The phylogeny within the order Choreotrichida is reconstructed using (i) morphologic, ontogenetic, and ultrastructural evidence for the cladistic approach and (ii) the small subunit ribosomal RNA (SSrRNA) gene sequences, including the new sequence of Rimostrombidium lacustris. The morphologic cladograms and the gene trees converge rather well for the Choreotrichida, demonstrating that hyaline and agglutinated loricae do not characterize distinct lineages, i.e., both lorica types can be associated with the most highly developed ciliary pattern. The position of Rimostrombidium lacustris within the family Strobilidiidae is corroborated by the genealogical analyses. The diagnosis of the genus Tintinnidium is improved, adding cytological features, and the genus is divided into two subgenera based on the structure of the somatic kineties. The diagnosis of the family Lohmanniellidae and the genus Lohmanniella are improved, and Rimostrombidium glacicolum Petz, Song and Wilbert, 1995 is affiliated.

Oligotrich ciliates are an abundant and important component of planktonic food webs. Classified together, based on morphology and the arrangement of the oral ciliature, they include sensu lato halteriids, strombidiids, strobilidiids, and tintinnids. The oral ciliature of tintinnids and strobilidiids, the subclass Choreotrichia, is composed of a closed circle of external polykinetids with internal polykinetids, leading to the cytostome. Strombidiid and halteriid species, the subclass Oligotrichia, have an open circle of adoral polykinetids and separate ventral polykinetids. Previous molecular phylogenetic analyses of spirotrichs have placed Halteria grandinella among the stichotrichs. Apart from H. grandinella, only one other small subunit ribosomal RNA (SSrRNA) gene sequence of an oligotrich ciliate (Strombidium purpureum) has been published. Fourteen additional species representative of the major oligotrich orders were collected to determine their phylogenetic relationships. These SSrRNA gene sequences include two strombidiids (Strombidium spp.), one strobilidiid (Strobilidium caudatum), and 11 tintinnid species (i.e. Codonellopsis americana, Eutintinnus pectinis, Eutintinnus sp., Favella panamensis, Metacylis angulata, Metacylis sp., Rhabdonella hebe, Tintinnidium mucicola, Tintinnopsis dadayi, Tintinnopsis fimbriata and Tintinnopsis tocatinensis). The subclasses Oligotrichia (excluding Halteria) and Choreotrichia form a monophyletic group and are sister group to the subclass Stichotrichia with high nodal support. The choreotrichs are confirmed as a monophyletic subclass whereas the halteriids do not cluster with the strombidiids, but within the subclass Stichotrichia. Therefore, the family Halteriidae is transferred to the order Sporadotrichida in the subclass Stichotrichia. The strombidiids form a sister clade to the subclass Choreotrichia. Since there are no clear morphological or genetical synapomorphies for these two groups combined, the separate subclasses Choreotrichia and Oligotrichia are retained. The genetic relationships among tintinnid genera do not correspond to relationships based on lorica morphology: species with agglomerated and hyaline loricae do not form separate branches but are interrelated.

Morphological data from life, protargol impregnation, and scanning electron microscopy were combined with genetic data not only to describe the marine plankton ciliates Pelagostrobilidium neptuni (Montagnes and Taylor, 1994) Petz, Song, and Wilbert, 1995 and Strombidium biarmatum nov. spec., but also to elucidate their phylogenetic relationships. Additionally, the ontogenesis of P. neptuni was studied and the diagnosis of the genus Pelagostrobilidium was improved due to further data from the newly affiliated species P. epacrum (Lynn and Montagnes, 1988) nov. comb. (basionym: Strobilidium epacrum Lynn and . The phylogenetic analysis of the small subunit ribosomal RNA genes matched the morphologic and ontogenetic assigning of P. neptuni to the choreotrichid family Strobilidiidae. The considerable genetic distance of d = 0.074 between P. neptuni and Strobilidium caudatum corroborated the morphological differences and thus the maintenance of the genus Pelagostrobilidium. Strombidium biarmatum nov. spec. is a typical member of the genus, except for the two types of extrusomes ("trichites"): ~12 × 0.5 μm, needle-shaped ones attached anterior to the girdle kinety and ~6 × 0.5 μm, rod-shaped ones at the distal end of the intermembranellar ridges. Its flask-shaped resting cysts have several strong spines. In accordance with the morphologic data, S. biarmatum is placed within the order Oligotrichida by gene sequence analysis. The great genetic distances within the oligotrichids support the diversity found in morphologic and ontogenetic studies.

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