Family METOPIDAE Kahl, 1927

Anterior part of body uniquely twisted to left, and posterior part sometimes tailed and/or bearing tuft of longer (caudal) cilia; buccal membranelles large, yet rather inconspicuous; uniform somatic ciliation, but often not dense; macronucleus compact, centrally located; widely distributed as fresh-water polysaprobic forms, but some also marine and psammophilic or inquilinic in echinoids.

[Note that Caenomorpha has now been removed to a separate suborder (below): Kahl (1932b) and Corliss (1961), as well as others - until the work of Jankowski (1964a,b) failed to recognize the cardinal differences between it and Metopus.]

Bothrostoma Stokes, 1887. Few species.
Brachonella Jankowski, 1964. Several species.
Metopus Claparède & Lachmann, 1858 (syn. Metopides [hom.] ). Very many ( > 50) species.
Palmarella Jankowski, 1975 (for Palmarium Gajewskaja, 1925 [hom.] ). Few species.
Spirorhynchus da Cunha, 1915 (syn. of Metopus?). Few species.
Tesnospira Jankowski, 1964. Single species.
Tropidoatractus Levander, 1894. Single species.