Pentastomida

The Pentastomida are an enigmatic group of parasitic arthropods commonly known as tongue worms due to the resemblance of the species of the genus Linguatula to a vertebrate tongue. They are traditionally seen as crustaceans, even if that position has been questioned.

About 130 species of pentastomids are known; all are obligate parasites with correspondingly degenerate anatomy. Adult tongue worms vary from about 1 to 14 cm in length, and parasitise the respiratory tracts of vertebrates. They have five anterior appendages. One is the mouth; the others are two pairs of hooks, which they use to attach to the host. This arrangement led to their scientific name, meaning "five openings", but although the appendages are similar in some species, only one is a mouth.

Alternative names for the Pentastomida include Pentastoma (strictly a genus name), Linguatulida, and Acanthotheca.

Biology
Historically significant accounts of tongue worm biology and systematics include early work by Josef Aloys Frölich, Alexander von Humboldt, Karl Asmund Rudolphi, Karl Moriz Diesing and Rudolph Leuckart.

Other important summaries have been published by Louis Westenra Sambon, Richard Heymons and John Riley, and a review of their evolutionary relationships with a bibliography up to 1969 was published by J. T. Self.

Anatomy
Pentastomids are worm-like animals ranging from 1 to 14 cm in length. The anterior end of the body bears five protuberances, four of which are clawed legs, while the fifth bears the mouth. The body is segmented and covered in a chitinous cuticle. The digestive tract is simple and tubular, since the animal feeds entirely on blood, although the mouth is somewhat modified as a muscular pump.

The nervous system is similar to that of other arthropods, including a ventral nerve cord with ganglia in each segment. Although the body contains a haemocoel, no circulatory, respiratory, or excretory organs are present.

Life cycle
Pentastomids live in the upper respiratory tract of reptiles, birds, and mammals, where they lay eggs. They are gonochoric (having two sexes), and employ internal fertilisation. The eggs are either coughed out by the host or leave the host body through the digestive system. The eggs are then ingested by an intermediate host, which is commonly either a fish or a small herbivorous mammal.

The larva hatches in the intermediate host and breaks through the wall of the intestine. It then forms a cyst in the intermediate host's body. The larva is initially rounded in form, with four or six short legs, but moults several times to achieve the adult form. The pentastomid reaches the main host when the intermediate host is eaten by the main host, and crawls into the respiratory tract from the oesophagus.

Human infestation
Tongue worms occasionally parasitise humans. While a report exists of Sebekia inducing dermatitis, the two genera responsible for most internal human infestation are Linguatula and Armillifer. Visceral pentastomiasis can be caused by Linguatula serrata, Armillifer armillatus, Armillifer moniliformis, Armillifer grandis, and Porocephalus crotali.

The terms associated with infections can vary:
 * Linguatula disease can be called linguatuliasis or linguatulosis.
 * Porocephalus disease can be called porocephaliasis or porocephalosis.
 * Armillifer disease can also be called porocephalosis. (An alternate name for Armillifer moniliformis is Porocephalus moniliformis.)
 * "Pentastomiasis" can refer to any infection of Pentastomida.

Porocephalus and Armillifer (which are all cylindrical and all inhabit snakes) have much more in common with each other than they do with Linguatula (which is flat and inhabits dogs and wolves).

Affinities
The affinities of tongue worms have long proved controversial. Historically, they were initially compared to various groups of parasitic worms. Once the arthropod-like nature of their cuticle was recognised, similarities were drawn with mites, particularly gall mites (Eriophyidae). Although gall mites are much smaller than tongue worms, they also have a long, segmented body and only two pairs of legs. Later work drew comparisons with millipedes and centipedes (Myriapoda), with velvet worms (Onychophora) and water bears (Tardigrada). Some authors interpreted tongue worms as essentially intermediate between annelids and arthropods, while others suggested that they deserved a phylum of their own. Tongue worms grow by moulting, which suggests they belong to Ecdysozoa, while other work has identified the arthropod-like nature of their larvae. In general, the two current alternative interpretations are: pentastomids are highly modified and parasitic crustaceans, probably related to fish lice, or they are an ancient group of stem-arthropods, close to the origins of Arthropoda.

Crustaceans
The possibility that tongue worms are crustaceans can be traced back to the work of Pierre-Joseph Van Beneden, who compared them to parasitic copepods. The modern form of this hypothesis dates from Karl Georg Wingstrand's study of sperm morphology, which recognised similarities in sperm structure between tongue worms and fish lice (Argulidae) – a group of maxillopod crustaceans which live as parasites on fish and occasionally amphibians. John Riley and colleagues also offered a detailed justification for the inclusion of the tongue worms among the crustaceans. The fish louse model received significant further support from the molecular work of Lawrence G. Abele and colleagues. A number of subsequent molecular phylogenies have corroborated these results, and the name Ichthyostraca has been proposed for a (Pentastomida + Branchiura) clade. Thus a number of important standard works and databases on crustaceans now include the pentastomids as members of this group.

Stem-arthropods
Critics of the Ichthyostraca hypothesis have pointed out that even parasitic crustaceans can still be recognised as crustaceans based on their larvae; but that tongue worms and their larvae do not express typical characters for Crustacea or even Euarthropoda. An alternative model notes the extremely ancient Cambrian origins of these animals and interprets tongue worms as stem-group arthropods. A recent morphological analysis recovered Pentastomida outside the arthropods, as sister group to a clade including nematodes, priapulids and similar ecdysozoan 'worm' groups. Adding fossils, they suggested an extinct animal called Facivermis could be closely related to tongue worms. However it should be stressed that these authors did not explicitly test pentastomid/crustacean relationships.

Fossil record
Exceptionally preserved, three-dimensional and phosphatised fossils from the Upper Cambrian Orsten of Sweden and the Cambrian/Ordovician boundary of Canada have been identified as pentastomids. Four fossil genera have been identified from the Cambrian so far: Aengapentastomum, Bockelericambria, Haffnericambria and Heymonsicambria. These fossils suggest that pentastomids evolved very early and raise questions about whether these animals were parasites at this time, and if so, on which hosts. Conodonts have sometimes been mentioned as possible hosts in this context. A fifth genus, Invavita, is from Silurian-aged marine strata of England: fossil specimens of Invavita are found firmly attached to their ostracod hosts of the species Nymphatelina gravida. It possessed a head, a worm-like body, and two pairs of limbs.

Classification
This article follows Martin and Davis in placing the Pentastomida in the class Maxillopoda within the subphylum Crustacea. The subclass contains these extant orders, superfamilies, families, genera, and species:


 * †Aengapentastomum Waloszek, Repetksi & Maas 2006
 * †Aengapentastomum andresi Waloszek, Repetksi & Maas 2006


 * †Boeckelericambria Walossek & Müller 1994
 * †Boeckelericambria pelturae Walossek & Müller 1994


 * †Haffnericambria Walossek & Müller 1994
 * †Haffnericambria trolmeniensis Walossek & Müller 1994


 * †Heymonsicambria Walossek & Müller 1994
 * †Heymonsicambria ahlgreni Castellani et al. 2011
 * †Heymonsicambria gossmannae Walossek & Müller 1994
 * †Heymonsicambria kinnekullensis Walossek & Müller 1994
 * †Heymonsicambria repetskii Walossek & Müller 1994
 * †Heymonsicambria scandica Walossek & Müller 1994
 * †Heymonsicambria taylori Walossek, Repetski & Müller 1994


 * Eupentastomida Waloszek, Repetski & Mass 2006
 * Cephalobaenida Heymons, 1935
 * Cephalobaenidae Heymons, 1922
 * Bothropsiella Cavalieri 1967 [species inquirendum]
 * Bothropsiella cornuta Cavalieri 1967
 * Cephalobaena Heymons, 1922
 * Cephalobaena tetrapoda Heymons, 1922
 * †Invavita Siveter et al. 2015
 * †Invavita piratica Siveter et al. 2015
 * Reighardiida Almeida & Christoffersen, 1999
 * Reighardiidae Heymons, 1926
 * Hispania Martínez et al., 2004
 * Hispania vulturis Martínez et al., 2004
 * Reighardia Ward, 1899
 * Reighardia lomviae Dyck, 1975
 * Reighardia sternae (Diesing, 1864)
 * Raillietiellida Almeida & Christoffersen, 1999
 * Raillietiellidae Sambon, 1922
 * Raillietiella Sambon, 1910
 * Raillietiella aegypti Ali, Riley & Self, 1982
 * Raillietiella affinis Bovien, 1927
 * Raillietiella agcoi Tubangui & Masiluñgan, 1936
 * Raillietiella ampanihyensis Gretillat, Brygoo & Domergue, 1962
 * Raillietiella amphiboluri Mahon, 1954
 * Raillietiella belohaensis McAllister, Riley, Freed & Freed, 1993
 * Raillietiella bicaudata Heymons & Vitzthum, 1935
 * Raillietiella boulengeri (Vaney & Sambon, 1910)
 * Raillietiella bufonis Ali, Riley & Self, 1982
 * Raillietiella cartagenensis Ali, Riley & Self, 1985
 * Raillietiella chamaeleonis Gretillat & Brygoo, 1959
 * Raillietiella colubrilineati (Leuckart, 1860)
 * Raillietiella congolensis Fain, 1961
 * Raillietiella crotali Ali, Riley & Self, 1984
 * Raillietiella freitasi (Motta & Gomes, 1968)
 * Raillietiella furcocercum (Diesing, 1836)
 * Raillietiella gehyrae Bovien, 1927
 * Raillietiella gigliolii Hett, 1924
 * Raillietiella gowrii Rajalu & Rajendran, 1970
 * Raillietiella hebitihamata Self & Kuntz, 1960
 * Raillietiella hemidactyli Hett, 1934
 * Raillietiella indica Gedoelst, 1921
 * Raillietiella kochi Heymons, 1926
 * Raillietiella mabuiae Heymons, 1922
 * Raillietiella maculatus Rao & Hiregaudar, 1962
 * Raillietiella maculilabris Ali, Riley & Self, 1984
 * Raillietiella madagascariensis McAllister et al., 1993
 * Raillietiella mediterranea (Hett, 1915)
 * Raillietiella monarchus Ali, Riley & Self, 1984
 * Raillietiella morenoi Abreu-Acosta et al., 2006
 * Raillietiella mottae Almeida, Freire & Lopes, 2008
 * Raillietiella namibiensis Riley & Heideman 1998
 * Raillietiella orientalis (Hett, 1915)
 * Raillietiella piscator Nair, 1967
 * Raillietiella rileyi Krishnasamy et al., 1995
 * Raillietiella schoutedeni Fain, 1960
 * Raillietiella scincoides Ali, Riley & Self, 1984
 * Raillietiella spiralis Hett, 1924
 * Raillietiella teagueselfi Riley, McAllister & Freed, 1988
 * Raillietiella tetrapoda (Gretillat, Brygoo & Domergue, 1962)
 * Raillietiella trachea Riley, Oaks & Gilbert, 2003
 * Raillietiella venteli (Motta, 1965)
 * Yelirella Spratt, 2010
 * Yelirella petauri (Spratt, 2003) Spratt 2010
 * Porocephalida Heymons 1935
 * Linguatuloidea Haldeman 1851
 * Linguatulidae Haldeman, 1851
 * Linguatula Frölich, 1789
 * Linguatula arctica Riley, Haugerud & Nilssen, 1987
 * Linguatula multiannulata Haffner & Rack in Haffner, Rack & Sachs, 1969
 * Linguatula recurvata (Diesing, 1850)
 * Linguatula serrata Frölich, 1789
 * Neolinguatula Haffner & Rack in Haffner, Rack & Sachs, 1969
 * Neolinguatula nuttalli (Sambon, 1922)
 * Subtriquetridae Fain, 1961
 * Subtriquetra Sambon, 1922
 * Subtriquetra megacephalum (Baird, 1853)
 * Subtriquetra rileyi Junker, Boomker & Booyse, 1998
 * Subtriquetra shipleyi Hett, 1924
 * Subtriquetra subtriquetra (Diesing, 1836)
 * Porocephaloidea Sambon 1922
 * Porocephalidae Sambon, 1922
 * Armillifer Sambon, 1922
 * Armillifer aborealis Riley & Self, 1981
 * Armillifer agkistrodontis Self & Kuntz, 1966
 * Armillifer armillatus (Wyman, 1845)
 * Armillifer australis Heymons, 1935
 * Armillifer grandis (Hett, 1915)
 * Armillifer mazzai (Sambon, 1922)
 * Armillifer moniliformis (Diesing, 1836)
 * Armillifer yoshidai Kishida, 1928
 * Cubirea Kishida, 1928
 * Cubirea annulata (Baird, 1853)
 * Cubirea pomeroyi (Woodland, 1921)
 * Elenia Heymons, 1932
 * Elenia australis Heymons, 1932
 * Gigliolella Chabaud & Choquet, 1954
 * Gigliolella brumpti (Giglioli, 1922)
 * Kiricephalus Sambon, 1922
 * Kiricephalus clelii Riley & Self, 1980
 * Kiricephalus coarctatus (Diesing, 1850)
 * Kiricephalus constrictor Riley & Self, 1980
 * Kiricephalus gabonensis Riley & Self, 1980
 * Kiricephalus pattoni (Stephens, 1908)
 * Kiricephalus tortus (Shipley, 1898)
 * Parasambonia Stunkard & Gandal, 1968
 * Parasambonia bridgesi Stunkard & Gandal, 1968
 * Parasambonia minor Riley & Self, 1982
 * Porocephalus Humboldt, 1812
 * Porocephalus basiliscus Riley & Self, 1979
 * Porocephalus benoiti Fain, 1960
 * Porocephalus bifurcatus
 * Porocephalus clavatus (Wyman, 1845)
 * Porocephalus crotali Humboldt, 1812
 * Porocephalus dominicana Riley & Walters, 1980
 * Porocephalus stilesi Sambon in Vaney & Sambon, 1910
 * Porocephalus subuliferum (Leuckart, 1860)
 * Porocephalus taiwana Qiu, Ma, Fan & Lu, 2005
 * Porocephalus tortugensis Riley & Self, 1979
 * Waddycephalus Sambon, 1922
 * Waddycephalus calligaster Riley & Self, 1981
 * Waddycephalus komodoensis Riley & Self, 1981
 * Waddycephalus longicauda Riley & Self, 1981
 * Waddycephalus porphyriacus Riley & Self, 1981
 * Waddycephalus punctulatus Riley & Self, 1981
 * Waddycephalus radiata Riley & Self, 1981
 * Waddycephalus scutata Riley & Self, 1981
 * Waddycephalus superbus Riley & Self, 1981
 * Waddycephalus teretiusculus (Baird, 1862)
 * Waddycephalus vitiensis Heymons, 1932
 * Sebekidae Sambon, 1922
 * Alofia Giglioli in Sambon, 1922
 * Alofia ginae Giglioli in Sambon, 1922
 * Alofia indica (von Linstow, 1906)
 * Alofia merki Giglioli 'in'' Sambon, 1922
 * Alofia nilotici Riley & Huchzermeyer, 1995
 * Alofia parva Riley & Huchzermeyer, 1995
 * Alofia platycephalum (Lohrmann, 1889)
 * Alofia simpsoni Riley, 1994
 * Alofia travassosi (Heymons, 1932)
 * Diesingia Sambon, 1922
 * Diesingia kachugensis (Shipley, 1910)
 * Diesingia megastomum (Diesing, 1836)
 * Selfia Riley, 1994
 * Selfia porosus Riley, 1994
 * Leiperia Sambon, 1922
 * Leiperia australiensis Riley & Huchzermeyer, 1996
 * Leiperia cincinnalis (Sambon in Vaney & Sambon, 1910)
 * Leiperia gracilis (Diesing, 1836)
 * Sambonia Noc & Giglioli, 1922
 * Sambonia clavata (Lohrmann, 1889)
 * Sambonia parapodum Self & Kuntz, 1966
 * Sambonia solomenensis (Self & Kuntz, 1957)
 * Sambonia varani (Self & Kuntz, 1957)
 * Sambonia wardi (Sambon in Vaney & Sambon, 1910)
 * Agema Riley et al., 1997
 * Agema silvaepalustris Riley et al., 1997
 * Pelonia Junker & Boomker, 2002
 * Pelonia africana Junker & Boomker, 2002
 * Sebekia Sambon, 1922
 * Sebekia cesarisi Giglioli in Sambon, 1922
 * Sebekia divestei Giglioli in Sambon, 1922
 * Sebekia johnstoni Riley, Spratt & Winch, 1990
 * Sebekia microhamus Self & Rego, 1985
 * Sebekia minor (Wedl, 1861)
 * Sebekia mississippiensis Overstreet, Self & Vliet, 1985
 * Sebekia multiannulata Riley, Spratt & Winch, 1990
 * Sebekia novaeguineae Riley, Spratt & Winch, 1990
 * Sebekia okavangoensis Riley & Huchzermeyer, 1995
 * Sebekia oxycephalum (Diesing, 1836)
 * Sebekia purdieae Riley, Spratt & Winch, 1990
 * Sebekia trinitatis Riley, Spratt & Winch, 1990