The Acanthocephala are distinguished from other taxa by a series of autapomorphies: a hooked proboscis, a retractor, a receptacle with six nuclei, a receptacle constrictor or protrusor, paired retinacula, a special sexual apparatus, a keratinous eggshale with four covers and an endoparasitic life cycle with a mandibulate intermediate host and a gnathostome definitive host. Presomal sense organs as apical and lateral sense organs are absent in the acanthocephalan basal pattern.
Within the Acanthocephala the "Palaeacanthocephala" are paraphyletic. Eoacanthocephala and Archiacanthocephala both are monophyletic. The former have a newly described binucleate epidermis cone in the proboscis apex and an unpaired cement gland with eight nuclei. Species of the Archiacanthocephala exhibit two apical sense organs, paired apical sense nerves and a support "cell" with four nuclei and four processes as autapomorphies. Furthermore the Archiacanthocephala have thick walled eggs and evolved a terrestrial life cycle with a tracheate intermediate host.
Paired lateral sense organs, paired apical sense nerves and a binucleate support "cell" with two processes are evolved in the stem lineage of some "Palaeacanthocephala", all Eoacanthocephala and all Archiacanthocephala. Probably Eoacanthocephala and Archiacanthocephala are sistergroups sharing among others median lacunar channels as a synapomorphic feature.
Within the Archiacanthocephala the Aporhynchida are monophyletic because of a reduced receptacle and an extended proboscis. The Moniliformida show a spiral-shaped receptacle protrusor as an evolutionary novelty. The "Gigantorhynchida" are paraphyletic. The Oligacanthorhynchida exhibit a receptacle muscle with four nuclei, a binucleate ventral midventral longitudinal muscle and an unpaired apical sense organ as autapomorphies. Only Moniliformida, "Gigantorhynchida" and Oligacanthorhynchida have paired cellular lateral receptacle flexors and a binucleate muscle plate innervated by anterior proboscis nerves. "Gigantorhynchida" share with the Oligacanthorhynchida the existence of a caudally extended receptacle containing a filling syncytium described for the first time.
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