53-412: Nereididae (formerly spelled Nereidae ) are a family of polychaete worms. It contains about 500 – mostly marine – species grouped into 42 genera. They may be commonly called ragworms or clam worms . The prostomium of Nereididae bears a pair of palps that are differentiated into two units. The proximal unit is much larger than the distal unit. Parapodia are mostly biramous (only
106-479: A "single common ancestor" organism. Paraphyly is common in speciation , whereby a mother species (a paraspecies ) gives rise to a daughter species without itself becoming extinct. Research indicates as many as 20 percent of all animal species and between 20 and 50 percent of plant species are paraphyletic. Accounting for these facts, some taxonomists argue that paraphyly is a trait of nature that should be acknowledged at higher taxonomic levels. Cladists advocate
159-592: A cell nucleus, a plesiomorphy ) from its excluded descendants. Also, some systematists recognize paraphyletic groups as being involved in evolutionary transitions, the development of the first tetrapods from their ancestors for example. Any name given to these hypothetical ancestors to distinguish them from tetrapods—"fish", for example—necessarily picks out a paraphyletic group, because the descendant tetrapods are not included. Other systematists consider reification of paraphyletic groups to obscure inferred patterns of evolutionary history. The term " evolutionary grade "
212-407: A common ancestor are said to be monophyletic . A paraphyletic group is a monophyletic group from which one or more subsidiary clades (monophyletic groups) are excluded to form a separate group. Philosopher of science Marc Ereshefsky has argued that paraphyletic taxa are the result of anagenesis in the excluded group or groups. A cladistic approach normally does not grant paraphyletic assemblages
265-477: A distinct form to breed ( epitoky ). Ragworms are important food sources for a number of shore birds. Ragworms such as Hediste diversicolor are commonly used as bait in sea angling . They are a popular bait for all types of wrasse and pollock . They are also used as fish feed in aquaculture . Ragworms, such as Tylorrhynchus heterochetus , are considered a delicacy in Vietnam where they are used in
318-527: 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
371-419: A group of dinosaurs (part of Diapsida ), both of which are "reptiles". Osteichthyes , bony fish, are paraphyletic when circumscribed to include only Actinopterygii (ray-finned fish) and Sarcopterygii (lungfish, etc.), and to exclude tetrapods ; more recently, Osteichthyes is treated as a clade, including the tetrapods. The " wasps " are paraphyletic, consisting of the narrow-waisted Apocrita without
424-439: A kind of lizard). Put another way, viviparity is a synapomorphy for Theria within mammals, and an autapomorphy for Eulamprus tympanum (or perhaps a synapomorphy, if other Eulamprus species are also viviparous). Groupings based on independently-developed traits such as these examples of viviparity represent examples of polyphyly , not paraphyly. The following list recapitulates a number of paraphyletic groups proposed in
477-560: A more inclusive clade, it often makes sense to study the paraphyletic group that remains without considering the larger clade. For example, the Neogene evolution of the Artiodactyla (even-toed ungulates, like deer, cows, pigs and hippopotamuses - Cervidae , Bovidae , Suidae and Hippopotamidae , the families that contain these various artiodactyls, are all monophyletic groups) has taken place in environments so different from that of
530-557: 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
583-440: 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 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
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#1732798473018636-419: A pair of strong jaws on the distal portion and usually with conical teeth on one or more areas of both portions. Most genera have no gills (if present, they are usually branched and arise on mid-anterior segments of body). The larval body consists of four segments. Ragworms' teeth are made of a very tough, yet lightweight material. Unlike bone and tooth enamel , this is not mineralised with calcium , but
689-424: A phylogenetic species concept that does not consider species to exhibit the properties of monophyly or paraphyly, concepts under that perspective which apply only to groups of species. They consider Zander's extension of the "paraphyletic species" argument to higher taxa to represent a category error When the appearance of significant traits has led a subclade on an evolutionary path very divergent from that of
742-466: A polychaete's death. Although biomineralisation 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
795-421: 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
848-399: 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
901-433: Is allowed as a synonym of Magnoliopsida. Phylogenetic analysis indicates that the monocots are a development from a dicot ancestor. Excluding monocots from the dicots makes the latter a paraphyletic group. Among animals, several familiar groups are not, in fact, clades. The order Artiodactyla ( even-toed ungulates ) as traditionally defined is paraphyletic because it excludes Cetaceans (whales, dolphins, etc.). Under
954-1111: Is formed by a histidine rich protein, with bound zinc ions. Research on this material could lead to applications in engineering. Nereididae are currently considered a monophyletic taxon. Their closest neighbours in polychaete phylogenetic tree are Chrysopetalidae and Hesionidae (the superfamily Nereidoidea). Nereididae are divided into 42 genera, but the relationships between them are as yet unclear. The family contains traditionally three subfamilies - Namanereidinae, Gymnonereinae and Nereidinae. Subfamily Gymnonereidinae Banse, 1977 Subfamily Namanereidinae Hartman, 1959 Subfamily Nereidinae Blainville, 1818 Subfamily Nereididae incertae sedis : Ragworms are predominantly marine organisms that may occasionally swim upstream to rivers and even climb to land (for example Lycastopsis catarractarum ). They are commonly found in all water depths, foraging in seaweeds, hiding under rocks or burrowing in sand or mud. Ragworms are mainly omnivorous but many are active carnivores . Nereids breed only once before dying ( semelparity ), and most of them morph into
1007-426: 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,
1060-412: 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
1113-629: Is rather arbitrary, since the character states of common ancestors are inferences, not observations. These terms were developed during the debates of the 1960s and 1970s accompanying the rise of cladistics . Paraphyletic groupings are considered problematic by many taxonomists, as it is not possible to talk precisely about their phylogenetic relationships, their characteristic traits and literal extinction. Related terms are stem group , chronospecies , budding cladogenesis, anagenesis, or 'grade' groupings. Paraphyletic groups are often relics from outdated hypotheses of phylogenic relationships from before
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#17327984730181166-425: 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
1219-433: 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
1272-436: Is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping (a clade ) includes a common ancestor and all of its descendants. The terms are commonly used in phylogenetics (a subfield of biology ) and in the tree model of historical linguistics . Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies . If many subgroups are missing from
1325-482: Is sometimes used for paraphyletic groups. Moreover, the concepts of monophyly , paraphyly, and polyphyly have been used in deducing key genes for barcoding of diverse group of species. Current phylogenetic hypotheses of tetrapod relationships imply that viviparity , the production of offspring without the external laying of a fertilized egg, developed independently in the lineages that led to humans ( Homo sapiens ) and southern water skinks ( Eulampus tympanum ,
1378-514: The Cetacea (whales, dolphins, and porpoises) that the Artiodactyla are often studied in isolation even though the cetaceans are a descendant group. The prokaryote group is another example; it is paraphyletic because it is composed of two Domains (Eubacteria and Archaea) and excludes (the eukaryotes ). It is very useful because it has a clearly defined and significant distinction (absence of
1431-581: The ICN ) abandoned consideration of bacterial nomenclature in 1975; currently, prokaryotic nomenclature is regulated under the ICNB with a starting date of 1 January 1980 (in contrast to a 1753 start date under the ICBN/ICN). Among plants, dicotyledons (in the traditional sense) are paraphyletic because the group excludes monocotyledons . "Dicotyledon" has not been used as a botanic classification for decades, but
1484-660: The ants and bees . The sawflies ( Symphyta ) are similarly paraphyletic, forming all of the Hymenoptera except for the Apocrita, a clade deep within the sawfly tree. Crustaceans are not a clade because the Hexapoda (insects) are excluded. The modern clade that spans all of them is the Tetraconata . One of the goals of modern taxonomy over the past fifty years has been to eliminate paraphyletic "groups", such as
1537-438: 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
1590-410: 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
1643-474: 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
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1696-656: 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
1749-546: 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
1802-477: The dish chả rươi . In rice-growing areas of China, these worms are called 禾虫 (Mandarin: hé chóng, Cantonese: woh4 chuhng4). They are harvested from the rice fields and are often cooked with eggs. Polychaeta 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
1855-450: The examples given here, from formal classifications. Species have a special status in systematics as being an observable feature of nature itself and as the basic unit of classification. Some articulations of the phylogenetic species concept require species to be monophyletic, but paraphyletic species are common in nature, to the extent that they do not have a single common ancestor. Indeed, for sexually reproducing taxa, no species has
1908-658: 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 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
1961-464: 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 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
2014-480: The fact that a monophyletic group includes organisms consisting of all the descendants of a unique common ancestor. By comparison, the term polyphyly , or polyphyletic , uses the Ancient Greek prefix πολύς ( polús ), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. Groups that include all the descendants of
2067-584: The first two pairs are uniramous). Peristomium fused with the first body segment, with usually two pairs of tentacular cirri . The first body segment with 1-2 pairs tentacular cirri without aciculae . Compound setae are present. Notopodia are distinct (rarely reduced), usually with more flattened lobes, notosetae compound falcigers and/or spinigers (rarely notosetae absent). They have two prostomial antennae (absent in Micronereis ). Their pharynx , when everted, clearly consists of two portions, with
2120-439: 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
2173-462: 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. Paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping
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2226-622: The literature, and provides the corresponding monophyletic taxa. The concept of paraphyly has also been applied to historical linguistics , where the methods of cladistics have found some utility in comparing languages. For instance, the Formosan languages form a paraphyletic group of the Austronesian languages because they consist of the nine branches of the Austronesian family that are not Malayo-Polynesian and are restricted to
2279-568: The named group, it is said to be polyparaphyletic. The term received currency during the debates of the 1960s and 1970s accompanying the rise of cladistics , having been coined by zoologist Willi Hennig to apply to well-known taxa like Reptilia ( reptiles ), which is paraphyletic with respect to birds . Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor except for birds. Other commonly recognized paraphyletic groups include fish , monkeys , and lizards . The term paraphyly , or paraphyletic , derives from
2332-465: 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
2385-458: 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 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 ,
2438-449: 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
2491-429: 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
2544-696: The ranks of the ICZN Code , the two taxa are separate orders. Molecular studies, however, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetaceans the Artiodactyls are paraphyletic. The class Reptilia is paraphyletic because it excludes birds (class Aves ). Under a traditional classification, these two taxa are separate classes. However birds are sister taxon to
2597-567: The rise of cladistics. The prokaryotes (single-celled life forms without cell nuclei) are a paraphyletic grouping, because they exclude the eukaryotes , a descendant group. Bacteria and Archaea are prokaryotes, but archaea and eukaryotes share a common ancestor that is not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed by Edouard Chatton in 1937 and was generally accepted after being adopted by Roger Stanier and C.B. van Niel in 1962. The botanical code (the ICBN, now
2650-790: 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
2703-460: The status of "groups", nor does it reify them with explanations, as in cladistics they are not seen as the actual products of evolutionary events. A group whose identifying features evolved convergently in two or more lineages is polyphyletic (Greek πολύς [ polys ], "many"). More broadly, any taxon that is not paraphyletic or monophyletic can be called polyphyletic. Empirically, the distinction between polyphyletic groups and paraphyletic groups
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#17327984730182756-477: The two Ancient Greek words παρά ( pará ), meaning "beside, near", and φῦλον ( phûlon ), meaning "genus, species", and refers to the situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) are left apart from all other descendants of a unique common ancestor. Conversely, the term monophyly , or monophyletic , builds on the Ancient Greek prefix μόνος ( mónos ), meaning "alone, only, unique", and refers to
2809-409: 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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