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37-551: See text Palola is a genus of polychaetes belonging to the family Eunicidae . The genus has almost cosmopolitan distribution , except northernward regions. Species: This annelid -related article is a stub . You can help Misplaced Pages by expanding it . Polychaetes 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

74-527: 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

111-585: A mesentery. Peritoneal folds develop from the ventral and dorsal mesentery of the embryo. CT scan is a fast (15 seconds) and efficient way in visualising the peritoneal spaces. Although ultrasound is good at visualizing peritoneal collections and ascites, without ionising radiation, it does not provide a good overall assessment of all the peritoneal cavities. MRI scan is also increasingly used to visualise peritoneal diseases, but requires long scan time (30 to 45 minutes) and prone to motion artifacts due to respiration and peristalsis and chemical shift artifacts at

148-557: 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

185-440: 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

222-466: 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

259-421: 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

296-399: 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

333-478: Is different from the intraperitoneal space (located within the abdominal cavity but wrapped in peritoneum). The structures within the intraperitoneal space are called "intraperitoneal" (e.g., the stomach and intestines ), the structures in the abdominal cavity that are located behind the intraperitoneal space are called " retroperitoneal " (e.g., the kidneys ), and those structures below the intraperitoneal space are called "subperitoneal" or "infraperitoneal" (e.g.,

370-426: 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,

407-412: 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

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444-425: 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

481-433: 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

518-474: Is the high number of arteries and veins in the peritoneal cavity. Through the mechanism of diffusion , waste products are removed from the blood. Peritonitis is the inflammation of the peritoneum. It is more commonly associated to infection from a punctured organ of the abdominal cavity. It can also be provoked by the presence of fluids that produce chemical irritation, such as gastric acid or pancreatic juice . Peritonitis causes fever, tenderness, and pain in

555-419: The bladder ). The peritoneum is one continuous sheet, forming two layers and a potential space between them: the peritoneal cavity . The outer layer, the parietal peritoneum , is attached to the abdominal wall and the pelvic walls . The tunica vaginalis , the serous membrane covering the male testis , is derived from the vaginal process , an outpouching of the parietal peritoneum. The inner layer,

592-438: 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

629-785: The omental foramen . The mesentery is the part of the peritoneum through which most abdominal organs are attached to the abdominal wall and supplied with blood and lymph vessels and nerves. In addition, in the pelvic cavity there are several structures that are usually named not for the peritoneum, but for the areas defined by the peritoneal folds: The structures in the abdomen are classified as intraperitoneal, mesoperitoneal, retroperitoneal or infraperitoneal depending on whether they are covered with visceral peritoneum and whether they are attached by mesenteries (mensentery, mesocolon). Structures that are intraperitoneal are generally mobile, while those that are retroperitoneal are relatively fixed in their location. Some structures, such as

666-410: 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

703-413: The visceral peritoneum , is wrapped around the visceral organs, located inside the intraperitoneal space for protection. It is thinner than the parietal peritoneum. The mesentery is a double layer of visceral peritoneum that attaches to the gastrointestinal tract . There are often blood vessels, nerves, and other structures between these layers. The space between these two layers is technically outside of

740-469: 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

777-656: 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

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814-427: The bowel-mesentery interface. Those with peritoneal carcinomatosis, acute pancreatitis, and intraabdominal sepsis may not tolerate prolonged MRI scan. In one form of dialysis , called peritoneal dialysis , a glucose solution is sent through a tube into the peritoneal cavity. The fluid is left there for a prescribed amount of time to absorb waste products, and then removed through the tube. The reason for this effect

851-530: 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

888-647: 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

925-460: 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

962-428: The greater sac and the lesser sac. The peritoneal space in males is closed, while the peritoneal space in females is continuous with the extraperitoneal pelvis through openings of the fallopian tubes , the uterus , and the vagina . Peritoneal folds are omentums, mesenteries and ligaments ; they connect organs to each other or to the abdominal wall. There are two main regions of the peritoneal cavity, connected by

999-507: The kidneys, are "primarily retroperitoneal", while others such as the majority of the duodenum, are "secondarily retroperitoneal", meaning that structure developed intraperitoneally but lost its mesentery and thus became retroperitoneal. The peritoneum develops ultimately from the mesoderm of the trilaminar embryo . As the mesoderm differentiates, one region known as the lateral plate mesoderm splits to form two layers separated by an intraembryonic coelom . These two layers develop later into

1036-439: 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

1073-401: 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. Peritoneum The abdominal cavity (the space bounded by the vertebrae , abdominal muscles , diaphragm , and pelvic floor )

1110-465: 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

1147-458: 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 ,

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1184-401: The peritoneal sac, and thus not in the peritoneal cavity. The potential space between these two layers is the peritoneal cavity , filled with a small amount (about 50 mL) of slippery serous fluid that allows the two layers to slide freely over each other. The right paracolic gutter is continuous with the right and left subhepatic spaces. The epiploic foramen allows communication between

1221-449: 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

1258-429: 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

1295-790: 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

1332-423: The visceral and parietal layers found in all serous cavities , including the peritoneum. As an embryo develops, the various abdominal organs grow into the abdominal cavity from structures in the abdominal wall. In this process they become enveloped in a layer of peritoneum. The growing organs "take their blood vessels with them" from the abdominal wall, and these blood vessels become covered by peritoneum, forming

1369-409: 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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