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50-456: Chaetopteridae Polychaeta ( / ˌ p ɒ l ɪ ˈ k iː t ə / ) is a paraphyletic class of generally marine annelid worms , commonly called bristle worms or polychaetes ( / ˈ p ɒ l ɪ ˌ k iː t s / ). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae , which are made of chitin . More than 10,000 species are described in this class. Common representatives include

100-732: A 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe Nereus at the bottom of the Challenger Deep , the deepest known spot in the Earth's oceans. Only 168 species (less than 2% of all polychaetes) are known from fresh waters. Polychaetes are segmented worms, generally less than 10 cm (4 in) in length, although ranging at the extremes from 1 mm (0.04 in) to 3 m (10 ft), in Eunice aphroditois . They can sometimes be brightly coloured, and may be iridescent or even luminescent . Each segment bears

150-407: A carbon dioxide-enriched atmosphere, and share an enhanced capacity to produce endocardial infections, especially in young children. Fusobacterium is a pathogen. The laryngopharynx, ( Latin : pars laryngea pharyngis ), also known as hypopharynx , is the caudal part of the pharynx; it is the part of the throat that connects to the esophagus. It lies inferior to the epiglottis and extends to

200-451: A distinct deviation from this general design. At no point in larval growth stages does the metatrochophore take on the clearly segmented form of the typical nectochaete larva. The 15 segments of Chaetopterus are formed by subdivision of existing anlage . The Chaetopteridae have several genera with peculiar and well-studied filter-feeding mechanisms. The genera Chaetopterus , Mesochaetopterus , and Spiochaetopterus feed using

250-524: A few in terrestrial environments. They are extremely variable in both form and lifestyle, and include a few taxa that swim among the plankton or above the abyssal plain . Most burrow or build tubes in the sediment, and some live as commensals . A few species, roughly 80 (less than 0.5% of species), are parasitic. These include both ectoparasites and endoparasites . Ectoparasitic polychaetes feed on skin, blood, and other secretions, and some are adapted to bore through hard, usually calcerous surfaces, such as

300-425: A large buccal funnel . These larvae are often long lived and effectively disseminate , although are constrained geographically to their appropriate ranges for successful adult growth. The most common form of larval developmental plan for polychaetes is the trochophore larvae. The trochophore will add segments sequentially from a posterior growth zone to produce a nectochaete larva. Chaetopterus represents

350-557: A pair of gonads in every segment, but most species exhibit some degree of specialisation. The gonads shed immature gametes directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species, or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species copulate , but most fertilize their eggs externally. The fertilized eggs typically hatch into trochophore larvae, which float among

400-400: A pair of paddle-like and highly vascularized parapodia , which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called chaetae , project from the parapodia. However, polychaetes vary widely from this generalized pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along

450-421: A simple columnar epithelium covered by a thin cuticle . Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the body cavity . Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it

500-401: A thin mucus net suspended across the upper portion of their tube. The mucus net is secreted by a hooplike structure called the aliform notopodia arch. The net can grow at a rate as great as one millimeter per second as water currents generated by the notopodial fans pass plankton through the net. When the net grows large enough it contacts the ciliated cup, which rolls up the net. When

550-417: Is pharyngeal ( / ˌ f æ r ɪ n ˈ dʒ iː əl / or / f ə ˈ r ɪ n dʒ i əl / ). All vertebrates have a pharynx, used in both feeding and respiration. The pharynx arises during development in all vertebrates through a series of six or more outpocketings on the lateral sides of the head. These outpocketings are pharyngeal arches , and they give rise to a number of different structures in

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600-445: Is absent. Being soft-bodied organisms , the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts , and the mineralized tubes that some of them secrete. Most important biomineralising polychaetes are serpulids , sabellids , and cirratulids . Polychaete cuticle does have some preservation potential ; it tends to survive for at least 30 days after a polychaete's death. Although biomineralisation

650-400: Is also responsible for the ear-cough reflex in which stimulation of the ear canal results in a person coughing. The pharynx moves food from the mouth to the esophagus. It also moves air from the nasal and oral cavities to the larynx . It is also used in human speech, as pharyngeal consonants are articulated here, and it acts as a resonating chamber during phonation. Inflammation of

700-426: Is more continuous. The mouth of polychaetes is located on the peristomium , the segment behind the prostomium , and varies in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species,

750-426: Is noted. When the moray bites prey , it first bites normally with its oral jaws, capturing the prey. Immediately thereafter, the pharyngeal jaws are brought forward and bite down on the prey to grip it; they then retract, pulling the prey down the eel's esophagus, allowing it to be swallowed. Invertebrates also have a pharynx. Invertebrates with a pharynx include the tardigrades , annelids and arthropods , and

800-411: Is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed, and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water. A similar strategy is employed by

850-452: Is part of the digestive system and the conducting zone of the respiratory system . (The conducting zone—which also includes the nostrils of the nose , the larynx , trachea , bronchi , and bronchioles —filters, warms and moistens air and conducts it into the lungs ). The human pharynx is conventionally divided into three sections: the nasopharynx , oropharynx , and laryngopharynx . In humans, two sets of pharyngeal muscles form

900-425: Is relatively large, compared with that of other annelids, and lies in the upper part of the head. An endocrine gland is attached to the ventral posterior surface of the brain, and appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots, statocysts , and numerous additional sensory nerve endings, most likely involved with the sense of touch, also occur on

950-413: Is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the nonmineralised Burgess shale shows this need not always be the case. Their preservation potential is similar to that of jellyfish . Taxonomically, polychaetes are thought to be paraphyletic , meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from

1000-437: The coelomic fluid that fills their body cavities. The blood may be colourless, or have any of three different respiratory pigments. The most common of these is haemoglobin , but some groups have haemerythrin or the green-coloured chlorocruorin , instead. The nervous system consists of a single or double ventral nerve cord running the length of the body, with ganglia and a series of small nerves in each segment. The brain

1050-413: The lugworm ( Arenicola marina ) and the sandworm or clam worm Alitta . Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain , to forms which tolerate the extremely high temperatures near hydrothermal vents . Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to

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1100-490: The pharyngeal plexus and by the recurrent laryngeal nerve . The vascular supply to the laryngopharynx includes the superior thyroid artery , the lingual artery and the ascending pharyngeal artery . The primary neural supply is from both the vagus and glossopharyngeal nerves. The vagus nerve provides an auricular branch also termed "Arnold's nerve" which also supplies the external auditory canal, thus laryngopharyngeal cancer can result in referred ear pain . This nerve

1150-409: The plankton , and eventually metamorphose into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg. However, some polychaetes exhibit remarkable reproductive strategies. Some species reproduce by epitoky . For much of

1200-403: The 4th and 6th cervical vertebrae , the superior boundary of the laryngopharynx is at the level of the hyoid bone . The laryngopharynx includes three major sites: the pyriform sinus , postcricoid area, and the posterior pharyngeal wall. Like the oropharynx above it, the laryngopharynx serves as a passageway for food and air and is lined with a stratified squamous epithelium . It is innervated by

1250-469: The Alciopids' complex eyes which rival cephalopod and vertebrate eyes. Many species show bioluminescence ; eight families have luminous species. The head also includes a pair of antennae , tentacle-like palps , and a pair of pits lined with cilia , known as "nuchal organs". These latter appear to be chemoreceptors , and help the worm to seek out food. The outer surface of the body wall consists of

1300-425: The auditory tube , somewhat triangular in shape and bounded behind by a firm prominence, the torus tubarius or cushion, caused by the medial end of the cartilage of the tube that elevates the mucous membrane . Two folds arise from the cartilaginous opening: The oropharynx lies behind the oral cavity, extending from the uvula to the level of the hyoid bone . It opens anteriorly, through the isthmus faucium , into

1350-653: The body. Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish " chloragogen " tissue, similar to that found in oligochaetes , which appears to function in metabolism, in a similar fashion to that of the vertebrate liver . The cuticle is constructed from cross-linked fibres of collagen and may be 200 nm to 13 mm thick. Their jaws are formed from sclerotised collagen, and their setae from sclerotised chitin . Polychaetes are predominantly marine, but many species also live in freshwater, and

1400-700: The bottom, but others have adapted to many different ecological niches , including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism , and parasitism , requiring various modifications to their body structures. The head, or prostomium , is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision, including

1450-545: The deep sea worm Syllis ramosa , which lives inside a sponge . The rear ends of the worm develop into "stolons" containing the eggs or sperm; these stolons then become detached from the parent worm and rise to the sea surface, where fertilisation takes place. Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte , a rich, sedimentary deposit in Greenland tentatively dated to

1500-418: The developing pharynx of amphioxi and hemichordates . However, the vertebrate pharynx is unique in that it gives rise to endoskeletal support through the contribution of neural crest cells. Pharyngeal jaws are a "second set" of jaws contained within the pharynx of many species of fish, distinct from the primary (oral) jaws. Pharyngeal jaws have been studied in moray eels where their specific action

1550-412: The head for members of this family. Chaetopteridae larvae are the largest among the polychaete worms. The larvae will range in size from 0.4 mm to 2.5 mm (largest polychaete larvae reported having a maximum length of 12 mm; the late stage of an unknown phyllodocid species). Chaetopteridae larvae are barrel-like in form with one to two ciliated bands at the midsection. They also have

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1600-435: The late Atdabanian (early Cambrian ). The oldest found is Phragmochaeta canicularis . Many of the more famous Burgess Shale organisms, such as Canadia , may also have polychaete affinities. Wiwaxia , long interpreted as an annelid, is now considered to represent a mollusc. An even older fossil, Cloudina , dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus

1650-446: The layout presented here. As comparatively few polychaete taxa have been subject to cladistic analysis, some groups which are usually considered invalid today may eventually be reinstated. These divisions were shown to be mostly paraphyletic in recent years. Chaetopteridae See text The Chaetopteridae are a family of marine filter-feeding polychaete worms that live in vertical or U-shaped tubes in tunnels buried in

1700-423: The location where this common pathway diverges into the respiratory ( laryngeal ) and digestive ( esophageal ) pathways. At that point, the laryngopharynx is continuous with the esophagus posteriorly. The esophagus conducts food and fluids to the stomach; air enters the larynx anteriorly. During swallowing, food has the "right of way", and air passage temporarily stops. Corresponding roughly to the area located between

1750-400: The middle ear to the pharynx, opens into the nasopharynx at the pharyngeal opening of the auditory tube. The opening and closing of the auditory tubes serves to equalize the barometric pressure in the middle ear with that of the ambient atmosphere. The anterior aspect of the nasopharynx communicates through the choanae with the nasal cavities. On its lateral wall is the pharyngeal opening of

1800-459: The mouth as the free apex of the rostral epiglottis lies dorsal to the soft palate in a normal horse. In ruminants , the tonsils are a compact mass that points towards the lumen of the pharynx. Pharyngeal arches are characteristic features of vertebrates whose origin can be traced back through chordates to basal deuterostomes who also share endodermal outpocketings of the pharyngeal apparatus. Similar patterns of gene expression can be detected in

1850-432: The mouth, while in its lateral wall, between the palatoglossal arch and the palatopharyngeal arch , is the palatine tonsil . The anterior wall consists of the base of the tongue and the epiglottic vallecula ; the lateral wall is made up of the tonsil, tonsillar fossa, and tonsillar (faucial) pillars; the superior wall consists of the inferior surface of the soft palate and the uvula. Because both food and air pass through

1900-465: The parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in

1950-467: The pharyngeal tonsils, are lymphoid tissue structures located in the posterior wall of the nasopharynx. Waldeyer's tonsillar ring is an annular arrangement of lymphoid tissue in both the nasopharynx and oropharynx. The nasopharynx is lined by respiratory epithelium that is pseudostratified, columnar, and ciliated. Polyps or mucus can obstruct the nasopharynx, as can congestion due to an upper respiratory infection. The auditory tube , which connects

2000-407: The pharynx and determine the shape of its lumen . They are arranged as an inner layer of longitudinal muscles and an outer circular layer. The upper portion of the pharynx, the nasopharynx, extends from the base of the skull to the upper surface of the soft palate . It includes the space between the internal nares and the soft palate and lies above the oral cavity. The adenoids , also known as

2050-449: The pharynx is modified into a lengthy proboscis . The digestive tract is a simple tube, usually with a stomach part way along. The smallest species, and those adapted to burrowing, lack gills , breathing only through their body surfaces. Most other species have external gills, often associated with the parapodia. A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply

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2100-437: The pharynx, a flap of connective tissue called the epiglottis closes over the glottis when food is swallowed to prevent aspiration . The oropharynx is lined by non-keratinized squamous stratified epithelium. The HACEK organisms ( H aemophilus , A ctinobacillus actinomycetemcomitans , C ardiobacterium hominis , E ikenella corrodens , K ingella ) are part of the normal oropharyngeal flora, which grow slowly, prefer

2150-404: The pharynx, or pharyngitis , is the painful inflammation of the throat. Pharyngeal cancer is a cancer that originates in the neck and/or throat. Waldeyer's tonsillar ring is an anatomical term collectively describing the annular arrangement of lymphoid tissue in the pharynx. Waldeyer's ring circumscribes the naso- and oropharynx, with some of its tonsillar tissue located above and some below

2200-429: The polychaetes include the clitellates ( earthworms and leeches ), sipunculans , and echiurans . The Pogonophora and Vestimentifera were once considered separate phyla, but are now classified in the polychaete family Siboglinidae . Much of the classification below matches Rouse & Fauchald, 1998, although that paper does not apply ranks above family. Older classifications recognize many more (sub)orders than

2250-410: The roll becomes large the net is disconnected from the aliform notopodia and is rolled into a ball before the ciliated mid- dorsal groove transports it to the mouth. Molecular analysis suggests that this group is basal within the annelids, below the sipunculid worms. The World Register of Marine Species lists the following genera as being in the family: Pharynx In humans, the pharynx

2300-402: The sedimentary or hard substrate of marine environments. The worms are highly adapted to the hard tube they secrete. Inside the tube the animal is segmented and regionally specialized, with highly modified appendages on different segments for cutting the tunnel, feeding, or creating suction for the flow of water through the tube home. The modified segments for feeding are on the 12th segment from

2350-784: The shells of mollusks. These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals). The mobile forms ( Errantia ) tend to have well-developed sense organs and jaws, while the stationary forms ( Sedentaria ) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms . Underwater polychaetes have eversible mouthparts used to capture prey. A few groups have evolved to live in terrestrial environments, like Namanereidinae with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange. Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have

2400-447: The skeletal, muscular, and circulatory systems. The structure of the pharynx varies across the vertebrates. It differs in dogs, horses, and ruminants. In dogs, a single duct connects the nasopharynx to the nasal cavity. The tonsils are a compact mass that points away from the lumen of the pharynx. In the horse, the auditory tube opens into the guttural pouch and the tonsils are diffuse and raised slightly. Horses are unable to breathe through

2450-571: The soft palate (and to the back of the oral cavity). It is believed that Waldeyer's ring prevents the invasion of microorganisms from going into the air and food passages and this helps in the defense mechanism of the respiratory and alimentary systems. The word pharynx ( / ˈ f ær ɪ ŋ k s / ) is derived from the Greek φάρυγξ phárynx , meaning "throat". Its plural form is pharynges / f ə ˈ r ɪ n dʒ iː z / or pharynxes / ˈ f ær ɪ ŋ k s ə z / , and its adjective form

2500-409: The year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches, the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half, responsible for breeding, is known as the epitoke. Each of the epitoke segments

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