Stem-arthropods
Critics of the Ichthyostraca hypothesis have pointed out that even parasitic crustaceans can still be recognised as crustaceans based on their larvae; but that tongue worms and their larvae do not express typical characters for Crustacea or even Euarthropoda. An alternative model notes the extremely ancient Cambrian origins of these animals and interprets tongue worms as stem-group arthropods. A recent morphological analysis recovered Pentastomida outside the arthropods, as sister group to a clade including nematodes, priapulids and similar ecdyzoan 'worm' groups. Adding fossils, they suggested an extinct animal called Facivermes could be closely related to tongue worms. However it should be stressed that these authors did not explicitly test pentastomid/crustacean relationships.
Read more about this topic: Pentastomida, Affinities