Priapulida

Priapulids are cylindrical worm-like animals, ranging from 0.2 to 0.3 to 39 centimetres (0.08–0.12 to 15.35 in) long, with a median anterior mouth quite devoid of any armature or tentacles. They show both radial and bilateral symmetry. The gonads, protonephridia and ventral nerve cord are bilateral, while the introvert, pharynx and brain show radial symmetry, and appears to be a secondary trait. Also the larvae show internal and external characteristics of radial symmetry. The adult body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. In addition it is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. The body has a chitinous cuticle that is moulted as the animal grows. Members of the family Chaetostephanidae also secretes a gelatinous tube, open in both ends, which they live in. There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a coelom; it is probably a blood-space or hemocoel. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it. The nervous system retains a basiepidermal configuration with a connection with the ectoderm, forming part of the body wall. There are no specialized sense organs, but there are sensory nerve endings in the body, especially on the proboscis. Their male and female organs are closely associated with the excretory protonephridia. They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts. The perigenital area of the genus Tubiluchus exhibit sexual dimorphism. Reproduction and development For the species Priapulus caudatus, the 80 μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern. Development is remarkably slow, with the first cleavage taking place 15 hours after fertilization, gastrulation after several days and hatching of the first 'lorica' larvae after 15 to 20 days. The species Meiopriapulus fijiensis have direct development. In current systematics, they are described as protostomes, despite having a deuterostomic development. Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained. == Fossil record ==

'' in the Burgess Shale (Middle Cambrian) of a priapulid tooth (Ottoia, Cambrian); from Smith et al. 2015 Stem-group priapulids are known from the Middle Cambrian Burgess Shale, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. In addition, isolated microfossils (corresponding to the various teeth and spines that line the pharynx and introvert) are widespread in Cambrian deposits, allowing the distribution of priapulids – and even individual species – to be tracked widely through Cambrian oceans. Trace fossils that are morphologically almost identical to modern priapulid burrows (Treptichnus pedum) officially mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time. Crown-group priapulid body fossils are first known from the Carboniferous. ==Phylogeny==

External phylogeny }} Internal phylogeny }} }} }} ==Classification==

There are 22 known extant species: '', a prehistoric priapulid. Phylum Priapulida Théel 1906 • Order Halicryptomorpha Salvini-Plawen 1974 [Adrianov & Malakhov 1995; Salvini-Plawen 1974; Eupriapulida Lemburg, 1999] • Family Halicryptidae Salvini-Plawen 1974 • Genus Halicryptus • Species H. higginsi (Shirley & Storch, 1999) • Species H. spinulosus (von Siebold, 1849) • Order Priapulomorpha Adrianov & Malakhov 1995 (assigned its own order by ) • Family Priapulidae Gosse 1855 [Xiaoheiqingidae (sic) Hu 2002] • Genus Acanthopriapulus • Species A. horridus (Théel, 1911) • Genus Priapulopsis • Species P. australis (de Guerne, 1886) • Species P. bicaudatus (Danielssen, 1869) • Species P. cnidephorus (Salvini-Plawen, 1973) • Genus Priapulus • Species P. abyssorum (Menzies, 1959) • Species P. caudatus (Lamarck, 1816) • Species P. tuberculatospinosus (Baird, 1868) • Family Tubiluchidae van der Land 1970 [Meiopriapulidae Adrianov & Malakhov 1995] • Genus Tubiluchus • Species T. arcticus (Adrianov, Malakhov, Tchesunov & Tzetlin, 1989) • Species T. australensis (van der Land, 1985) • Species T. corallicola (van der Land, 1968) • Species T. lemburgi (Schmidt-Rhaesa, Rothe & Martínez, 2013) • Species T. pardosi (Schmidt-Rhaesa, Panpeng & Yamasaki, 2017) • Species T. philippinensis (van der Land, 1985) • Species T. remanei (van der Land, 1982) • Species T. soyoae (Schmidt-Rhaesa, Panpeng & Yamasaki, 2017) • Species T. troglodytes (Todaro & Shirley, 2003) • Species T. vanuatensis (Adrianov & Malakhov, 1991) • Genus Meiopriapulus • Species M. fijiensis (Morse, 1981) • Order Seticoronaria • Family Chaetostephanidae Por & Bromley 1974 [Chaetostephanidae Salvini-Plawen 1974] • Genus Maccabeus • Species M. cirratus (Malakhov, 1979) • Species M. tentaculatus (Por, 1973) Extinct groups Stem-group †Scalidophora • Order †Ancalagonida Adrianov & Malakhov 1995 [Fieldiida Adrianov & Malakhov 1995] • Family †Ancalagonidae Conway Morris 1977 • Genus †Ancalagon Conway Morris 1977 • Family †Fieldiidae Conway Morris 1977 • Genus †Fieldia Walcott 1912 Stem-group †Palaeoscolecida • Family †Selkirkiidae Conway Morris 1977 • Genus †Selkirkia Walcott 1911 non Hemsley 1884 • Order †Ottoiomorpha Adrianov & Malakhov 1995 • Genus †Scolecofurca Conway Morris 1977 • Family †Ottoiidae Walcott 1911 • Genus †Ottoia Walcott 1911 • Family †Corynetidae Huang, Vannier & Chen 2004 • Genus †Corynetis Luo & Hu 1999 [Anningvermis Huang, Vannier & Chen 2004] • Family †Miskoiidae Walcott 1911 • Genus †Miskoia Walcott 1911 • Genus †Louisella Conway Morris 1977 == References ==