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 term ...

' + 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 f ...

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 do n ...

. 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 unde ...

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 The Echiura, or spoon worms, are a small group of marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into segments, but echiurans have secondarily los ...

and

Sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. The name ''Sipuncula'' is from the genus name ''Sipunculus'', and comes from the Latin ' ...

, they were once placed in the taxon

Gephyrea Gephyrea is a now-dismantled taxon for a group of non-annulated worms, considered intermediate between annelids and holothurians, containing the three modern phyla Echiura, Sipuncula The Sipuncula or Sipunculida (common names sipunculid worms ...

, but consistent morphological and molecular evidence supports their belonging to Ecdysozoa, which also includes

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

s and

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

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 ar ...

'' 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 Loricifera (from Latin, '' lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals that had been determined to be 37 described species, in 9 genera, bu ...

, with which they constitute the

Scalidophora Scalidophora is a group of marine pseudocoelomate protostomes that was proposed on morphological grounds to unite three phyla: the Kinorhyncha, the Priapulida and the Loricifera. The three phyla have four characters in common — chitinous cu ...

clade named after the spines covering the introvert ( scalids). They feed on slow-moving invertebrates, such as

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worms. Priapulid-like fossils are known at least as far back as the Middle

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...

. 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 r ...

; it is probably a blood-space or

hemocoel The blood circulatory system is a organ system, 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 vascula ...

. 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 ...

respiratory The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...

systems, but the body cavity does contain

phagocytic Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

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 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 struc ...

, 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 th ...

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 sympatheti ...

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 Stimulus (physiology), stimuli. (For example, in the human body, the brain which is part of the cen ...

organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

, 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 The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...

. 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, unsegment ...

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 The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest foss ...

, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. In addition, isolated

microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

(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 ''Treptichnus'' (formerly named ''Phycodes'', ''Manykodes'' by J. Dzik, and also known as ''Trichophycus''See e.gfossiilid.info: paleodiversity in Baltoscandia: Trichophycus pedum/ref>) is the preserved burrow of an animal. As such, it is regar ...

'') 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 ''Selkirkia'' is a genus of predatory, tubicolous priapulid worms known from the Middle Cambrian Burgess Shale, Ogygopsis Shale and Puncoviscana Formation. 142 specimens of ''Selkirkia'' are known from the Greater Phyllopod bed, where they compr ...

'' 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 ''Ottoia'' is a stem-group archaeopriapulid worm known from Cambrian fossils. Although priapulid-like worms from various Cambrian deposits are often referred to ''Ottoia'' on spurious grounds, the only clear ''Ottoia'' macrofossils come from the ...

Walcott 1911 ***Genus †''

Ottoia ''Ottoia'' is a stem-group archaeopriapulid worm known from Cambrian fossils. Although priapulid-like worms from various Cambrian deposits are often referred to ''Ottoia'' on spurious grounds, the only clear ''Ottoia'' macrofossils come from th ...

'' 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 ...

'' 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 Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a phylum of unsegmented ...

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 Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worm ...

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