Affinities
The affinities of tongue worms have long proved controversial. Historically, they were initially compared to various groups of parasitic worms. Once the arthropod-like nature of their cuticle was recognised, similarities were drawn with mites, particularly gall mites (Eriophyidae). Although gall mites are much smaller than tongue worms they also have a long, segmented body and only two pairs of legs. Later work drew comparisons with millipedes and centipedes (Myriapoda), with velvet worms (Onychophora) and water bears (Tardigrada). Some authors 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, In general, there are two current alternative interpretations: 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.
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