The phylogenetic relationships of the pentastomida: The case for their inclusion within the crustacea

Pentastomids are a distinct systematic assemblage of worm-like animals which, as adults, are parasitic in the respiratory tracts of vertebrates. Recent evidence regarding the phylogenetic affinities of the group is assessed. Evidence favouring a descent from annelids, or from an annelid-like ancestor of the arthropods, is dismissed because it is based on gross comparative anatomy and fails to take into account the many adaptive features of these highly specialised endoparasites. Theories according pentastomids the status of an independent phylum, sharing a similar rank with tardigrades and onychophorans are dismissed similarly.Three crucial items of evidence, embryogenesis, the structure of the integument, and gametogenesis are considered to confirm previous hypotheses of genuine arthropod homologies. Spermatogenesis particularly, clearly establishes pentastomids as a crustacean sub-class, closely allied to the Branchiura. We postulate that the pentastomid progenitor was originally a parasite of fish which subsequently became adapted to an endoparasitic existence in aquatic reptiles through predation.