Priapulid
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Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' '
Priapus In Greek mythology, Priapus (; ) 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 priapism. He becam ...
' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
of unsegmented marine
worm Worms are many different distantly related bilateria, bilateral animals that typically have a long cylindrical tube-like body, no limb (anatomy), limbs, and usually no eyes. Worms vary in size from microscopic to over in length for marine ...
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 arthropod mouthparts, mouthparts used for feeding and sucking. In vertebrates, a pr ...
may resemble the shape of a human
penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...
. They live in the mud, except for a few tropical meiobenthic species which live in medium- to coarse-grained sands, and are found in comparatively shallow waters to deep waters and no warmer than 12–13°C. 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 toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
, anoxia and low
salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
. ''Halicryptus spinulosus'' appears to prefer brackish shallow waters. 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 (5,390 per square foot). Together with
Echiura The Echiura, or spoon worms, are a small group of ocean, marine animals. Once treated as a separate phylum, they are now considered to belong to Annelida. Annelids typically have their bodies divided into Segmentation (biology), segments, but e ...
and
Sipuncula The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of marine annelid worms, that have secondarily lost their segmentation. Sipuncula was once considered a phylum of unsegmented ...
, they were once placed in the taxon
Gephyrea Gephyrea is a now-dismantled class of marine worms, containing the three modern taxa Echiura, Sipuncula, and Priapulida. Also ''Sternaspis (annelid), Sternaspis'', the first described genus in the family Sternaspidae, was at some point assumed to b ...
, but consistent morphological and molecular evidence supports their belonging to
Ecdysozoa Ecdysozoa () is a group of protostome animals, including Arthropoda (insects, chelicerates (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phylum (biology), phyla. The grouping of these animal phyla into a single ...
, which also includes
arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...
s and
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
s. Fossil findings show that the mouth design of the
stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...
-arthropod '' Pambdelurion'' 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 (, ' "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 commonly called mud dragons. Modern species are or less, but Cambrian forms could reac ...
and
Loricifera Loricifera (from Latin, ''wikt:lorica, lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals with 43 described species and approximately 100 more that hav ...
, with which they constitute the
Scalidophora Scalidophora is a group of marine pseudocoelomate ecdysozoans 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 cuti ...
clade named after the spines covering the introvert ( scalids). They feed on slow-moving invertebrates, such as
polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...
worms. Some analyses suggest that Priapulida may represent a basal lineage within Ecdysozoa, leading to their classification as "living fossils". Priapulid-like fossils are known at least as far back as the Middle
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
. 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 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 inernal 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. Family Priapulidae have species with a tail or a pair of caudal appendages. A slender tail or tail filament is also found in family Tubiluchidae. Appendages are absent in the remaining families. The body has a
chitin Chitin (carbon, C8hydrogen, H13oxygen, O5nitrogen, N)n ( ) is a long-chain polymer of N-Acetylglucosamine, ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is the second most abundant polysaccharide in nature (behind only cell ...
ous 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 The coelom (or celom) is the main body cavity in many 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, i ...
; it is probably a blood-space or
hemocoel In vertebrates, the circulatory system is a organ system, system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of ...
. There are no
vascular Vascular can refer to: * blood vessels, the vascular system in animals * vascular tissue Vascular tissue is a complex transporting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue ...
or
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 gr ...
systems, but the body cavity does contain
phagocytic Phagocytosis () is the process by which a cell (biology), 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 ph ...
amoebocytes and cells containing the respiratory pigment haemerythrin. 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 tract is the largest of the body's systems, after the cardiovascular system. ...
is straight, consisting of an eversible
pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...
, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. Three of the five extant families have gone through a significant miniaturization and become detritivores (Tubiluchidae and Meiopriapulidae) and filter feeders (Chaetostephanidae). The two remaining families Priapulidae and Halicryptidae are larger carnivores that feed on other animals, although some species also consume detritus as larvae. The shape of the teeth reflect these different lifestyles, and seem to be adapted mainly towards grasping prey or raking detritus from the sediment into the mouth. 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 complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...
consists of a nerve ring around the pharynx and a prominent cord running the length of the body with
ganglia A ganglion (: ganglia) 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 a ...
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 the o ...
, 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 surroundings through the detection of Stimulus (physiology), stimuli. Although, in some cultures, five human senses were traditio ...
organs, but there are sensory nerve endings in the body, especially on the proboscis. The priapulids are gonochoristic, 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 Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...
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

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 Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left–r ...
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 fos ...
, 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 called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...
that are morphologically almost identical to modern priapulid burrows (''
Treptichnus pedum ''Treptichnus'' (formerly named ''Phycodes'', ''Manykodes'' by Jerzy Dzik, 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, ...
'') officially mark the start of the
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
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: 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 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 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'' (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 haetostephanidae 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 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'' Conway Morris 1977 **Family † Ottoiidae Walcott 1911 ***Genus †'' Ottoia'' Walcott 1911 **Family † Corynetidae Huang, Vannier & Chen 2004 ***Genus †''
Corynetis ''Corynetis'' is a genus of archaeopriapulid known from the Chengjiang biota, and a senior synonym of ''Anningvermis multispinosus''. References Priapulida Prehistoric protostome genera Cambrian genus extinctions {{Cambrian-animal- ...
'' Luo & Hu 1999 'Anningvermis'' Huang, Vannier & Chen 2004**Family † Miskoiidae Walcott 1911 ***Genus †'' Miskoia'' Walcott 1911 ***Genus †'' Louisella'' Conway Morris 1977


References


External links

* {{Authority control Ecdysozoa phyla Extant Pennsylvanian first appearances