Later work[citation needed] drew comparisons with millipedes and centipedes (Myriapoda), with velvet worms (Onychophora) and water bears (Tardigrada).

Some authors[citation needed] interpreted tongue worms as essentially intermediate between annelids and arthropods, while others suggested that they deserved a phylum of their own.

Tongue worms grow by moulting, which suggests they belong to Ecdysozoa, while other work has identified the arthropod-like nature of their larvae.

[12] In general, the two current alternative interpretations are: pentastomids are highly modified and parasitic crustaceans, probably related to fish lice, or they are an ancient group of stem-arthropods, close to the origins of Arthropoda.

The discovery that tongue worms are crustaceans can be traced back to the work of Pierre-Joseph Van Beneden,[13] who compared them to parasitic copepods.

The modern form of this hypothesis dates from Karl Georg Wingstrand's study of sperm morphology,[14] which recognised similarities in sperm structure between tongue worms and fish lice (Argulidae) – a group of maxillopod crustaceans which live as parasites on fish and occasionally amphibians.

[16] A number of subsequent molecular phylogenies have corroborated these results,[17][18][19] and the name Ichthyostraca has been proposed for a (Pentastomida + Branchiura) clade.

An alternative model notes the extremely ancient Cambrian origins of these animals and interprets tongue worms as stem-group arthropods.

[23] Adding fossils, they suggested an extinct animal called Facivermis could be closely related to tongue worms.

Exceptionally preserved, three-dimensional and phosphatised fossils from the Upper Cambrian Orsten of Sweden[24] and the Cambrian/Ordovician boundary of Canada[25] have been identified as pentastomids.

[35] While a report exists of Sebekia inducing dermatitis,[36][37] the two genera responsible for most internal human infestation are Linguatula and Armillifer.