Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' '

Priapus In Greek mythology, Priapus (; grc, Πρίαπος, ) is a minor rustic fertility god, protector of livestock, fruit plants, gardens and male genitalia. Priapus is marked by his oversized, permanent erection, which gave rise to the medical ter ...

' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...

of unsegmented marine

worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...

s. The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert (eversible)

proboscis A proboscis () is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In invertebrates, the term usually refers to tubular mouthparts used for feeding and sucking. In vertebrates, a proboscis is an elong ...

may resemble the shape of a human

penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males d ...

. They live in the mud and in comparatively shallow waters up to deep. Some species show a remarkable tolerance for

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The under ...

and anoxia. They can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of ''Priapulus caudatus'' per square meter has been recorded, while the density of its larvae can be as high as 58,000 per square meter. Together with Echiura and Sipuncula, they were once placed in the taxon Gephyrea, but consistent morphological and molecular evidence supports their belonging to

Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda ( insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Aguinaldo ''et al.'' in 1997, based mainly on phylogenetic ...

, which also includes

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

s and

nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...

s. Fossil findings show that the mouth design of the

stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

-arthropod ''

Pambdelurion ''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...

'' is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods, even if modern arthropods no longer possess it. Among Ecdysozoa, their nearest relatives are

Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Modern species are or less, ...

and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert ( scalids). They feed on slow-moving invertebrates, such as polychaete worms. Priapulid-like fossils are known at least as far back as the Middle

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

. They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. 22 extant species of priapulid worms are known, half of them being of meiobenthic size.

Anatomy

Priapulids are cylindrical worm-like animals, ranging from 0.2–0.3 to 39 centimetres ( 0.08–0.12 to 15.35 in) long, with a median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. The body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Some species may also have a tail or a pair of caudal appendages. The body has a

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

ous cuticle that is moulted as the animal grows. There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a

coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...

; it is probably a blood-space or

hemocoel The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...

. There are no

vascular The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...

or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the

respiratory pigment A respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There ar ...

haemerythrin Hemerythrin (also spelled haemerythrin; grc, αἷμα, haîma, blood, grc, ἐρυθρός, erythrós, red) is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiop ...

. The

alimentary canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

is straight, consisting of an eversible

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. The anus is terminal, although in ''Priapulus'' one or two hollow ventral diverticula of the body-wall stretch out behind it. The

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...

consists of a nerve ring around the pharynx and a prominent cord running the length of the body with

ganglia A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympathe ...

and longitudinal and transversal neurites consistent with an orthogonal organisation. The nervous system retains a basiepidermal configuration with a connection with the

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from t ...

, forming part of the body wall. There are no specialized

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

organs, but there are sensory nerve endings in the body, especially on the proboscis. The priapulids are

gonochoristic In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism c ...

, having two separate sexes (i.e. male and female). 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

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegmen ...

s 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

Priapulid development has been reappraised recently because early studies reported abnormal development caused by high temperature of embryo culture. 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 The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...

have been maintained.

Fossil record

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 A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils ...

that are morphologically almost identical to modern priapulid burrows ('' Treptichnus pedum'') officially mark the start of the

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 living species: Ottoia reconstruction

Stem-group †Scalidophora *Order † Ancalagonida Adrianov & Malakhov 1995 ieldiida 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 ''Scolecofurca'' is a genus of stem-group priapulid worm dating from the Middle Cambrian period approximately 505 million years ago. It is known from a single fossil specimen from the Raymond Quarry in the Burgess Shale The Burgess Shale ...

'' 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 ''Louisella'' is a genus of worm known from the Middle Cambrian Burgess Shale. It was originally described by Charles Walcott in 1911 as a holothurian echinoderm, and represents a senior synonym of ''Miskoia'', which was originally described as a ...

'' Conway Morris 1977 Phylum Priapulida Théel 1906 *Order Halicryptomorpha Salvini-Plawen 1974 small>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 Meiopriapulomorpha **Family Meiopriapulidae ***Genus Meiopriapulus ****Species M. fijiensis (Morse, 1981) *Order Priapulomorpha Adrianov & Malakhov 1995 (assigned its own order by Adrianov A. V, Malakhov V. V. 2001. Symmetry of priapulids (Priapulida). 1. Symmetry of adults. 247:99–110.) **Family Priapulidae Gosse 1855 iaoheiqingidae (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 Tubiluchidae is one of the two families of priapulimorphidan priapulid worms. Taxa The family includes only the genus ''Tubiluchus'' van der Land 1968, and in some classifications ''Meiopriapulus''. A full list of the included species: Ref ...

van der Land 1970 eiopriapulidae 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 (Scmidt-Rhaesa, Panpeng & Yamasaki, 2017) ****Species T. philippinensis (van der Land, 1985) ****Species T. remanei (van der Land, 1982) ****Species T. soyoae (Scmidt-Rhaesa, Panpeng & Yamasaki, 2017) ****Species T. troglodytes (Todaro & Shirley, 2003) ****Species T. vanuatensis (Adrianov & Malakhov, 1991) *Order Seticoronaria **Family

Chaetostephanidae ''Maccabeus'' is the sole genus of seticoronarian priapulid worms. References

Priapulida Ecdysozoa families {{Protostome-stub ...

Por & Bromley 1974 haetostephanidae Salvini-Plawen 1974***Genus Maccabeus ****Species M. cirratus (Malakhov, 1979) ****Species M. tentaculatus (Por, 1973)

References

External links

* {{Authority control Animal phyla Extant Pennsylvanian first appearances