications testing a diversity of toxicants from heavy metals to detergents to hydrocarbons . Recent reviews discuss the various end-points used (Gilron & Lynn, 1996; Sauvant, Pepin, & Piccinni, 1999). These include growth inhibition (Dive et al., 1991; Miyoshi et al., 2003; Schultz, 1997; Schultz & Dumont, 1977; Zilberg & Sinai, 2006), survival (Komala, 1993; Madoni & Romeo, 2006; Sartory & Lloyd, 1976; Schlenk & Moore, 1994), respiration (Slabbert, Smith, & Morgan, 1983), chemosensory behavior (Berk, Gunderson, & Derk, 1985; Gilron et al., 1999; Roberts & Berk, 1990), mutagenicity (Smith-Sonneborn, 1981), and ingestion rate (Juchelka & Snell, 1995). Recently, viability has 15.7 Other Features 325 326 15. Subphylum 2. INTRAMACRONUCLEATA: Class 9. OLIGOHYMENOPHOREA been assessed using fluorescent dyes indicative of the integrity of particular cell functions (Dayeh et al., 2004; Dias et al., 2003; Wang, Zhang, & Wang, 2000). Toxicants , such as heavy metals and oils , are likely to impact communities, both in natural habi- tats (Caron & Sieburth, 1981) and in waste-water treatment plants (Madoni et al., 1996). Nevertheless, the scuticociliate Uronema can acquire tolerance to heavy metals (Berk et al., 1978), and this is likely due to the induction of metallothioneins , cysteine- rich metal-binding proteins, which have been iden- tified in Tetrahymena (Fu & Miao, 2006; Santovito et al., 2001). Nyberg and Bishop (1983) concluded that selective forces have operated to generate the variation in tolerance to heavy metals , particularly copper and mercury , among stocks of Paramecium primaurelia . Finally, Tetrahymena may be engineered to play a new role in the manufacturing of proteins for human use. Gaertig et al. (1999) were able to express the surface antigen of Ichthyophthirius multifiliis on the cell surface of Tetrahymena , demonstrating that Tetrahymena might be used as the \u201cvehicle\u201d to establish immunity to \u201c Ich \u201d in fishes. Of more relevance to humans, Peterson et al. (2002) targeted a protein from the malaria par- asite Plasmodium falciparum to the cell surface of Tetrahymena , demonstrating that this ciliate might be used in vaccine development against malaria . As a last example, Weide et al. (2006) have used Tetrahymena as an expression system for human enzymes and shown that a functional enzyme results. Is it too much of a stretch to imagine oligo- hymenophoreans , like Tetrahymena , playing crucial roles in future human health applications?