Molecular phylogenies of Oligotrichea currently do not contain all genera and families and display topologies which are often incongruent with morphological findings. In ciliates, the somatic kinetids are rather conserved, i.e., their ultrastructures, particularly the fibrillar associates, often characterise the main groups, except for the choreotrichids. Four different kinetid types are found in protargol-stained choreotrichids and used for reconstructing the taxon's evolution (the "Kinetid Transformation Hypothesis"). Proof for this hypothesis requires transmission electron microscopic studies, which are very rare in the choreotrichids and oligotrichids. Such an approach provides insights into the ultrastructural variability of somatic kinetids in spirotrichs and may also detect apomorphies characterising certain choreotrichid families. In the model tintinnid Schmidingerella meunieri, the ultrastructure of the three kinetid types in the somatic ciliature is studied in cryofixed cells. The data support the "Kinetid Transformation Hypothesis" regarding tintinnids with a ventral kinety. This first detailed study on kinetids in tintinnids and choreotrichids in general reveals totally new kinetid types in ciliates: beyond the three common associates, they are characterised by two or three conspicuous microtubular ribbons extending on the kinetids' left sides. These extraordinary ribbons form together with the overlapping postciliary ribbons a unique network in the cortex of the anterior cell portion. The evolutionary constrains which might have fostered the development of such structures are discussed for the Oligotrichea, the choreotrichids, and tintinnids as their first occurrence is currently uncertain. Additionally, the kinetids in tintinnids, aloricate choreotrichids, oligotrichids, hypotrichs, and euplotids are compared.

Keywords: basal bodies; fibrillar associates; phylogeny; somatic infraciliature; tintinnids; ultrastructure.