The Synhymeniida are an order in the phylum Ciliophora which prossess a cytopharyngeal apparatus consisting of a basket of well-developed nematodesmata bound proximally by fibrous material. This type of oral structure is somtimes called a cyrtos. The Synhymeniida are distinguished from other cyrtos-bearing ciliates by the presence of a band of cilia (called either the synhymenium or frange) that partially encircles the anterior end of the cell on the ventral surface. In some classifications of the Ciliophora, this group has been proposed to be the pivotal group that links the lower prostomial ciliates with the higher hymenostomial ciliates (Corliss 1979). In other classifications, the synhymeniids are alinged with the class Nassophorea but are not considered to be closely related to other cyrtos-bearing ciliates in the order Cyrtophorida, which are in a different class, the Phyllopharyngea (Small and Lynn 1985).
For the past 100 years, ciliates with a cyrtos have been alternatively combined into a single taxon, and divided into separate taxa. Schewiakoff (1896) initially proposed the group Hypostomata for all ciliates with a cyrtos and the taxon is recognized in many of the popular classification schemes of the early part of this century (e.g., tribe Hypostomata in Kahl (1930-1935)). In classifications from 1950-1970 (e.g. Faure-Fremiet (1950); Kudo (1966); Corliss (1961)), ciliates with an anteriorly-located cell mouth with relatively unspecialized circumoral cilia (commonly called prostomial ciliates) were considered primitive ciliates that represent the ancestral stock for the evolution of the rest of the Ciliophora. The ciliates in which the cytostome was a cyrtos that had shifted to the ventral surface were considered to be "more advanced" than the prostomes but "more primitive" than and ancestral to the ciliates with complex oral structures. In these classifications, some of the cyrtos-bearing ciliates were placed in the Cyrtophorina but those with reduced body cilia (e.g. the Microthoracidae) were put in a separate order, the Trichostomatida. Beginning in the late 1960s, ultrastructural morphology of many ciliates was reported. This new information resulted in revised classification schemes and new scenarios for the evolution of the ciliates. In Corliss (1979) and de Puytorac et al. (1974) for example, all ciliates with a ventral cytostome and a cyrtos were transferred into the resurrected Hypostomata (now a subclass). The hypostomes were divided into three orders: the Synhymeniida, Nassulida, and Cyrtophorida. In these classification, the synhymeniids are considered to be the ancestors to both the nassulids (which in turn gave rise to the higher ciliates) and the cyrtophorids (which, in turn, give rise to the suctorians). Other systematists drew different conclusions. Small and Lynn (1981, 1985) and Lynn and Small (1988), produced a revised classification which emphasized the pattern of the microtubules and fibers associated with the basal body of the cilium. In their classification, the synhymeniids are placed in the class Nassophorea together with some of the other cyrtos-bearing ciliates such as the Nassulida, Microthoracida, and Propeniculida (e.g. Pseudomicrothorax) but also with ciliates that do not have a cyrtos such as the Peniculida (e.g. Paramecium) and the euplotine Hypotrichia (e.g. Euplotes; but see also Lynn and Small (1988). This class is defined as having a unique kinetid pattern; a tangential transverse ribbon of microtubules, slightly divergent, well-developed postciliary microtubules ribbon, and an anteriorly-directed kinetodesmal fiber (Lynn 1981, 1991, Small and Lynn 1985). Even though the ultrastructure of the synhymeniids has never been described and it is not known if they have this pattern or not, they are placed in this group. Small and Lynn combined the remaining ciliates with a cyrtos in the order Cyrtophorida in a different class, the Phyllopharyngea. The kinetid pattern of this class consists of: a reduced (or absent) radial ribbon of transverse microtubules, moderately convergent postciliary microtubules, and a short laterally- to slightly anterolaterally-directed kinetodesmal fibril (Lynn 1981). The present investigation reveals the first information about the ultrastructural features of a synhymeniid ciliate. This new knowledge allows comparisons to be made about the ultrastructural morphology of the synhymeniids and other cyrtos-bearing ciliates. The monokinetid of the synhymeniid, Zosterodasys agamalievi, shares some characteristics with members of the Nassulida (sensu Corliss (1979)) such as divergent postciliary microtubules and an anteriorly-directed kinetodesmal fibril (Lynn 1981, 1991, Small and Lynn 1985). However, like ciliates in the order Cyrtophorida (sensu Corliss (1979)), the transverse microtubules in Z. agamalievi are radially-oriented (Lynn 1981). In addition, in both the Cyrtophorida and Z. amagalievi, the postciliary microtubules overlap, forming postciliodesmata, although the arrangement of microtubules is different. In the Cyrtophorida, the posticiliary microtubules of each kinetosome form triads (Hoffman and Bardele 1987, Soltynska 1971) whereas in Z. agamalievi, they form a double row. The somatic monokinetid of Z. agamalievi lacks the subkinetal ribbons of microtubules and dense transverse fibril which are associated with the kinetosomes of the Cyrtophorida (Hoffmann and Bardele 1987, Soltynska 1971, Small and Lynn 1985). Unlike either the Nassulida or the Cyrtophorida, a fibrous rootlet plus microtubules is associated with the somatic monokinetid of Z. agamalievi. Invaginations of the inner alveolar membrane called alveolocysts (Peck 1971) are present in nassulids (Eisler and Bardele 1983) but are absent in Z. agamalievi. In addition, the 'B-cartwheel' structure described in the proximal region of the kinetosome of the ciliates in the order Nassulida, suborder Nassulina (sensu Corliss (1979) (Eisler 1988, Eisler and Bardele 1983, Tucker 1971) was not observed in Z. agamalievi. The proximal region of the kinetosome in Z. agamalievi appears empty as it does in ciliates in the order Nassulida, suborder Microthoracina (sensu Corliss 1979) (Njine and Didier 1980, Peck 1977). The electron-opaque bodies which are observed in the cortex of Z. agamalievi have not been reported in any nassulid, however, similar looking bodies have been reported in the cyrtophorid, Brooklynella (Lom and Corliss 1971). These electron-opaque bodies also resemble structures which have been described in some heterotrichs karyorelictids, pleurostomatids, and haptorids (Bohatier et al. 1978, Bohatier and Njine 1973, de Puytorac and Njine 1970, Kennedy 1965, Williams et al. 1981). Oral ciliature, present in both the orders Nassulida (de Puytorac and Njine 1980, Eisler and Bardele 1983, Grain et al. 1978, Tucker 1968, 1971) and Cyrtophorida (Hofmann and Bardele 1987, Hofmann 1987, Soltynska 1971), was not observed in the feeding stage of the life cycle of Z. agamalievi, which was used in this study. Ultrasructure of the synhymenium reveals that it possesses a kinetodesmal fiber, indicating that it is a component of the somatic ciliature. The oral structure of the synhymeniid, Z. agamalievi, appears to have fewer elements than other cyrtos-bearing ciliates (see Eisler's 1988 review of oral structure). Like ciliates in the order Nassulida, suborders Nassulina (Eisler 1988, Grain et al. 1978, Tucker 1968) and Microthoracina (Eisler 1988, Hausmann and Peck 1978, 1979), and the order Cyrtophorida (Eisler 1988, Kaczanowska and Moraczewski 1985, Pyne and Tuffrau 1970, Soltynska 1971), the Synhymeniida cyrtos is equipped with well-developed nematodesmata. The nematodesmata are capped distally with capitula in both the Synhymeniida and the Cyrtophorida (Kaczanowska and Moraczewski 1985, Lom and Corliss 1971, Soltynska 1971). In the Synhymeniida, the pharyngeal tube is bound distally by a fibrous sheath and is lined by microtubules. Similar microtubules have been described in the Cyrtophorida (Eisler 1988, Hofmann 1987, Kaczanowska and Moraczewski 1985, Pyne and Tuffrau 1970, Soltynska 1971), Nassulina (de Puytorac and Njine 1980, Eisler 1988, Grain et al. 1978, Tucker 1968), and Chonotrichida (Eisler 1988). A fibrous sheath is also present in the Nassulina, but according to Tucker's (1968) diagrammatic reconstruction, it is restricted to the proximal region of the cyrtos. Not observed in the Synhymeniida, but present in the Cyrtophorida (Eisler 1988, Hofmann 1987), Nassulina (de Puytorac and Njine 1980, Eisler 1988, Grain et al., 1978, Tucker 1968), and Chonotrichida (Eisler 1988) is a second set of microtubules lining the cytopharynx which are called subcytostomal microtubules by some (de Puytorac and Njine 1980, Eisler 1988, Grain et al., 1978, Tucker 1968) and distal microtubules by others (Hofmann 1987). A third set of microtubules, the nematodesmal lamella, is present in the cyrtos of the suborders Nassulina (de Puytorac and Njine 1980, Eisler 1988, Grain et al., 1978, Tucker 1968), and Microthoracina (Eisler 1988, Hausmann and Peck 1978, 1979), but was not observed in the cyrtos of the order Synhymeniida. These are several other components of the nassuline cyrtos which are absent in the Synhymeniida. These include microtubules crests, an inner fibrillar ring, an outer fibrillar ring, and a dense annulus. Additionally, the cytopharynx of nassulines is corrugated distally (Tucker 1968). Whether or not Z. agamalievi has a primitively simple or a secondarily reduced oral complex can not be determined until homology of these structures has been tested by congruence in a phylogenetic analysis (Lipscomb 1992, Patterson 1982).