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ultrastructural studies on micronuclear and macronuclear division in the clevelandellid Nyctotherus cordiformis . Macronuclei divide by intramacronuclear microtubules that are primarily responsible for the elongation of the macronucleus, which is also accompanied on its out- side by scattered extramacronuclear microtubules (Eichenlaub-Ritter & Tucker, 1984; Hamelmann, Eichenlaub-Ritter, & Ruthmann, 1986). Micronuclear mitosis is an endomitosis, typical of ciliates (Raikov, 1982). There may be three \u201cclasses\u201d of micro tubules, identified by their differing responses to drugs and temperature, which function to accomplish micronu- clear mitosis : (1) manchette microtubules underlying the nuclear envelope; (2) interpolar and kinetochore microtubules, which function during anaphase; and (3) stembody microtubules, which function during telophase to separate the putative micronuclei to each progeny cell (Eichenlaub-Ritter & Ruthmann, 1982a, 1982b). Microtubules in the dividing nuclei may have more than the canonical 13-protofila- ments (Eichenlaub-Ritter, 1985; Eichenlaub-Ritter & Tucker). Conjugation has been studied in only a few examples of armophoreans since the description of it in Nyctotheroides (= Nyctotherus ) by Wichterman (1936). It is not established what factors stimulate conjugation in free-living forms. Wichterman (1936) observed it occurring only in transforming tadpoles of the frog Hyla versicolor . This lead to specula- tion that gonadotropins or some other physiological signal derived from the host may cue these ciliates to begin conjugation. However, Sandon (1941a) observed conjugation in Paranyctotherus isolated from the adult clawed frog Xenopus laevis , sug- gesting that other factors are involved. Affa\u2019a and Amiet (1994) have confirmed that conjugation can occur in all stages of the frog life cycle \u2013 tadpoles , transforming individuals, and adults. Gonadotropin injections induced conjugation in Prosicuophora , even when immature stages were treated (Affa\u2019a, 1986b). Thus, it is unlikely that one single factor stimulates conjugation . Fusion of the conjugants occurs in the anterior region, and in some Metopus species total conjuga- tion may occur (Noland, 1927). The micronuclei of each partner typically undergo three maturation divisions \u2013 two meiotic divisions followed by a mitosis of one of the four haploid products (Raikov, 1972; Martín-González et al., 1987). In the total conjugation of Metopus , the cytoplasm of one con- jugant flows into the partner carrying the gametic nucleus or nuclei with it. However, the old macro- nucleus is left in the cortical shell of the disgarded partner (Noland, 1927). Following fusion of the gametic nuclei to form the synkaryon , armopho- reans typically have one post-synkaryon division with one nucleus becoming the new micronucleus and the other becoming the new macronucleus. In species with more than one macronucleus, there may be additional post-synkaryon divisions (see Martín-González et al., 1987). Development of the macronuclear anlage in armophoreans is an extremely long process: Golikova (1965) recorded it taking up to 2 weeks in Nyctotheroides (= Nyctotherus ) while Noland (1927) observed a mininimum of 1 week in Metopus . In both these genera, it appears that polytene chromo- somes are formed at one stage during anlage devel- opment. Golikova (1965) concluded that one giant polytene chromosome may form in Nyctotheroides by the end-to-end joining of the individual chromo- somes. This giant chromosome later fragments both transversely and longitudinally to yield the macro- nuclear chromosomes (Vinnikova & Golikova, 1978). Ultimately, the macronuclear chromosomes fragment into gene-sized pieces as happens in the Class SPIROTRICHEA (see Chapter 7 ), a fact that Riley and Katz (2001) have confirmed by molecular analyses of the macronuclear DNA of both armophorids and clevelandellids. 8.7 Other Features The free-living armophorids have been recog- nized for some time as strong indicators of anoxic aquatic environments (e.g., Bick, 1972; Foissner, 1988a; Sládec\u2c7ek, 1973). They are commonly found in soils (Foissner, 1987) and have been recorded from a variety of municipal landfill sites in the United Kingdom, where they undergo an encyst- ment - excystment cycle in response to starvation and water loss (Finlay & Fenchel, 1991). 8.7 Other Features 185