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112-911: Ascidiacea , commonly known as the ascidians or sea squirts , is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders . Ascidians are characterized by a tough outer "tunic" made of a polysaccharide . Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea (salps, doliolids and pyrosomes) and Appendicularia (larvaceans) swim freely like plankton , sea squirts are sessile animals after their larval phase: they then remain firmly attached to their substratum , such as rocks and shells. There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual

224-401: A lamina propria , and a thin layer of smooth muscle called the muscularis mucosa . Beneath the mucosa lies the submucosa , consisting of fibrous connective tissue . Meissner's plexus is in this layer interior to the oblique muscle layer. Outside of the submucosa lies the muscular layer. It consists of three layers of muscular fibres, with fibres lying at angles to each other. These are

336-479: 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

448-592: 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 "

560-471: A colony into subcolonies. Subsequent zooid replication can lead to coalescence and circulatory fusion of the subcolonies. Closely related colonies which are proximate to each other may also fuse if they coalesce and if they are histocompatible . Ascidians were among the first animals to be able to immunologically distinguish self from non-self as a mechanism to prevent unrelated colonies from fusing to them and parasitizing them. Sea squirt eggs are surrounded by

672-407: 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

784-419: A fibrous vitelline coat and a layer of follicle cells that produce sperm-attracting substances. In fertilization , the sperm passes through the follicle cells and binds to glycosides on the vitelline coat. The sperm's mitochondria are left behind as the sperm enters and drives through the coat; this translocation of the mitochondria might provide the necessary force for penetration. The sperm swims through

896-512: A fin along the tail in the larva. The larva also has a statocyst and a pigmented cup above the mouth, which opens into a pharynx lined with small clefts opening into a surrounding atrium. The mouth and anus are originally at opposite ends of the animal, with the mouth only moving to its final (posterior) position during metamorphosis. The larva selects and settles on appropriate surfaces using receptors sensitive to light, orientation to gravity, and tactile stimuli. When its anterior end touches

1008-419: 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

1120-410: A high triglyceride content remain in the stomach the longest. Since enzymes in the small intestine digest fats slowly, food can stay in the stomach for 6 hours or longer when the duodenum is processing fatty chyme. However, this is still a fraction of the 24 to 72 hours that full digestion typically takes from start to finish. Although the absorption in the human digestive system is mainly a function of

1232-439: 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

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1344-434: A long time, under the name of Acopa. Brusca et al . treat Ascidiacea as a monophyletic group including pelagic Thaliacea. Appendicularia Stolidobranchia ( ascidians ) Thaliacea 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 is said to be paraphyletic with respect to

1456-549: 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

1568-473: A mouth. The pharynx itself is ciliated and contains numerous perforations, or stigmata, arranged in a grid-like pattern around its circumference. The beating of the cilia sucks water through the siphon, and then through the stigmata. A long ciliated groove, or endostyle , runs along one side of the pharynx, and a projecting ridge along the other. The endostyle may be homologous with the thyroid gland of vertebrates, despite its differing function. The pharynx

1680-424: A new colony. In some, the pharyngeal part of the animal degenerates, and the abdomen breaks up into patches of germinal tissue, each combining parts of the epidermis, peritoneum, and digestive tract, and capable of growing into new individuals. In yet others, budding begins shortly after the larva has settled onto the substrate. In the family Didemnidae , for instance, the individual essentially splits into two, with

1792-424: 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

1904-430: A process called gastric emptying , rhythmic mixing waves force about 3 mL of chyme at a time through the pyloric sphincter and into the duodenum. Release of a greater amount of chyme at one time would overwhelm the capacity of the small intestine to handle it. The rest of the chyme is pushed back into the body of the stomach, where it continues mixing. This process is repeated when the next mixing waves force more chyme into

2016-434: A set of genes that encode proteins homologous to those employed in the repair of DNA interstrand crosslinks in humans. The exceptional filtering capability of adult sea squirts causes them to accumulate pollutants that may be toxic to embryos and larvae as well as impede enzyme function in adult tissues. This property has made some species sensitive indicators of pollution. Over the last few hundred years, most of

2128-409: A surface, papillae (small, finger-like nervous projections) secrete an adhesive for attachment. Adhesive secretion prompts an irreversible metamorphosis : various organs (such as the larval tail and fins) are lost while others rearrange to their adult positions, the pharynx enlarges, and organs called ampullae grow from the body to permanently attach the animal to the substratum. The siphons of

2240-672: Is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale ( Yunnan , South China ). There are also two enigmatic species from the Ediacaran period with some affinity to the ascidians – Ausia from the Nama Group of Namibia and Burykhia from the Onega Peninsula, White Sea of northern Russia . They are also recorded from Lower Jurassic (Bonet and Benveniste-Velasquez, 1971; Buge and Monniot, 1972) and

2352-442: Is a narrow tubular region, the proventriculus , lined by fundic glands, and connecting the true stomach to the crop . Beyond lies the powerful muscular gizzard , lined by pyloric glands, and, in some species, containing stones that the animal swallows to help grind up food. In insects , there is also a crop. The insect stomach is called the midgut . Information about the stomach in echinoderms or molluscs can be found under

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2464-404: Is a precursor to pepsin- the highly acidic environment converts the pepsinogen to pepsin), and neuroendocrine cells that secrete serotonin . Glands differ where the stomach meets the esophagus and near the pylorus. Near the gastroesophageal junction lie cardiac glands , which primarily secrete mucus. They are fewer in number than the other gastric glands and are more shallowly positioned in

2576-519: Is a term that is also used to describe the removal of the heart . A gastrectomy may be carried out because of gastric cancer or severe perforation of the stomach wall. Fundoplication is stomach surgery in which the fundus is wrapped around the lower esophagus and stitched into place. It is used to treat gastroesophageal reflux disease (GERD) . The word stomach is derived from Greek stomachos ( στόμαχος ), ultimately from stoma ( στόμα ) 'mouth'. Gastro- and gastric (meaning 'related to

2688-433: 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

2800-542: Is an acidic fluid containing hydrochloric acid and digestive enzymes. The glands contains a number of cells, with the function of the glands changing depending on their position within the stomach. Within the body and fundus of the stomach lie the fundic glands . In general, these glands are lined by column-shaped cells that secrete a protective layer of mucus and bicarbonate . Additional cells present include parietal cells that secrete hydrochloric acid and intrinsic factor , chief cells that secrete pepsinogen (this

2912-578: Is called a zooid ) forming large colonies. Sea squirts feed by taking in water through a tube, the oral siphon. The water enters the mouth and pharynx , flows through mucus-covered gill slits (also called pharyngeal stigmata ) into a water chamber called the atrium, then exits through the atrial siphon. Some authors now include the thaliaceans in Ascidiacea, making it monophyletic . Sea squirts are rounded or cylindrical animals ranging from about 0.5 to 10 cm (0.20 to 3.94 in) in size. One end of

3024-412: Is formed from a plate that rolls up to form a neural tube . The number of cells within the central nervous system is very small. The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates. Although there is no true brain, the largest ganglion

3136-525: Is from the celiac trunk , and venous drainage is by the portal venous system . Lymph from these organs is drained to the prevertebral celiac nodes at the origin of the celiac artery from the aorta . In the human digestive system , a bolus (a small rounded mass of chewed up food) enters the stomach through the esophagus via the lower esophageal sphincter . The stomach releases proteases (protein-digesting enzymes such as pepsin ), and hydrochloric acid , which kills or inhibits bacteria and provides

3248-447: Is in response to food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs to push food into the small intestine only when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food. Epidermal growth factor (EGF) results in cellular proliferation, differentiation, and survival. EGF is a low-molecular-weight polypeptide first purified from

3360-420: Is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates. Sea squirts lack special sense organs, although the body wall incorporates numerous individual receptors for touch, chemoreception , and

3472-435: Is not absolute allowing some self-fertilization. It was speculated that self-incompatibility evolved to avoid inbreeding depression, but that selfing ability was retained to allow reproduction at low population density. Botryllus schlosseri is a colonial tunicate able to reproduce both sexually and asexually. B. schlosseri is a sequential (protogynous) hermaphrodite, and in a colony, eggs are ovulated about two days before

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3584-434: Is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve . The stomach can also sense, independently of tongue and oral taste receptors, glucose , carbohydrates , proteins , and fats . This allows the brain to link nutritional value of foods to their tastes. This syndrome defines the association between thyroid disease and chronic gastritis, which

3696-629: 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

3808-548: 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

3920-482: 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 ,

4032-407: Is supplied by the right gastric artery inferiorly and the left gastric artery superiorly, which also supplies the cardiac region. The greater curvature is supplied by the right gastroepiploic artery inferiorly and the left gastroepiploic artery superiorly. The fundus of the stomach, and also the upper portion of the greater curvature, is supplied by the short gastric arteries , which arise from

4144-421: Is surrounded by an atrium, through which water is expelled through a second, usually smaller, siphon. Cords of connective tissue cross the atrium to maintain the general shape of the body. The outer body wall consists of connective tissue, muscle fibres, and a simple epithelium directly underlying the tunic. The pharynx forms the first part of the digestive system. The endostyle produces a supply of mucus which

4256-404: Is then passed into the rest of the pharynx by the beating of flagella along its margins. The mucus then flows in a sheet across the surface of the pharynx, trapping planktonic food particles as they pass through the stigmata, and is collected in the ridge on the dorsal surface. The ridge bears a groove along one side, which passes the collected food downwards and into the oesophageal opening at

4368-503: 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

4480-569: 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

4592-638: 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

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4704-516: The ecosystem of several natural sub-tidal areas by smothering native animal species. Sea squirts are the natural prey of many animals, including nudibranchs , flatworms , molluscs , rock crabs , sea stars , fish, birds, and sea otters . Some are also eaten by humans in many parts of the world, including Japan , Korea , Chile , and Europe (where they are sold under the name " sea violet "). As chemical defenses, many sea squirts intake and maintain an extremely high concentration of vanadium in

4816-414: The nervous system . Chyme from the stomach is slowly released into the duodenum through coordinated peristalsis and opening of the pyloric sphincter. The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by the various digestive hormones of the digestive system: Other than gastrin, these hormones all act to turn off the stomach action. This

4928-405: The pharynx , while the abdomen contains most of the other bodily organs, and the postabdomen contains the heart and gonads . In many sea squirts, the postabdomen, or even the entire abdomen, are absent, with their respective organs being located more anteriorly. As its name implies, the pharyngeal region is occupied mainly by the pharynx. The large buccal siphon opens into the pharynx, acting like

5040-473: The physiological and structural changes of development. The dramatic rearrangement of egg cytoplasm following fertilization, called ooplasmic segregation, determines the dorsoventral and anteroposterior axes of the embryo. There are at least three types of sea squirt egg cytoplasm : ectoplasm containing vesicles and fine particles, endoderm containing yolk platelets , and myoplasm containing pigment granules, mitochondria , and endoplasmic reticulum . In

5152-484: The tail of the pancreas , splenic artery , left kidney , left suprarenal gland , transverse colon and its mesocolon , and the left crus of diaphragm , and the left colic flexure . The term was introduced around 1896 by Philip Polson of the Catholic University School of Medicine, Dublin. However this was brought into disrepute by surgeon anatomist J Massey. The lesser curvature of the human stomach

5264-510: The Tertiary from France (Deflandre-Riguard, 1949, 1956; Durand, 1952; Deflandre and Deflandre-Rigaud, 1956; Bouche, 1962; Lezaud, 1966; Monniot and Buge, 1971; Varol and Houghton, 1996). Older (Triassic) records are ambiguous. From the Early Jurassic, the species Didemnum cassianum, Quadrifolium hesselboi, Palaeoquadrum ullmanni and other indet genera are recorded. The representatives of

5376-502: The Tunicata, as shown in the following cladogram . Aplousobranchia ( ascidians ) Appendicularia Stolidobranchia ( ascidians ) Phlebobranchia ( ascidians ) Thaliacea Recent studies have suggested an alternate phylogeny, placing Appendicularia as sister to the rest of Tunicata , and Thaliacea nested inside Ascidiacea. A grouping of Thaliacea and Ascidiacea to the exclusion of Appendicularia had already been suggested for

5488-409: The acidic pH of 2 for the proteases to work. Food is churned by the stomach through peristaltic muscular contractions of the wall – reducing the volume of the bolus, before looping around the fundus and the body of stomach as the boluses are converted into chyme (partially digested food). Chyme slowly passes through the pyloric sphincter and into the duodenum of the small intestine , where

5600-419: The animal hemisphere while the endoplasm ends up in the vegetal hemisphere. Ciona intestinalis is a hermaphrodite that releases sperm and eggs into the surrounding seawater almost simultaneously. It is self-sterile, and thus has been used for studies on the mechanism of self-incompatibility. Self/non-self-recognition molecules play a key role in the process of interaction between sperm and the vitelline coat of

5712-442: The animal's chordate affinities. One group though, the molgulid ascidians, have evolved tailless species on at least four separate occasions, and even direct development. A notochord is formed early in development and always consists of a row of exactly 40 cells. The nerve tube enlarges in the main body, and will eventually become the cerebral ganglion of the adult. The tunic develops early in embryonic life and extends to form

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5824-543: The atrial siphon. Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally. There are four different types of blood cell: lymphocytes , phagocytic amoebocytes, nephrocytes and morula cells . The nephrocytes collect waste material such as uric acid and accumulate it in renal vesicles close to

5936-403: The atrial siphons opening into it, although the buccal siphons all remain separate. A series of glands lie on the outer surface of the intestine, opening through collecting tubules into the stomach, although their precise function is unclear. The heart is a curved muscular tube lying in the postabdomen, or close to the stomach. Each end opens into a single vessel, one running to the endostyle, and

6048-413: The base of the pharynx. The esophagus runs downwards to a stomach in the abdomen, which secretes enzymes that digest the food. An intestine runs upwards from the stomach parallel to the oesophagus and eventually opens, through a short rectum and anus , into a cloaca just below the atrial siphon. In some highly developed colonial species, clusters of individuals may share a single cloaca, with all

6160-445: The basis for the adult gastrointestinal tract. The sac is surrounded by a network of vitelline arteries and veins . Over time, these arteries consolidate into the three main arteries that supply the developing gastrointestinal tract: the celiac artery , superior mesenteric artery , and inferior mesenteric artery . The areas supplied by these arteries are used to define the foregut , midgut , and hindgut . The surrounded sac becomes

6272-402: The blood, have a very low pH of the tunic due to acid in easily ruptured bladder cells, and (or) produce secondary metabolites harmful to predators and invaders. Some of these metabolites are toxic to cells and are of potential use in pharmaceuticals . Ascidians are soft-bodied animals, and for this reason, their fossil record is almost entirely lacking. The earliest reliable ascidians

6384-485: The body is always firmly fixed to rock, coral, or some similar solid surface. The lower surface is pitted or ridged, and in some species has root-like extensions that help the animal grip the surface. The body wall is covered by a smooth thick tunic, which is often quite rigid. The tunic consists of cellulose , along with proteins and calcium salts. Unlike the shells of molluscs, the tunic is composed of living tissue and often has its own blood supply. In some colonial species,

6496-400: The cardia followed by the fundus, the body and the pylorus. The cardia is defined as the region following the "z-line" of the gastroesophageal junction , the point at which the epithelium changes from stratified squamous to columnar . Near the cardia is the lower esophageal sphincter. The stomach bed refers to the structures upon which the stomach rests in mammals. These include

6608-403: The coincidental release of sperm from other individuals. A fertilized egg spends 12 hours to a few days developing into a free-swimming tadpole -like larva , which then takes no more than 36 hours to settle and metamorphose into a juvenile. As a general rule, the larva possesses a long tail, containing muscles, a hollow dorsal nerve tube and a notochord , both features clearly indicative of

6720-481: The contents of the stomach contained; the lower esophageal sphincter (found in the cardiac region), at the junction of the esophagus and stomach, and the pyloric sphincter at the junction of the stomach with the duodenum. The stomach is surrounded by parasympathetic (inhibitor) and sympathetic (stimulant) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior , superior and inferior , celiac and myenteric), which regulate both

6832-696: The detection of light. Almost all ascidians are hermaphrodites and conspicuous mature ascidians are sessile . The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. Broadly speaking, the ascidians can be divided into species which exist as independent animals (the solitary ascidians) and those which are interdependent (the colonial ascidians). Different species of ascidians can have markedly different reproductive strategies, with colonial forms having mixed modes of reproduction. Solitary ascidians release many eggs from their atrial siphons; external fertilization in seawater takes place with

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6944-439: The digesting and readsorbing ability, and lastly, similar ability to form iodotyrosines by peroxidase activity, where iodide acts as an electron donor in the presence of H 2 O 2 . In the following years, many researchers published reviews about this syndrome. A series of radiographs can be used to examine the stomach for various disorders. This will often include the use of a barium swallow . Another method of examination of

7056-408: The digestive tract. The morula cells help to form the tunic, and can often be found within the tunic substance itself. In some species, the morula cells possess pigmented reducing agents containing iron ( hemoglobin ), giving the blood a red colour, or vanadium ( hemovanadin ) giving it a green colour. In that case the cells are also referred to as vanadocytes . The ascidian central nervous system

7168-464: The dorsal mesentery thins and forms the greater omentum , which is attached to the greater curvature of the stomach. The ventral mesentery forms the lesser omentum, and is attached to the developing liver . In the adult, these connective structures of omentum and mesentery form the peritoneum , and act as an insulating and protective layer while also supplying organs with blood and lymph vessels as well as nerves. Arterial supply to all these structures

7280-411: The duodenum. Gastric emptying is regulated by both the stomach and the duodenum. The presence of chyme in the duodenum activates receptors that inhibit gastric secretion. This prevents additional chyme from being released by the stomach before the duodenum is ready to process it. The fundus stores both undigested food and gases that are released during the process of chemical digestion. Food may sit in

7392-494: The egg. It appears that self/non-self recognition in ascidians such as C. intestinalis is mechanistically similar to self-incompatibility systems in flowering plants. Self-incompatibility promotes out-crossing, and thus provides the adaptive advantage at each generation of masking deleterious recessive mutations (i.e. genetic complementation). Ciona savignyi is highly self-fertile. However, non-self sperm out-compete self-sperm in fertilization competition assays. Gamete recognition

7504-613: The embryo develops directly into the juvenile without developing a tailed larva. Colonial ascidians reproduce both asexually and sexually . Colonies can survive for decades. An ascidian colony consists of individual elements called zooids . Zooids within a colony are usually genetically identical and some have a shared circulation. Different colonial ascidian species produce sexually derived offspring by one of two dispersal strategies – colonial species are either broadcast spawners (long-range dispersal) or philopatric (very short-range dispersal). Broadcast spawners release sperm and ova into

7616-497: The esophagus opening directly into the intestine. These animals all consume diets that require little storage of food, no predigestion with gastric juices, or both. The gastric lining is usually divided into two regions, an anterior portion lined by fundic glands and a posterior portion lined with pyloric glands. Cardiac glands are unique to mammals , and even then are absent in a number of species. The distributions of these glands vary between species, and do not always correspond with

7728-450: 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

7840-411: 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 the named group, it

7952-429: The extraction of nutrients begins. Gastric juice in the stomach also contains pepsinogen . Hydrochloric acid activates this inactive form of enzyme into the active form, pepsin. Pepsin breaks down proteins into polypeptides . Within a few moments after food enters the stomach, mixing waves begin to occur at intervals of approximately 20 seconds. A mixing wave is a unique type of peristalsis that mixes and softens

8064-473: 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

8176-417: The first phase of ooplasmic segregation, the myoplasmic actin-filament network contracts to rapidly move the peripheral cytoplasm (including the myoplasm) to the vegetal pole , which marks the dorsal side of the embryo. In the second phase, the myoplasm moves to the subequatorial zone and extends into a crescent, which marks the future posterior of the embryo. The ectoplasm with the zygote nucleus ends up at

8288-418: The food with gastric juices to create chyme. The initial mixing waves are relatively gentle, but these are followed by more intense waves, starting at the body of the stomach and increasing in force as they reach the pylorus. The pylorus, which holds around 30 mL of chyme, acts as a filter, permitting only liquids and small food particles to pass through the mostly, but not fully, closed pyloric sphincter . In

8400-501: The fundus of the stomach for a while before being mixed with the chyme. While the food is in the fundus, the digestive activities of salivary amylase continue until the food begins mixing with the acidic chyme. Ultimately, mixing waves incorporate this food with the chyme, the acidity of which inactivates salivary amylase and activates lingual lipase . Lingual lipase then begins breaking down triglycerides into free fatty acids, and mono- and diglycerides. The breakdown of protein begins in

8512-559: The genus Cystodytes (family Polycitoridae) have been described from the Pliocene of France by Monniot (1970, 1971) and Deflandre-Rigaud (1956), and from Eocene of France by Monniot and Buge (1971), and lately from the Late Eocene of S Australia by Łukowiak (2012). The ascidians were on morphological evidence treated as sister to the Thaliacea and Appendicularia , but molecular evidence has suggested that ascidians could be polyphyletic within

8624-430: The growth of the colonies derived from their metamorphosis is significantly lower. These findings suggest that self-fertilization gives rise to inbreeding depression associated with developmental deficits that are likely caused by expression of deleterious recessive mutations. Many colonial sea squirts are also capable of asexual reproduction, although the means of doing so are highly variable between different families. In

8736-406: The inner oblique, middle circular, and outer longitudinal layers. The presence of the inner oblique layer is distinct from other parts of the gastrointestinal tract, which do not possess this layer. The stomach contains the thickest muscular layer consisting of three layers, thus maximum peristalsis occurs here. The outer longitudinal layer is responsible for moving the semi-digested food towards

8848-424: The intestines. In humans, many bariatric surgery procedures involve the stomach, in order to lose weight. A gastric band may be placed around the cardia area, which can adjust to limit intake. The anatomy of the stomach may be modified , or the stomach may be bypassed entirely . Surgical removal of the stomach is called a gastrectomy , and removal of the cardia area is a called a cardiectomy . "Cardiectomy"

8960-405: The island of Taiwan . Stomach The stomach is located between the esophagus and the small intestine . The pyloric sphincter controls the passage of partially digested food ( chyme ) from the stomach into the duodenum , the first and shortest part of the small intestine, where peristalsis takes over to move this through the rest of the intestines. In the human digestive system ,

9072-593: The juvenile ascidian become orientated to optimise current flow through the feeding apparatus. Sexual maturity can be reached in as little as a few weeks. Since the larva is more advanced than its adult, this type of metamorphosis is called 'retrogressive metamorphosis'. This feature is a landmark for the 'theory of retrogressive metamorphosis or ascidian larva theory'; the true chordates are hypothesized to have evolved from sexually mature larvae. Some ascidians, especially in Molgulidae family, have direct development in which

9184-605: 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

9296-738: The mouse submandibular gland, but since then found in many human tissues including the submandibular gland , and the parotid gland . Salivary EGF, which also seems to be regulated by dietary inorganic iodine , also plays an important physiological role in the maintenance of oro-esophageal and gastric tissue integrity. The biological effects of salivary EGF include healing of oral and gastroesophageal ulcers, inhibition of gastric acid secretion, stimulation of DNA synthesis, and mucosal protection from intraluminal injurious factors such as gastric acid, bile acids, pepsin, and trypsin and from physical, chemical, and bacterial agents. The human stomach has receptors responsive to sodium glutamate and this information

9408-414: The mucosa. There are two kinds - either simple tubular glands with short ducts or compound racemose resembling the duodenal Brunner's glands . Near the pylorus lie pyloric glands located in the antrum of the pylorus. They secrete mucus, as well as gastrin produced by their G cells . About 20,000 protein-coding genes are expressed in human cells and nearly 70% of these genes are expressed in

9520-416: The mucosa; pepsinogen and gastric lipase , expressed in gastric chief cells ; and a gastric ATPase and gastric intrinsic factor , expressed in parietal cells . In the early part of the development of the human embryo , the ventral part of the embryo abuts the yolk sac . During the third week of development, as the embryo grows, it begins to surround parts of the yolk sac. The enveloped portions form

9632-411: The normal stomach. Just over 150 of these genes are more specifically expressed in the stomach compared to other organs, with only some 20 genes being highly specific. The corresponding specific proteins expressed in stomach are mainly involved in creating a suitable environment for handling the digestion of food for uptake of nutrients. Highly stomach-specific proteins include gastrokine-1 expressed in

9744-427: The other to the dorsal surface of the pharynx. The vessels are connected by a series of sinuses, through which the blood flows. Additional sinuses run from that on the dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic. Nitrogenous waste , in the form of ammonia , is excreted directly from the blood through the walls of the pharynx, and expelled through

9856-417: The parent colony (often within meters). The combined effect of short sperm range and philopatric larval dispersal results in local population structures of closely related individuals/inbred colonies. Generations of colonies which are restricted in dispersal are thought to accumulate adaptions to local conditions, thereby providing advantages over newcomers. Trauma or predation often results in fragmentation of

9968-414: The peak of sperm emission. Thus self-fertilization is avoided, and cross-fertilization is favored. Although avoided, self-fertilization is still possible in B. schlosseri . Self-fertilized eggs develop with a substantially higher frequency of anomalies during cleavage than cross-fertilized eggs (23% vs. 1.6%). Also, a significantly lower percentage of larvae derived from self-fertilized eggs metamorphose, and

10080-457: The perivitelline space, finally reaching the egg plasma membrane and entering the egg. This prompts rapid modification of the vitelline coat, through processes such as the egg's release of glycosidase into the seawater, so no more sperm can bind and polyspermy is avoided. After fertilization, free calcium ions are released in the egg cytoplasm in waves, mostly from internal stores. The temporary large increase in calcium concentration prompts

10192-444: The pharynx growing a new digestive tract and the original digestive tract growing a new pharynx. Apurinic/apyrimidinic (AP) sites are a common form of DNA damage that inhibit DNA replication and transcription . AP endonuclease 1 (APEX1), an enzyme produced by C. intestinalis , is employed in the repair of AP sites during early embryonic development. Lack of such repair leads to abnormal development. C. intestinalis also has

10304-401: The philopatrically dispersed ascidians: sperm from a nearby colony (or from a zooid of the same colony) enter the atrial siphon and fertilization takes place within the atrium. Embryos are then brooded within the atrium where embryonic development takes place: this results in macroscopic tadpole-like larvae. When mature, these larvae exit the atrial siphon of the adult and then settle close to

10416-433: The primitive gut. Sections of this gut begin to differentiate into the organs of the gastrointestinal tract, and the esophagus, and stomach form from the foregut. As the stomach rotates during early development, the dorsal and ventral mesentery rotate with it; this rotation produces a space anterior to the expanding stomach called the greater sac, and a space posterior to the stomach called the lesser sac. After this rotation

10528-504: The pylorus of the stomach through muscular shortening. To the outside of the muscular layer lies a serosa , consisting of layers of connective tissue continuous with the peritoneum . Smooth mucosa along the inside of the lesser curvature forms a passageway - the gastric canal that fast-tracks liquids entering the stomach, to the pylorus. The mucosa lining the stomach is lined with gastric pits , which receive gastric juice , secreted by between 2 and 7 gastric glands . Gastric juice

10640-675: 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

10752-563: 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

10864-409: The same regions as in humans. Furthermore, in many non-human mammals, a portion of the stomach anterior to the cardiac glands is lined with epithelium essentially identical to that of the esophagus. Ruminants , in particular, have a complex stomach, the first three chambers of which are all lined with esophageal mucosa. In birds and crocodilians , the stomach is divided into two regions. Anteriorly

10976-475: The secretory activity of the stomach and the motor (motion) activity of its muscles. The stomach is distensible , and can normally expand to hold about one litre of food. In a newborn human baby the stomach will only be able to hold about 30 millilitres. The maximum stomach volume in adults is between 2 and 4 litres, although volumes of up to 15 litres have been observed in extreme circumstances. The human stomach can be divided into four sections, beginning at

11088-403: The simplest forms, the members of the colony are linked only by rootlike projections from their undersides known as stolons . Buds containing food storage cells can develop within the stolons and, when sufficiently separated from the 'parent', may grow into a new adult individual. In other species, the postabdomen can elongate and break up into a string of separate buds, which can eventually form

11200-449: The small intestine, some absorption of certain small molecules nevertheless does occur in the stomach through its lining. This includes: The parietal cells of the human stomach are responsible for producing intrinsic factor , which is necessary for the absorption of vitamin B12 . B12 is used in cellular metabolism and is necessary for the production of red blood cells , and the functioning of

11312-442: The splenic artery. The two sets of gastric lymph nodes drain the stomach. Like the other parts of the gastrointestinal wall , the human stomach wall from inner to outer, consists of a mucosa , submucosa , muscular layer , subserosa and serosa . The inner part of the stomach wall is the gastric mucosa a mucous membrane that forms the lining of the stomach. the membrane consists of an outer layer of columnar epithelium ,

11424-460: 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

11536-413: The stomach lies between the esophagus and the duodenum (the first part of the small intestine ). It is in the left upper quadrant of the abdominal cavity . The top of the stomach lies against the diaphragm . Lying behind the stomach is the pancreas . A large double fold of visceral peritoneum called the greater omentum hangs down from the greater curvature of the stomach. Two sphincters keep

11648-432: The stomach through the actions of hydrochloric acid, and the enzyme pepsin . The stomach can also produce gastric lipase , which can help digesting fat. The contents of the stomach are completely emptied into the duodenum within two to four hours after the meal is eaten. Different types of food take different amounts of time to process. Foods heavy in carbohydrates empty fastest, followed by high-protein foods. Meals with

11760-484: The stomach') are both derived from Greek gaster ( γαστήρ ) 'belly'. Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the esophageal and duodenal openings remain relatively constant. As a result, the organ always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys , hagfishes , chimaeras , lungfishes , and some teleost fish have no stomach at all, with

11872-414: The stomach, is the use of an endoscope . A gastric emptying study is considered the gold standard to assess the gastric emptying rate. A large number of studies have indicated that most cases of peptic ulcers , and gastritis , in humans are caused by Helicobacter pylori infection, and an association has been seen with the development of stomach cancer . A stomach rumble is actually noise from

11984-490: The thyroid cells, such as primitive gastroenteric cells, migrated and specialized in uptake of iodide and in storage and elaboration of iodine compounds during vertebrate evolution. In fact, the stomach and thyroid share iodine-concentrating ability and many morphological and functional similarities, such as cell polarity and apical microvilli, similar organ-specific antigens and associated autoimmune diseases, secretion of glycoproteins (thyroglobulin and mucin) and peptide hormones,

12096-453: The tunics of adjacent individuals are fused into a single structure. The upper surface of the animal, opposite to the part gripping the substratum, has two openings, or siphons. When removed from the water, the animal often violently expels water from these siphons, hence the common name of "sea squirt". The body itself can be divided into up to three regions, although these are not clearly distinct in most species. The pharyngeal region contains

12208-477: 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

12320-410: The water column and fertilization occurs near to the parent colonies. Some species are also viviparous. Resultant zygotes develop into microscopic larvae that may be carried great distances by oceanic currents. The larvae of sessile forms which survive eventually settle and complete maturation on the substratum- then they may bud asexually to form a colony of zooids. The picture is more complicated for

12432-456: The world's harbors have been invaded by non-native sea squirts that have been introduced by accident from the shipping industry. Several factors, including quick attainment of sexual maturity, tolerance of a wide range of environments, and a lack of predators , allow sea squirt populations to grow rapidly. Unwanted populations on docks , ship hulls, and farmed shellfish cause significant economic problems, and sea squirt invasions have disrupted

12544-420: Was first described in the 1960s. This term was coined also to indicate the presence of thyroid autoantibodies or autoimmune thyroid disease in patients with pernicious anemia, a late clinical stage of atrophic gastritis. In 1993, a more complete investigation on the stomach and thyroid was published, reporting that the thyroid is, embryogenetically and phylogenetically, derived from a primitive stomach, and that

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