Cosmotropical species reported in the genus Acarnus, are demonstrated to show small, but consistent interregional morphological differences, leading to the conclusion that these "species" are very probably complexes of closely related species derived from former wide-spread (Tethyan and/or Gondwanan) ancestors. On the basis of this analysis, six regional Acarnus forms are described as new species. The confused taxonomy of some species is clarified on the basis of reexamination of type specimens. The phylogenetic history of the genus is reconstructed with cladistic techniques, using morphological characters of original as well as unpublished material together covering almost all described taxa from all over their known ranges. The resultant cladogram is used for a discussion of the biogeographic history of the genus. Some disjunct patterns are apparent, which are possibly in conflict with the currently accepted history of the world oceans.