Apororhynchus
Apororhynchus is a genus of small parasitic spiny-headed (or thorny-headed) worms. It is the only genus in the family Apororhynchidae, which in turn is the only member of the order Apororhynchida.[3] A lack of features commonly found in the phylum Acanthocephala (primarily musculature) suggests an evolutionary branching from the other three orders of class Archiacanthocephala; however no genetic analysis has been completed to determine the evolutionary relationship between species. The distinguishing features of this order among archiacanthocephalans is a highly enlarged proboscis which contain small hooks. The musculature around the proboscis (the proboscis receptacle and receptacle protrusor) is also structured differently in this order. This genus contains six species that are distributed globally, being collected sporadically in Hawaii, Europe, North America, South America, and Asia. These worms exclusively parasitize birds by attaching themselves around the cloaca using their hook-covered proboscis. The bird hosts are of different orders, including owls, waders, and passerines. Infestation by an Apororhynchus species may cause enteritis and anemia.
The first species in this order to be described was Apororhynchus hemignathi which was originally named Arhynchus hemignathi by Arthur Shipley in 1896. The name Arhynchus[a] was chosen based on the characteristic absence of a proboscis in this species of Acanthocephala.[1] It was later renamed Apororhynchus (along with the family name Apororhynchidae) by Shipley in 1899 due to the name Arhynchus having been used by Dujean in 1834 for a beetle.[4]
Although Apororhynchus has not been included in phylogenetic analyses thus far due to insufficiency of morphological data, the lack of features such as an absence of a muscle plate, a midventral longitudinal muscle, lateral receptacle flexors, and an apical sensory organ when compared to the other three orders of class Archiacanthocephala indicate it is an early offshoot (basal).[5]
The genus Apororhynchus consists of ectoparasitic worms that attach themselves beneath the skin and around the anus of birds.[6][5] The distinguishing features of this order among acanthocephalans are a highly enlarged proboscis with limited motility and a reduced size of the hooks (or spines).[5] Apororhynchus species have short conical trunks and a reduced or absent neck.[7] The proboscis is large and globular with numerous deeply set spirally arranged rootless hooks usually not reaching the surface, or with no hooks.[7] They contain sets of muscles that are common to all Acanthocephala including a proboscis receptacle, a receptacle-surrounding muscle called a receptacle protrusor, retinacula (connective tissue that stabilizes tendons), a neck retractor, proboscis and receptacle retractors,[7] circular and longitudinal musculature under the metasomal (trunk) tegument, and a single muscular layer beneath the proboscis wall.[5]
Two regions of musculature are considerably different in Apororhynchus compared to the other acanthocephalan orders: the proboscis receptacle and receptacle protrusor are both reorganized in Apororhynchus with the muscles subdivided into strands extending from the cerebral ganglion, or nerve bundle, to the proboscis wall. These two muscles suspend the cerebral ganglion but are not involved in the eversion of the proboscis.[5] Additional anatomical features that can be used to distinguish this genus among other acanthocephalans include a cerebral ganglion located under the anterior wall of the proboscis, long and tubular lemnisci (bundles of sensory nerve fibers) that run along a central canal, the lack of any protonephridia (an organ which functions as a kidney), and the presence of eight pear-shaped cement glands used to temporarily close the posterior end of the female after copulation.[8][9]
