[Kingdom] Animalia Linnaeus, 1758 - animals
   Grade Plastidozoa [monotypic]
     [Phylum] Protozoa (Goldfuss, 1818) C.T. von Siebold, 1854 see Kingdom Protozoa Goldfuss, 1818
       Grade Stomatodea
         Class Ciliata see [unranked] ciliates
           Order Holotricha
             1 Genus Opalina Purkinje & Valentin, 1835
             2 Genus Trachelius Schrank, 1803