[ref. ID; 4125 (Jacques Berger and Denis H. Lynn, 1981)]
Small & Lynn (1981) suggested on the basis of preliminary observations that the plagiophylids belonged in their Class Oligohymenophorea. At that time, the data (Berger & Lake 1969) suggested that the plagiopylid somatic kinetid was similar to the oligohymophorean somatic kinetids: that is, typically a monokinetid with a divergent radial postciliary microtubular ribbon, a radial transverse radial postciliary microtubular ribbon, a radial transverse microtubular ribbon, and an anteriorly directed kinetodesmal fibril that overlaps with fibrils from other kinetids. Small & Lynn (1984), furthermore, have suggested that the plagiopylids belong in their own subclass within the Class Oligohymenophorea. Small & Lynn (1981) have placed the subclasses Hymenostomatia, Astomatia, Apostomatia, and Peritrichia in the Class Oligohymenophorea. The somatic kinetids of the first three subclasses are virtually identical. The Peritricha have long been considered phylogenetically derived from a hymenostome ancestral stock (Corliss 1979); the features of their stomatogenesis and oral structures have been reasonably used to support that view. Thus, this class has an obvious unity. What does the addition of the plagiopylids do to the phylogenetic coherence of this taxon? The plagiopylid somatic kinetid is a monokinetid with a divergent postciliary ribbon and an anteriorly directed.
These features are similar to the oligohymenophorean kinetid but are also shared by some taxa in the Class Nassophorea (Lynn 1981; Small & Lynn 1981). The feature that is clearly unique to the oligohymeopnorean kinetid is the radial transverse ribbon. The plagiopylids, rather than having this, have their own unique transverse microtubular ribbon, which travels longitudinally beneath the right surface of the cortical ridge adjacent to the one beneath which it originates. In addition to this unique kinetids feature, the extrusomes of plagiopylids bear some resemblance to the rod-like mucocysts of some scuticociliate taxa; these extrusomes exhibit very slight internal differentiation. The observation that after extrusion, they resemble a trichocyst surrounded by an outer sheath, however, characterizes these organelles also as very distinct. Trichocysts exhibit in situ a definite substructure that includes a distal tip and a larger striated basal portion. When extruded, trichocysts bear the distal tip and are never ensheathed (Hausmann 1978). The plagiopylid extrusomes do not fit clearly into either of these categories. The plagiopylids are certainly not related to the classical trichostome taxa (Corliss 1961) since their kinetids are quite different. Clearly, the plagiopylid kinetid exhibits features characteristic of the kinetid of most taxa in the Subphylum Cyrtophora, in which group the plagiopylids seem appropriately placed. On the basis of their unique cortical ultrastructure and their stomatogenesis (Faure-Fremiet 1950), we recommended that the plagiopylids be designated as incertae sedis in the Subphylum Cyrtophora.