The Systematic Position of Pseudomicrothorax dubius, Ciliate with a Unique Combination of Anatomical Features*

SYNOPSIS. The rather rarely found holotrichous ciliate Pseudomicrothorax dubius, generally considered in the most authoritative literature to be a member of the order Trichostomatida, more likely belongs in the Hymenostomatida. Its curious combination of trans‐ordinal characteristics, however, makes difficult exact systematic allocation of it and of the congeneric species P. agilis. The provocative morphological features include: a gymnostome‐like cytopharyngeal “basket”; a tetrahymenal buccal apparatus, with membranelles comparable with those of primitive hymenostomes; an advanced semi‐autonomous type of stomatogenesis; and sensory bristles, flattened rigid form, and restricted ciliature reminiscent of certain spirotrichs. Brief discussion is presented concerning the influence of modern ideas of ciliate classification upon what is considered the proper taxonomic position of the genus.