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.

[15] The fish louse model received significant further support from the molecular work of Lawrence G. Abele and colleagues.

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.