The musculature of the praesoma in Macracanthorhynchus hirudinaceus (Acanthocephala, Archiacanthocephala): re-examination and phylogenetic significance

The endoparasitic Archiacanthocephala (Acanthocephala) consist of the Aporhynchida, Moniliformida, Gigantorhynchida and Oligacanthocephala. In the present study the organisation of the praesoma in Macracanthorhynchus hirudinaceus (Archiacanthocephala, Oligacanthorhynchida) was investigated by light microscopy based on series of semithin sections (5 µm) with special emphasis on the musculature. The study was carried out to substantiate the ground pattern of the Acanthocephala and to elucidate the phylogenetic relationships within the Archiacanthocephala. A comparison of the presented morphology in M. hirudinaceus with literature data leads to the assumption that the muscle plate and the midventral longitudinal muscle evolutionarily originated from the circular musculature of the praesoma and the receptacle, respectively. Whereas the midventral longitudinal muscle probably represents an autapomorphy of the taxon Oligacanthorhynchida, a muscle plate can be regarded as an autapomorphy of a monophylum consisting of the Moniliformida, Gigantorhynchida and Oligacanthorhynchida. Moreover, the outer wall in species with a double-walled receptacle probably corresponds to the receptacle protrusor or receptacle constrictor in species with a single-walled receptacle, and thus not only a receptacle but also an additional surrounding muscle can be assumed for the ground pattern of the Acanthocephala. For a better comparability the discussion includes a tabular survey of the synonyms used in the literature.