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58-504: The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms . There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii . Two families— Harrimaniidae and Ptychoderidae —separated at least 370 million years ago. Until recently, it

116-402: A tornaria larva. These are very similar in appearance to the bipinnaria larvae of starfishes , with convoluted bands of cilia running around the body. Since the embryonic development of the blastula within the egg is also very similar to that of echinoderms , this suggests a close phylogenetic link between the two groups. After a number of days or weeks, a groove begins to form around

174-407: A collagenous tubular structure called a coenecium . The discovery of the stem group hemichordate Gyaltsenglossus shows that early hemichordates combined aspects of the two morphologically disparate classes. The body plan of hemichordates is characterized by a muscular organization. The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and

232-497: A depth of 10,000 ft. (3,050 m). The worms lie there with the proboscis sticking out of one opening in the burrow. Acorn worms are generally slow burrowers. To obtain food , many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms (this is known as deposit feeding). At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments (casts). Another method that some acorn worms use to obtain food

290-585: 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

348-399: A netrin that groups with netrin gene class 1 and 2. Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline. Hemichordata are divided into two classes:

406-462: A phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm. The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm. The Pterobranch genus most extensively studied

464-402: A primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes. Hemichordates have a nerve net and longitudinal nerves, but no brain. Some species biomineralize in calcium carbonate. Hemichordates have an open circulatory system . The heart vesicle is located dorsally within the proboscis complex, and does not contain any blood. Instead it moves

522-452: A similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm. Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of

580-445: A single blood vessel running underneath the digestive tract, from which smaller sinuses supply blood to the trunk, and back into the dorsal vessel. The blood of acorn worms is colourless and acellular. Acorn worms continually form new gill slits as they grow in size, with some older individuals of species like Balanoglossus aurantiacus having more than a hundred on each side. The microscopic species Meioglossus psammophilus has just

638-424: A single gill slit. The gills in some acorn worms have cartilaginous support structures. Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft and to the exterior through a dorso-lateral pore (see diagram below). Cilia push water through the slits, maintaining a constant flow. The tissues surrounding the slits are well supplied with blood sinuses. A plexus of nerves lies underneath

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696-520: Is Rhabdopleura from Plymouth, England and from Bermuda. The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava . Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly. P. flava’s early cleavage pattern

754-538: Is a phylum which consists of triploblastic , eucoelomate , and bilaterally symmetrical marine deuterostome animals , generally considered the sister group of the echinoderms . They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia . A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica . The class Graptolithina , formerly considered extinct,

812-481: Is assumed that the ancestors of acorn worms used to live in tubes like their relatives Pterobranchia , but that they eventually started to live a safer and more sheltered existence in sediment burrows instead. The body length normally range from 2 centimetres (0.79 in) to 2.5 metres (8 ft 2 in) ( Balanoglossus gigas ), but one species, Meioglossus psammophilus , only reach 0.6 millimetres (0.024 in). Due to secretions containing elements like iodine,

870-504: Is now placed within the pterobranchs, represented by a single living genus Rhabdopleura . Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest secreted tubes) and are deposit feeders, but some species are pharyngeal filter feeders , while the family are free living detritivores . Many are well known for their production and accumulation of various halogenated phenols and pyrroles . Pterobranchs are filter-feeders, mostly colonial, living in

928-407: Is perforated with gill slits (or pharyngeal slits), the oesophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile members of the acorn worm family Harrimaniidae . The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium. The mesosome extends into one pair (in

986-675: Is possible that the extinct organism Etacystis is a member of the Hemichordata, either within or with close affinity to the Pterobranchia. There are 130 described species of Hemichordata and many new species are being discovered, especially in the deep sea. A phylogenetic tree showing the position of the hemichordates is: Cephalochordata [REDACTED] Tunicata [REDACTED] Vertebrata / Craniata [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] The internal relationships within

1044-403: Is similar to that of S. kowalevskii . The first and second cleavages from the single cell zygote of P. flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in

1102-427: Is technically outside of 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

1160-474: 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

1218-401: Is to collect suspended particles of organic matter and microbes from the water. This is known as suspension feeding. Acorn worms are dioecious , having separate biological sexes, although at least some species are also capable of asexual reproduction in the form of fragmentation . They have paired gonads , which lie close to the pharynx and release the gametes through a small pore near to

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1276-655: The Enteropneusta , commonly called acorn worms, and the Pterobranchia , which includes the graptolites . A third class, Planctosphaeroidea , is proposed based on a single species known only from larvae. The phylum contains about 120 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic. It

1334-410: The blastula stage and goes on to gastrulation . The animal mesomeres of P. flava go on to give rise to the larva’s ectoderm , animal blastomeres also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues. Studies done on

1392-427: The deuterostomes , and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes . Studies have shown that the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system, they are controlling the development of the different body regions. The internal relationships within

1450-423: The kidneys ), and those structures below the intraperitoneal space are called "subperitoneal" or "infraperitoneal" (e.g., 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

1508-785: 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

1566-425: The vertebrae , abdominal muscles , diaphragm , and pelvic floor ) 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.,

1624-456: The Enteropneusta are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. Pterobranchia Stereobalanus Harrimaniidae Spengeliidae Torquaratoridae [REDACTED] Ptychoderidae Acorn worms are rarely seen by humans because of their lifestyle. They live in U-shaped burrows on the sea-bed, from the shoreline down to

1682-465: The animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred, the embryo has reached a 16 cell stage. P. flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and 4 larger macromeres. Further divisions occur until P. flava finishes

1740-412: The animals have an iodoform -like smell. Most acorn worms range from 9 to 45 centimetres (3.5 to 17.7 in) in length, with the largest species, Balanoglossus gigas , reaching 1.5 metres (5 ft) or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk. The creature's mouth is located at the collar behind

1798-602: The blood indirectly by pulsating against the dorsal blood vessel. Together with the echinoderms , the hemichordates form the Ambulacraria , which are the closest extant phylogenetic relatives of chordates . Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with a moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly. Hemichordates are

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1856-427: The blood system. Nonetheless, it does regularly pulsate, helping to push blood through the surrounding sinuses. From the central sinus in the collar, blood flows to a complex series of sinuses and peritoneal folds in the proboscis. This set of structures is referred to as a glomerulus and may have an excretory function, since acorn worms otherwise have no defined excretory system. From the proboscis, blood flows into

1914-413: The body during burrowing and crawling. Acorn worms have no eyes, ears or other special sense organs, except for the ciliary organ in front of the mouth, which appears to be involved in filter feeding and perhaps taste (3). There are, however, numerous nerve endings throughout the skin. Acorn worms have a Y-shaped nuchal skeleton that starts their proboscis and collar on their ventral side. The length of

1972-427: 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

2030-400: The cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in

2088-420: The ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii . Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii , such as

2146-401: The genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii , and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4 , which is homologous to Drosophila ’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on

2204-416: The genus Rhabdopleura ) or several pairs (in the genus Cephalodiscus ) of tentaculated arms used in filter feeding. The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of the colony, produced by asexual budding. In the genus Cephalodiscus , asexually produced individuals stay attached to the contractile stalk of

2262-445: The gill slits. The female lays a large number of eggs embedded in a gelatinous mass of mucus, which are then externally fertilized by the male before water currents break up the mass and disperse the individual eggs. In most species, the eggs hatch into planktonic larvae with elongated bodies covered in cilia. In some species, these develop directly into adults, but in others, there is a free-swimming intermediate stage referred to as

2320-428: 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

2378-415: The hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. Stereobalanus Harrimaniidae Spengeliidae Torquaratoridae Ptychoderidae [REDACTED] Cephalodiscida [REDACTED] Rhabdopleurida [REDACTED] † Dendroidea † Graptoloidea Peritoneum The abdominal cavity (the space bounded by

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2436-414: The horns of the nuchal skeleton varies between species. Acorn worms have a circulatory system with a heart that also functions as a kidney. Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish. Therefore, acorn worms are sometimes said to be a link between classical invertebrates and vertebrates . Some also have a postanal tail which may be homologous to

2494-507: 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

2552-487: The larval midsection, with the anterior portion eventually destined to become the proboscis, while the remainder forms the collar and trunk. The larvae eventually settle down and change into tiny adults to take on the burrowing lifestyle. A few species, such as Saccoglossus kowalevskii , lack even the planktonic larval stage, hatching directly as miniature adults. Hemichordata Hemichordata ( / ˌ h ɛ m ɪ k ɔːr ˈ d eɪ t ə / HEM -ih-kor- DAY -tə )

2610-507: The male testis , is derived from the vaginal process , an outpouching of the parietal peritoneum. The inner layer, 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

2668-423: The oesophagus connecting to the external surface, through which water from the food can be squeezed, helping to concentrate it. Digestion occurs in the intestine, with food material being pulled through by cilia, rather than by muscular action. Acorn worms breathe by drawing in oxygenated water through their mouth. The water then flows out the animal's gills which are on its trunk. Thus, the acorn worm breathes about

2726-477: The parent individual until completing their development. In the genus Rhabdopleura , zooids are permanently connected to the rest of the colony via a common stolon system. They have a diverticulum of the foregut called a stomochord , previously thought to be related to the chordate notochord , but this is most likely the result of convergent evolution rather than a homology . A hollow neural tube exists among some species (at least in early life), probably

2784-464: The post-anal tail of vertebrates. An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts. This means some of the original anatomy of the early chordate ancestors is still present even if it is not always visible. One theory is that the three-part body originates from an early common ancestor of all

2842-409: The posterior metasome. The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles. The mouth is located between the proboscis and the collar. The trunk is the longest part of the animal. It contains the pharynx, which

2900-410: The potential of the embryo at different stages have shown that at both the two and four cell stage of development P. flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage. Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to

2958-438: The proboscis. The skin is covered with cilia as well as glands that secrete mucus . Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms move only sluggishly, using ciliary action and peristalsis of the proboscis. Many acorn worms are detritus feeders , eating sand or mud and extracting organic detritus. Others feed on organic material suspended in

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3016-405: The proboscis. This diverticulum was once thought to be homologous with the notochord of chordates, hence the name "hemichordate" for the phylum. The mouth opens posteriorly into a pharynx with a row of gill slits along either side. The remainder of the digestive system consists of an oesophagus and intestine ; there is no stomach. In some families there are openings in the dorsal surface of

3074-416: The same way as fish. Acorn worms have an open circulatory system , in which the blood flows through the tissues sinuses . A dorsal blood vessel in the mesentery above the gut delivers blood to a sinus in the proboscis that contains a muscular sac acting as a heart . Unlike the hearts of most other animals, however, this structure is a closed fluid-filled vesicle whose interior does not connect directly to

3132-429: The skin, and is concentrated into both dorsal and ventral nerve cords. While the ventral cord runs only as far as the collar, the dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region. This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates. In acorn worms, it seems to be primarily involved with coordinating muscular action of

3190-441: The vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in

3248-423: 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

3306-442: The water, which they can draw into the mouth using the cilia on the gill bars. Research indicates that the rate of feeding of acorn worms that are detritus feeders is dependent on food availability and flow rate. A groove lined with cilia lies just in front of the mouth and directs suspended food into the mouth and may allow the animal to taste. The mouth cavity is tubular, with a narrow diverticulum or stomochord extending up into

3364-483: Was thought that all species lived in the sediment on the seabed , subsisting as deposit feeders or suspension feeders . However, the early 21st century has seen the description of a new family, the Torquaratoridae , evidently limited to the deep sea, in which most of the species crawl on the surface of the ocean bottom and alternatively rise into the water column, evidently to drift to new foraging sites. It

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