With the hypothesis that the Polyacanthocephala represent a different class within the phylum Acanthocephala. The more derived Palaeacanthocephala, including the Echinorhynchida and Polymorphida, are arranged in a paraphyletic assemblage. Both orders demonstrate high morphological diversity, which may explain why traditional identification keys have distinguished among the taxa according to their final hosts. The order Echinorhynchida infects teleost fishes, occasionally amphibians and reptiles whereas the Polymorphida include parasites of reptiles, birds, and marine mammals. The Echinorhynchida so far separate into 10 families and 339 valid species. The Polymorphida include only three families and a total of 255 valid species. Consequently, these species rich taxa include 83 genera and 594 species of Acanthocephalans, mainly from the aquatic environment. Herlyn et al. for the first time described paraphyly within the Palaeacanthocephala, indicating independent evolution within these widely distributed taxa. Similarly, molecular and morphological studies so far indicated that the family Rhadinorhynchidae is paraphyletic or polyphyletic, and that the genera should be reexamined and reclassified by using morphological, Rapamycin mTOR inhibitor ecological, and molecular characters, in agreement with the cladistic studies by Garcı´a-Varela and Nadler and Herlyn et al.. The present analyses place the two species Serrasentis sagittifer and Gorgorhynchoides bullocki, both Echinorhynchida, into the Polymorphida. Neither species demonstrates any morphological similarity. Conspicuous are the trunk hooks of Serrasentis that are arranged within rows, and the presence of four cement glands in the males. Gorgorhynchoides has trunk hooks on its praesoma and six cement gland in the males. Most interesting is the position of the polymorphid Plagoirhynchus cyndraecus, which is arranged between the Echinorhynchida and Polymorphida. This species uses birds as final hosts. The cylindrical trunk also has anterior hooks around a small bulb, and the males have also six cement glands. According to traditional classifications, this result questions the relationship of Serrasentis and Gorgorhynchoides to the other echinorhynchids. While only some echinorhynchid acanthocephalans have mainly irregularly arranged surface hooks on the trunk, the herewith recognized character of regularly arranged hooks on the trunk is one of the most common features within the polymorphids. Recent morphological assessment led to incongruent conclusions, due to difficulties in finding morphological characters that distinguish taxa, and to the partly subjective character states that often lack homologies with the outgroup. According to Garcı´a-Valera and Nadler, many families have been diagnosed based on character combinations rather than shared derived features. For several species, only a single record exists, caused by difficulties in sampling especially from the marine environment and in confirming the life cycles experimentally. Most previous molecular approaches include too few acanthocephalan sequences.