126-916: 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
252-478: A biopsy may be taken. If cancer of the esophagus is being investigated, other methods, including a CT scan , may also be used. The word esophagus ( British English : oesophagus ), comes from the Greek : οἰσοφάγος ( oisophagos ) meaning gullet . It derives from two roots (eosin) to carry and ( phagos ) to eat. The use of the word oesophagus, has been documented in anatomical literature since at least
378-533: A bolus of food to enter. Peristaltic contractions of the esophageal muscle push the food down the esophagus. These rhythmic contractions occur both as a reflex response to food that is in the mouth, and also as a response to the sensation of food within the esophagus itself. Along with peristalsis, the lower esophageal sphincter relaxes. The stomach produces gastric acid , a strongly acidic mixture consisting of hydrochloric acid (HCl) and potassium and sodium salts to enable food digestion . Constriction of
504-462: A mucous membrane consisting of a tough stratified squamous epithelium without keratin , a smooth lamina propria , and a muscularis mucosae . The epithelium of the esophagus has a relatively rapid turnover and serves a protective function against the abrasive effects of food. In many animals, the epithelium contains a layer of keratin, representing a coarser diet. There are two types of glands, with mucus-secreting esophageal glands being found in
630-439: A whale , which generally measures less than 10 cm (4 in) in diameter, although in larger baleen whales it may be up to 25 cm (10 in) when fully distended. A structure with the same name is often found in invertebrates, including molluscs and arthropods , connecting the oral cavity with the stomach. In terms of the digestive system of snails and slugs , the mouth opens into an esophagus, which connects to
756-479: 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
882-592: 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 "
1008-471: 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
1134-407: 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
1260-423: A corrosive substance, or a solid object is swallowed, it is most likely to lodge and damage one of these four points. These constrictions arise from particular structures that compress the esophagus. These constrictions are: The esophagus is surrounded at the top and bottom by two muscular rings, known respectively as the upper esophageal sphincter and the lower esophageal sphincter. These sphincters act to close
1386-419: 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
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#17327729537461512-512: 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
1638-419: 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
1764-439: 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
1890-434: 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
2016-461: A lot. A bleeding varix may cause a person to vomit blood , or suffer shock . To deal with a ruptured varix, a band may be placed around the bleeding blood vessel, or a small amount of a clotting agent may be injected near the bleed. A surgeon may also try to use a small inflatable balloon to apply pressure to stop the wound. IV fluids and blood products may be given in order to prevent hypovolemia from excess blood loss. Several disorders affect
2142-560: A more inclusive clade, it often makes sense to study the paraphyletic group that remains without considering the larger clade. For example, the Neogene evolution of the Artiodactyla (even-toed ungulates, like deer, cows, pigs and hippopotamuses - Cervidae , Bovidae , Suidae and Hippopotamidae , the families that contain these various artiodactyls, are all monophyletic groups) has taken place in environments so different from that of
2268-473: 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
2394-424: 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
2520-421: A person has had reflux for many years. Other strictures may include esophageal webs (which can also be congenital) and damage to the esophagus by radiotherapy, corrosive ingestion, or eosinophilic esophagitis. A Schatzki ring is fibrosis at the gastroesophageal junction. Strictures may also develop in chronic anemia , and Plummer-Vinson syndrome. Two of the most common congenital malformations affecting
2646-567: A person is immunocompromised . As of 2021 the causes of some forms of esophagitis, such as eosinophilic esophagitis , are not well-characterized, but may include Th2 -mediated atopies or genetic factors. There appear to be correlations between eosinophilic esophagitis, asthma (itself with an eosinophilic component), eczema , and allergic rhinitis , though it is not clear whether these conditions contribute to eosinophilic esophagitis or vice versa, or if they are symptoms of mutual underlying factors. Esophagitis can cause painful swallowing and
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#17327729537462772-424: 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
2898-434: 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
3024-505: A sphincter but does not have a distinct thickening like other sphincters. The upper esophageal sphincter surrounds the upper part of the esophagus. It consists of skeletal muscle but is not under voluntary control . Opening of the upper esophageal sphincter is triggered by the swallowing reflex . The primary muscle of the upper esophageal sphincter is the cricopharyngeal part of the inferior pharyngeal constrictor . The lower esophageal sphincter, or gastroesophageal sphincter, surrounds
3150-409: 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
3276-703: 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
3402-451: Is a spasm of the esophagus that can be one cause of chest pain. Such referred pain to the wall of the upper chest is quite common in esophageal conditions. Sclerosis of the esophagus, such as with systemic sclerosis or in CREST syndrome may cause hardening of the walls of the esophagus and interfere with peristalsis. Esophageal strictures are usually benign and typically develop after
3528-445: Is able to digest enough food and water. As of 2014 , the prognosis for esophageal cancer is still poor, so palliative therapy may also be a focus of treatment. Esophageal varices are swollen twisted branches of the azygous vein in the lower third of the esophagus. These blood vessels anastomose (join up) with those of the portal vein when portal hypertension develops. These blood vessels are engorged more than normal, and in
3654-433: 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
3780-584: 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
3906-412: 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
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4032-400: Is initially striated but then becomes smooth muscle in the caudal third or so. In canines and ruminants , however, it is entirely striated to allow regurgitation to feed young (canines) or regurgitation to chew cud (ruminants). It is entirely smooth muscle in amphibians, reptiles and birds. Contrary to popular belief, an adult human body would not be able to pass through the esophagus of
4158-420: 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
4284-424: Is most common in developed countries in those with Barrett's esophagus, and occurs in the cuboidal cells. In its early stages, esophageal cancer may not have any symptoms at all. When severe, esophageal cancer may eventually cause obstruction of the esophagus, making swallowing of any solid foods very difficult and causing weight loss. The progress of the cancer is staged using a system that measures how far into
4410-435: 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
4536-629: 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
4662-548: 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
4788-482: 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 ,
4914-421: 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
5040-404: 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
5166-463: Is thought to be one of the main contributors to the development of esophageal cancer . There are two main types of cancer of the esophagus . Squamous cell carcinoma is a carcinoma that can occur in the squamous cells lining the esophagus. This type is much more common in China and Iran . The other main type is an adenocarcinoma that occurs in the glands or columnar tissue of the esophagus. This
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5292-432: Is usually treated by managing the cause of the esophagitis - such as managing reflux or treating infection. Prolonged esophagitis, particularly from gastric reflux, is one factor thought to play a role in the development of Barrett's esophagus . In this condition, there is metaplasia of the lining of the lower esophagus, which changes from stratified squamous epithelia to simple columnar epithelia . Barrett's esophagus
5418-430: Is visible as a zig-zag line. Most of the muscle is smooth muscle although striated muscle predominates in its upper third. It has two muscular rings or sphincters in its wall, one at the top and one at the bottom. The lower sphincter helps to prevent reflux of acidic stomach content. The esophagus has a rich blood supply and venous drainage. Its smooth muscle is innervated by involuntary nerves ( sympathetic nerves via
5544-514: The Cetacea (whales, dolphins, and porpoises) that the Artiodactyla are often studied in isolation even though the cetaceans are a descendant group. The prokaryote group is another example; it is paraphyletic because it is composed of two Domains (Eubacteria and Archaea) and excludes (the eukaryotes ). It is very useful because it has a clearly defined and significant distinction (absence of
5670-581: The ICN ) abandoned consideration of bacterial nomenclature in 1975; currently, prokaryotic nomenclature is regulated under the ICNB with a starting date of 1 January 1980 (in contrast to a 1753 start date under the ICBN/ICN). Among plants, dicotyledons (in the traditional sense) are paraphyletic because the group excludes monocotyledons . "Dicotyledon" has not been used as a botanic classification for decades, but
5796-660: The ants and bees . The sawflies ( Symphyta ) are similarly paraphyletic, forming all of the Hymenoptera except for the Apocrita, a clade deep within the sawfly tree. Crustaceans are not a clade because the Hexapoda (insects) are excluded. The modern clade that spans all of them is the Tetraconata . One of the goals of modern taxonomy over the past fifty years has been to eliminate paraphyletic "groups", such as
5922-516: 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
6048-428: The erector spinae muscles and the vertebral column . The lower esophagus lies behind the heart and curves in front of the thoracic aorta . From the bifurcation of the trachea downwards, the esophagus passes behind the right pulmonary artery , left main bronchus , and left atrium . At this point, it passes through the diaphragm. The thoracic duct , which drains the majority of the body's lymph , passes behind
6174-424: The heart or limbs . The other cases occur singly. An X-ray of swallowed barium may be used to reveal the size and shape of the esophagus, and the presence of any masses. The esophagus may also be imaged using a flexible camera inserted into the esophagus, in a procedure called an endoscopy . If an endoscopy is used on the stomach, the camera will also have to pass through the esophagus. During an endoscopy,
6300-437: The midgut , hindgut and foregut . The surrounded sac becomes the primitive gut. Sections of this gut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach , and intestines . The esophagus develops as part of the foregut tube. The innervation of the esophagus develops from the pharyngeal arches . Food is ingested through the mouth and when swallowed passes first into
6426-449: The nucleus ambiguus , whereas fibers that supply the smooth muscle and lower esophageal sphincter have bodies situated in the dorsal motor nucleus . The vagus nerve plays the primary role in initiating peristalsis . The sympathetic trunk has a sympathetic function. It may enhance the function of the vagus nerve, increasing peristalsis and glandular activity, and causing sphincter contraction. In addition, sympathetic activation may relax
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#17327729537466552-414: The pharynx and then into the esophagus. The esophagus is thus one of the first components of the digestive system and the gastrointestinal tract . After food passes through the esophagus, it enters the stomach. When food is being swallowed, the epiglottis moves backward to cover the larynx , preventing food from entering the trachea . At the same time, the upper esophageal sphincter relaxes, allowing
6678-529: The pharynx to the stomach . The esophagus is a fibromuscular tube, about 25 cm (10 in) long in adults, that travels behind the trachea and heart , passes through the diaphragm , and empties into the uppermost region of the stomach . During swallowing, the epiglottis tilts backwards to prevent food from going down the larynx and lungs. The word oesophagus is from Ancient Greek οἰσοφάγος (oisophágos), from οἴσω (oísō), future form of φέρω (phérō, "I carry") + ἔφαγον (éphagon, "I ate"). The wall of
6804-405: 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
6930-473: 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
7056-454: The submucosa and esophageal cardiac glands, similar to cardiac glands of the stomach, located in the lamina propria and most frequent in the terminal part of the organ. The mucus from the glands gives a good protection to the lining. The submucosa also contains the submucosal plexus , a network of nerve cells that is part of the enteric nervous system . The muscular layer of the esophagus has two types of muscle. The upper third of
7182-731: The sympathetic trunk and parasympathetic nerves via the vagus nerve ) and in addition voluntary nerves ( lower motor neurons ) which are carried in the vagus nerve to innervate its striated muscle. The esophagus passes through the thoracic cavity into the diaphragm into the stomach. The esophagus may be affected by gastric reflux , cancer , prominent dilated blood vessels called varices that can bleed heavily, tears , constrictions, and disorders of motility. Diseases may cause difficulty swallowing ( dysphagia ), painful swallowing ( odynophagia ), chest pain , or cause no symptoms at all. Clinical investigations include X-rays when swallowing barium sulfate , endoscopy , and CT scans . Surgically,
7308-525: The yolk sac . During the second week of embryological development, as the embryo grows, it begins to surround parts of the sac. The enveloped portions form the basis for the adult gastrointestinal tract. The sac is surrounded by a network of vitelline arteries . 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
7434-522: 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
7560-507: 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
7686-419: 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
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#17327729537467812-442: 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
7938-543: 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
8064-403: 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
8190-413: 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
8316-402: 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
8442-485: 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,
8568-442: The cardiac sphincter, connects the stomach to the esophagus. This sphincter is very well developed in horses. This and the oblique angle at which the esophagus connects to the stomach explains why horses cannot vomit . The esophagus is also the area of the digestive tract where horses may have the condition known as choke . The esophagus of snakes is remarkable for the distension it undergoes when swallowing prey. In most fish,
8694-403: 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
8820-419: The course of the esophagus. The upper and middle parts of the esophagus drain into the azygos and hemiazygos veins , and blood from the lower part drains into the left gastric vein . All these veins drain into the superior vena cava , with the exception of the left gastric vein, which is a branch of the portal vein . Lymphatically, the upper third of the esophagus drains into the deep cervical lymph nodes ,
8946-417: The deeper red of the gastric mucosa, and the mucosal transition can be seen as an irregular zig-zag line, which is often called the z-line. Histological examination reveals abrupt transition between the stratified squamous epithelium of the esophagus and the simple columnar epithelium of the stomach . Normally, the cardia of the stomach is immediately distal to the z-line and the z-line coincides with
9072-696: 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
9198-408: 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
9324-494: 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
9450-613: 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
9576-424: The esophageal mucosa. The esophagus is innervated by the vagus nerve and the cervical and thoracic sympathetic trunk . The vagus nerve has a parasympathetic function, supplying the muscles of the esophagus and stimulating glandular contraction. Two sets of nerve fibers travel in the vagus nerve to supply the muscles. The upper striated muscle, and upper esophageal sphincter, are supplied by neurons with bodies in
9702-407: The esophageal wall the cancer has invaded, how many lymph nodes are affected, and whether there are any metastases in different parts of the body. Esophageal cancer is often managed with radiotherapy, chemotherapy, and may also be managed by partial surgical removal of the esophagus . Inserting a stent into the esophagus, or inserting a nasogastric tube , may also be used to ensure that a person
9828-416: The esophagus and the upper esophageal sphincter receive blood from the inferior thyroid artery , the parts of the esophagus in the thorax from the bronchial arteries and branches directly from the thoracic aorta , and the lower parts of the esophagus and the lower esophageal sphincter receive blood from the left gastric artery and the left inferior phrenic artery . The venous drainage also differs along
9954-414: The esophagus are an esophageal atresia where the esophagus ends in a blind sac instead of connecting to the stomach; and an esophageal fistula – an abnormal connection between the esophagus and the trachea. Both of these conditions usually occur together. These are found in about 1 in 3500 births. Half of these cases may be part of a syndrome where other abnormalities are also present, particularly of
10080-443: The esophagus are described here. For a more complete list, see esophageal disease . Inflammation of the esophagus is known as esophagitis . Reflux of gastric acids from the stomach, infection, substances ingested (for example, corrosives ), some medications (such as bisphosphonates ), and food allergies can all lead to esophagitis. Esophageal candidiasis is an infection of the yeast Candida albicans that may occur when
10206-429: The esophagus contains striated muscle , the lower third contains smooth muscle , and the middle third contains a mixture of both. Muscle is arranged in two layers: one in which the muscle fibers run longitudinal to the esophagus, and the other in which the fibers encircle the esophagus. These are separated by the myenteric plexus , a tangled network of nerve fibers involved in the secretion of mucus and in peristalsis of
10332-415: The esophagus focused in the neck, and was conducted in dogs by Theodore Billroth in 1871. In 1877 Czerny carried out surgery in people. By 1908, an operation had been performed by Voeckler to remove the esophagus, and in 1933 the first surgical removal of parts of the lower esophagus, (to control esophageal cancer ), had been conducted. The Nissen fundoplication , in which the stomach is wrapped around
10458-413: The esophagus from the lumen outwards consists of mucosa , submucosa (connective tissue), layers of muscle fibers between layers of fibrous tissue , and an outer layer of connective tissue. The mucosa is a stratified squamous epithelium of around three layers of squamous cells, which contrasts to the single layer of columnar cells of the stomach. The transition between these two types of epithelium
10584-469: The esophagus generally starts around the level of the sixth cervical vertebra behind the cricoid cartilage of the trachea , enters the diaphragm at about the level of the tenth thoracic vertebra , and ends at the cardia of the stomach, at the level of the eleventh thoracic vertebra . The esophagus is usually about 25 cm (10 in) in length. Many blood vessels serve the esophagus, with blood supply varying along its course. The upper parts of
10710-408: The esophagus is difficult to access in part due to its position between critical organs and directly between the sternum and spinal column. The esophagus is one of the upper parts of the digestive system . There are taste buds on its upper part. It begins at the back of the mouth, passing downward through the rear part of the mediastinum , through the diaphragm , and into the stomach. In humans,
10836-436: The esophagus is extremely short, primarily due to the length of the pharynx (which is associated with the gills ). However, some fish, including lampreys , chimaeras , and lungfish , have no true stomach, so that the esophagus effectively runs from the pharynx directly to the intestine , and is therefore somewhat longer. In many vertebrates, the esophagus is lined by stratified squamous epithelium without glands. In fish,
10962-411: The esophagus is often lined with columnar epithelium , and in amphibians , sharks and rays , the esophageal epithelium is ciliated , helping to wash food along, in addition to the action of muscular peristalsis. In addition, in the bat Plecotus auritus , fish and some amphibians, glands secreting pepsinogen or hydrochloric acid have been found. The muscle of the esophagus in many mammals
11088-420: The esophagus when food is not being swallowed. The upper esophageal sphincter is an anatomical sphincter, which is formed by the lower portion of the inferior pharyngeal constrictor, also known as the cricopharyngeal sphincter due to its relation with cricoid cartilage of the larynx anteriorly. However, the lower esophageal sphincter is not an anatomical but rather a functional sphincter, meaning that it acts as
11214-472: The esophagus with less than 50 genes being highly specific. The corresponding esophagus-specific proteins are mainly involved in squamous differentiation such as keratins KRT13 , KRT4 and KRT6C . Other specific proteins that help lubricate the inner surface of esophagus are mucins such as MUC21 and MUC22. Many genes with elevated expression are also shared with skin and other organs that are composed of squamous epithelia . The main conditions affecting
11340-405: The esophagus, curving from lying behind the esophagus on the right in the lower part of the esophagus, to lying behind the esophagus on the left in the upper esophagus. The esophagus also lies in front of parts of the hemiazygos veins and the intercostal veins on the right side. The vagus nerve divides and covers the esophagus in a plexus . The esophagus has four points of constriction. When
11466-450: 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
11592-411: 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
11718-434: The extensive venous plexus that exists between this vein and other veins, if portal hypertension occurs, the direction of blood drainage in this vein may reverse, with blood draining from the portal venous system, through the plexus. Veins in the plexus may engorge and lead to varices. Esophageal varices often do not have symptoms until they rupture. A ruptured varix is considered a medical emergency because varices can bleed
11844-480: The fact that a monophyletic group includes organisms consisting of all the descendants of a unique common ancestor. By comparison, the term polyphyly , or polyphyletic , uses the Ancient Greek prefix πολύς ( polús ), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. Groups that include all the descendants of
11970-417: 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
12096-570: 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
12222-430: 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
12348-456: The island of Taiwan . Oesophagus The esophagus ( American English ), oesophagus ( British English ), or œsophagus ( archaic spelling ) ( see spelling difference ) all / iː ˈ s ɒ f ə ɡ ə s , ɪ -/ ; pl. : ((o)e)(œ)sophagi or ( (o)e)(œ)sophaguses ), colloquially known also as the food pipe , food tube , or gullet , is an organ in vertebrates through which food passes, aided by peristaltic contractions , from
12474-599: 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
12600-622: The literature, and provides the corresponding monophyletic taxa. The concept of paraphyly has also been applied to historical linguistics , where the methods of cladistics have found some utility in comparing languages. For instance, the Formosan languages form a paraphyletic group of the Austronesian languages because they consist of the nine branches of the Austronesian family that are not Malayo-Polynesian and are restricted to
12726-402: The lower esophageal sphincter to stimulate its function and control reflux , was first conducted by Rudolph Nissen in 1955. In tetrapods , the pharynx is much shorter, and the esophagus correspondingly longer, than in fish. In the majority of vertebrates, the esophagus is simply a connecting tube, but in some birds , which regurgitate components to feed their young, it is extended towards
12852-422: The lower end to form a crop for storing food before it enters the true stomach. In ruminants , animals with four chambered stomachs, a groove called the sulcus reticuli is often found in the esophagus, allowing milk to drain directly into the hind stomach, the abomasum . In the horse the esophagus is about 1.2 to 1.5 m (4 to 5 ft) in length, and carries food to the stomach. A muscular ring, called
12978-420: The lower part of the esophagus at the junction between the esophagus and the stomach. It is also called the cardiac sphincter or cardioesophageal sphincter, named from the adjacent part of the stomach, the cardia . Dysfunction of the gastroesophageal sphincter causes gastroesophageal reflux , which causes heartburn , and, if it happens often enough, can lead to gastroesophageal reflux disease , with damage of
13104-421: The middle into the superior and posterior mediastinal lymph nodes, and the lower esophagus into the gastric and celiac lymph nodes . This is similar to the lymphatic drainage of the abdominal structures that arise from the foregut , which all drain into the celiac nodes. The upper esophagus lies at the back of the mediastinum behind the trachea , adjoining along the tracheoesophageal stripe , and in front of
13230-477: The motility of food as it travels down the esophagus. This can cause difficult swallowing, called dysphagia , or painful swallowing, called odynophagia . Achalasia refers to a failure of the lower esophageal sphincter to relax properly, and generally develops later in life. This leads to progressive enlargement of the esophagus, and possibly eventual megaesophagus . A nutcracker esophagus refers to swallowing that can be extremely painful. Diffuse esophageal spasm
13356-428: The muscle wall and cause blood vessel constriction. Sensation along the esophagus is supplied by both nerves, with gross sensation being passed in the vagus nerve and pain passed up the sympathetic trunk. The gastroesophageal junction (also known as the esophagogastric junction) is the junction between the esophagus and the stomach, at the lower end of the esophagus. The pink color of the esophageal mucosa contrasts to
13482-427: 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
13608-417: 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
13734-414: 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
13860-457: 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
13986-444: 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
14112-401: 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
14238-696: The ranks of the ICZN Code , the two taxa are separate orders. Molecular studies, however, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetaceans the Artiodactyls are paraphyletic. The class Reptilia is paraphyletic because it excludes birds (class Aves ). Under a traditional classification, these two taxa are separate classes. However birds are sister taxon to
14364-567: The rise of cladistics. The prokaryotes (single-celled life forms without cell nuclei) are a paraphyletic grouping, because they exclude the eukaryotes , a descendant group. Bacteria and Archaea are prokaryotes, but archaea and eukaryotes share a common ancestor that is not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed by Edouard Chatton in 1937 and was generally accepted after being adopted by Roger Stanier and C.B. van Niel in 1962. The botanical code (the ICBN, now
14490-403: 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
14616-404: The smooth muscle of the esophagus. The outermost layer of the esophagus is the adventitia in most of its length, with the abdominal part being covered in serosa . This makes it distinct from many other structures in the gastrointestinal tract that only have a serosa. In early embryogenesis , the esophagus develops from the endodermal primitive gut tube . The ventral part of the embryo abuts
14742-460: 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
14868-415: The stomach. Because of torsion , which is the rotation of the main body of the animal during larval development, the esophagus usually passes around the stomach, and opens into its back, furthest from the mouth. In species that have undergone de-torsion, however, the esophagus may open into the anterior of the stomach, which is the reverse of the usual gastropod arrangement. There is an extensive rostrum at
14994-510: The time of Hippocrates , who noted that "the oesophagus ... receives the greatest amount of what we consume." Its existence in other animals and its relationship with the stomach was documented by the Roman naturalist Pliny the Elder (AD23–AD79), and the peristaltic contractions of the esophagus have been documented since at least the time of Galen . The first attempt at surgery on
15120-453: 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
15246-477: 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
15372-455: The upper and lower esophageal sphincters helps to prevent reflux (backflow) of gastric contents and acid into the esophagus, protecting the esophageal mucosa. The acute angle of His and the lower crura of the diaphragm also help this sphincteric action. About 20,000 protein-coding genes are expressed in human cells and nearly 70% of these genes are expressed in the normal esophagus. Some 250 of these genes are more specifically expressed in
15498-486: The upper limit of the gastric folds of the cardia; however, when the anatomy of the mucosa is distorted in Barrett's esophagus the true gastroesophageal junction can be identified by the upper limit of the gastric folds rather than the mucosal transition. The functional location of the lower oesophageal sphincter is generally situated about 3 cm ( 1 + 1 ⁄ 4 in) below the z-line. The human esophagus has
15624-410: 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
15750-456: 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
15876-423: The worst cases may partially obstruct the esophagus. These blood vessels develop as part of a collateral circulation that occurs to drain blood from the abdomen as a result of portal hypertension , usually as a result of liver diseases such as cirrhosis . This collateral circulation occurs because the lower part of the esophagus drains into the left gastric vein, which is a branch of the portal vein. Because of
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