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The energy value of meat rises with the fat content. Except for certain vitamins, the composition of human breastmilk is fairly constant, regardless of the diet of the mother. The gastrointestinal tract digestive tract , digestional tract , GI tract , GIT , gut , or alimentary canal is an organ system within humans and other animals which takes in food, digests it to extract and absorb energy and nutrients, and expels the remaining waste as feces. Various pathogens can cause gastroenteritis an inflammation of the stomach and small intestine. Patients with chronic pain usually have abnormal physiologic parameters, such as elevated heart and respiratory rates, increased blood pressure, and dilated pupils. Fuel, containing energy, is taken into the engine, where it is burned, and a portion of the energy released is used to move the pistons.
Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is ammonia , which is removed by the external and internal water currents. There are no respiratory organs, and both cell layers absorb oxygen from and expel carbon dioxide into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some Anthozoa have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth.
This improves respiration after feeding and allows these animals, which use the cavity as a hydrostatic skeleton , to control the water pressure in the cavity without expelling undigested food.
Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove toxic. The animals produce large quantities of antioxidants to neutralize the excess oxygen. All cnidarians can regenerate , allowing them to recover from injury and to reproduce asexually. Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells.
This enables corals to recover even after apparently being destroyed by predators. Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa jellyfish and Cubozoa box jellies a larva swims until it finds a good site, and then becomes a polyp.
This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adults have gonads in the gastroderm , and these release ova and sperm into the water in the breeding season.
This phenomenon of succession of differently organized generations one asexually reproducing, sessile polyp, followed by a free-swimming medusa or a sessile polyp that reproduces sexually  is sometimes called "alternation of asexual and sexual phases" or "metagenesis", but should not be confused with the alternation of generations as found in plants.
Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa. Hydrozoa have a variety of life cycles. Some have no polyp stages and some e.
In some species, the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae. Meanwhile, life cycle reversal, in which polyps are formed directly from medusae without the involvement of sexual reproduction process, was observed in both Hydrozoa Turritopsis dohrnii  and Laodicea undulata  and Scyphozoa Aurelia sp.
Anthozoa have no medusa stage at all and the polyps are responsible for sexual reproduction. Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the phase of the moon. These mass spawnings may produce hybrids , some of which can settle and form polyps, but it is not known how long these can survive.
In some species the ova release chemicals that attract sperm of the same species. The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere blastula and then a depression forms at one end gastrulation and eventually becomes the digestive cavity.
However, in cnidarians the depression forms at the end further from the yolk at the animal pole , while in bilaterians it forms at the other end vegetal pole. Anthozoan larvae either have large yolks or are capable of feeding on plankton , and some already have endosymbiotic algae that help to feed them.
Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and metamorphose into polyps. Instead, these species rely on their medusae to extend their ranges. All known cnidaria can reproduce asexually by various means, in addition to regenerating after being fragmented.
Hydrozoan polyps only bud, while the medusae of some hydrozoans can divide down the middle. Scyphozoan polyps can both bud and split down the middle. In addition to both of these methods, Anthozoa can split horizontally just above the base. Asexual reproduction makes the daughter cnidarian a clone of the adult. Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata , but increasing awareness of their differences caused them to be placed in separate phyla.
Modern cnidarians are generally classified into four main classes: Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and the parasitic Myxozoa and Polypodiozoa are now recognized as highly derived cnidarians rather than more closely related to the bilaterians.
Stauromedusae, small sessile cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa. The Myxozoa , microscopic parasites , were first classified as protozoans. Some researchers classify the extinct conulariids as cnidarians, while others propose that they form a completely separate phylum.
Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates.
Nevertheless, major cnidarian groups contain species that have escaped these limitations. Hydrozoans have a worldwide range: Stauromedusae , although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic waters. Prey of cnidarians ranges from plankton to animals several times larger than themselves. Coral reefs form some of the world's most productive ecosystems.
Common coral reef cnidarians include both Anthozoans hard corals, octocorals, anemones and Hydrozoans fire corals, lace corals. The endosymbiotic algae of many cnidarian species are very effective primary producers , in other words converters of inorganic chemicals into organic ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently.
In addition, reefs provide complex and varied habitats that support a wide range of other organisms. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding. Few fossils of cnidarians without mineralized skeletons are known from more recent rocks, except in lagerstätten that preserved soft-bodied animals. A few mineralized fossils that resemble corals have been found in rocks from the Cambrian period, and corals diversified in the Early Ordovician.
Hydrozoa Hydra , siphonophores , etc. It is difficult to reconstruct the early stages in the evolutionary "family tree" of animals using only morphology their shapes and structures , because the large differences between Porifera sponges , Cnidaria plus Ctenophora comb jellies , Placozoa and Bilateria all the more complex animals make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics , which groups organisms according to similarities and differences in their biochemistry , usually in their DNA or RNA.
It is now generally thought that the Calcarea sponges with calcium carbonate spicules are more closely related to Cnidaria, Ctenophora comb jellies and Bilateria all the more complex animals than they are to the other groups of sponges. In , it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer mesoglea in Ctenophora, mesoderm in Bilateria.
However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora.
This grouping of Cnidaria and Bilateria has been labelled " Planulozoa " because it suggests that the earliest Bilateria were similar to the planula larvae of Cnidaria. Within the Cnidaria, the Anthozoa sea anemones and corals are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were sessile polyps with no medusa stage.
However, it is unclear how the other groups acquired the medusa stage, since Hydrozoa form medusae by budding from the side of the polyp while the other Medusozoa do so by splitting them off from the tip of the polyp. The traditional grouping of Scyphozoa included the Staurozoa , but morphology and molecular phylogenetics indicate that Staurozoa are more closely related to Cubozoa box jellies than to other "Scyphozoa".
Similarities in the double body walls of Staurozoa and the extinct Conulariida suggest that they are closely related. The position of Anthozoa nearest the beginning of the cnidarian family tree also implies that Anthozoa are the cnidarians most closely related to Bilateria, and this is supported by the fact that Anthozoa and Bilateria share some genes that determine the main axes of the body.
However, in Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle , which in bilaterians arises from the mesoderm.
They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians. In molecular phylogenetics analyses from onwards, important groups of developmental genes show the same variety in cnidarians as in chordates. The mitochondrial genome in the medusozoan cnidarians, unlike those in other animals, is linear with fragmented genes.
Jellyfish stings killed about 1, people in the 20th century,  and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in East and Southeast Asia. Coral reefs have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism.
However, they are vulnerable to over-fishing, mining for construction materials, pollution , and damage caused by tourism. Beaches protected from tides and storms by coral reefs are often the best places for housing in tropical countries. Reefs are an important food source for low-technology fishing, both on the reefs themselves and in the adjacent seas. However, human activities damage reefs in several ways: Some large jellyfish species of the Rhizostomae order are commonly consumed in Japan , Korea and Southeast Asia.
Jellyfish is very low in cholesterol and sugars , but cheap preparation can introduce undesirable amounts of heavy metals. Most stingings by C. A number of Myxozoans are commercially important pathogens in salmonid aquaculture. From Wikipedia, the free encyclopedia. Sponge , Ctenophore , and Bilateria. Journal of Evolutionary Biology.
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Medical Journal of Australia. Annals of Emergency Medicine. Chlorophyta Streptophyta Chlorokybophyceae Mesostigmatophyceae Spirotaenia. Some small bilaterians have no anus and dispose of solid wastes by other means for example, through the mouth.
However, the complete human digestive system is made up of the gastrointestinal tract plus the accessory organs of digestion the tongue , salivary glands , pancreas , liver and gallbladder. The whole human GI tract is about nine metres 30 feet long at autopsy. It is considerably shorter in the living body because the intestines, which are tubes of smooth muscle tissue , maintain constant muscle tone in a halfway-tense state but can relax in spots to allow for local distention and peristalsis.
The gastrointestinal tract contains trillions of microbes , with some 4, different strains of bacteria having diverse roles in maintenance of immune health and metabolism. These digestive hormones , including gastrin , secretin , cholecystokinin , and ghrelin , are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.
The structure and function can be described both as gross anatomy and as microscopic anatomy or histology. The tract itself is divided into upper and lower tracts, and the intestines small and large parts.
The upper gastrointestinal tract consists of the mouth , pharynx , esophagus , stomach , and duodenum. This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either "upper" or "lower" origin. Upon dissection , the duodenum may appear to be a unified organ, but it is divided into four segments based upon function, location, and internal anatomy.
The four segments of the duodenum are as follows starting at the stomach, and moving toward the jejunum: The suspensory muscle attaches the superior border of the ascending duodenum to the diaphragm.
The suspensory muscle is an important anatomical landmark which shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.
The lower gastrointestinal tract includes most of the small intestine and all of the large intestine. In humans, the small intestine is further subdivided into the duodenum , jejunum and ileum while the large intestine is subdivided into the, cecum , ascending, transverse, descending and sigmoid colon , rectum , and anal canal. The small intestine begins at the duodenum and is a tubular structure, usually between 6 and 7 m long.
There are three major divisions:. The large intestine also called the colon, consists of the cecum , rectum , and anal canal. It also includes the appendix , which is attached to the cecum. The colon is further divided into:.
The main function of the large intestine is to absorb water. The area of the large intestinal mucosa of an adult human is about 2 m 2. The gut is an endoderm -derived structure. At approximately the sixteenth day of human development, the embryo begins to fold ventrally with the embryo's ventral surface becoming concave in two directions: The result is that a piece of the yolk sac , an endoderm -lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut.
The yolk sac remains connected to the gut tube via the vitelline duct. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum. During fetal life, the primitive gut is gradually patterned into three segments: Although these terms are often used in reference to segments of the primitive gut, they are also used regularly to describe regions of the definitive gut as well.
Each segment of the gut is further specified and gives rise to specific gut and gut-related structures in later development. Components derived from the gut proper, including the stomach and colon , develop as swellings or dilatations in the cells of the primitive gut.
In contrast, gut-related derivatives — that is, those structures that derive from the primitive gut but are not part of the gut proper, in general develop as out-pouchings of the primitive gut.
The blood vessels supplying these structures remain constant throughout development. The gastrointestinal tract has a form of general histology with some differences that reflect the specialization in functional anatomy.
The mucosa is the innermost layer of the gastrointestinal tract. The mucosa surrounds the lumen , or open space within the tube. This layer comes in direct contact with digested food chyme.
The mucosa is made up of:. The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium. The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa.
It contains the submucosal plexus , an enteric nervous plexus , situated on the inner surface of the muscularis externa. The muscular layer consists of an inner circular layer and a longitudinal outer layer. The circular layer prevents food from traveling backward and the longitudinal layer shortens the tract.
The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch. Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food.
The muscularis externa of the stomach is composed of the inner oblique layer, middle circular layer and outer longitudinal layer. Between the circular and longitudinal muscle layers is the myenteric plexus. Activity is initiated by the pacemaker cells, myenteric interstitial cells of Cajal. The gut has intrinsic peristaltic activity basal electrical rhythm due to its self-contained enteric nervous system. The rate can be modulated by the rest of the autonomic nervous system.
The coordinated contractions of these layers is called peristalsis and propels the food through the tract. Food in the GI tract is called a bolus ball of food from the mouth down to the stomach. After the stomach, the food is partially digested and semi-liquid, and is referred to as chyme.
In the large intestine the remaining semi-solid substance is referred to as faeces. The outermost layer of the gastrointestinal tract consists of several layers of connective tissue. Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach , first part of the duodenum , all of the small intestine , caecum and appendix , transverse colon , sigmoid colon and rectum. In these sections of the gut there is clear boundary between the gut and the surrounding tissue.
These parts of the tract have a mesentery. Retroperitoneal parts are covered with adventitia. They blend into the surrounding tissue and are fixed in position. For example, the retroperitoneal section of the duodenum usually passes through the transpyloric plane. These include the esophagus , pylorus of the stomach, distal duodenum , ascending colon , descending colon and anal canal. In addition, the oral cavity has adventitia. Specific proteins expressed in the stomach and duodenum involved in defence include mucin proteins, such as mucin 6 and intelectin Finally, transit through the colon takes 12 to 50 hours with wide variation between individuals.
The gastrointestinal tract forms an important part of the immune system. There are additional factors contributing to protection from pathogen invasion. For example, low pH ranging from 1 to 4 of the stomach is fatal for many microorganisms that enter it.
Beneficial bacteria also can contribute to the homeostasis of the gastrointestinal immune system. For example Clostridia , one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.
This is due to the production of short-chain fatty acids during the fermentation of plant-derived nutrients such as butyrate and propionate. Basically, the butyrate induces the differentiation of Treg cells by enhancing histone H3 acetylation in the promoter and conserved non-coding sequence regions of the FOXP3 locus, thus regulating the T cells , resulting in the reduction of the inflammatory response and allergies.
The large intestine hosts several kinds of bacteria that can deal with molecules that the human body cannot otherwise break down. These bacteria also account for the production of gases at host-pathogen interface , inside our intestine this gas is released as flatulence when eliminated through the anus. However the large intestine is mainly concerned with the absorption of water from digested material which is regulated by the hypothalamus and the re absorption of sodium , as well as any nutrients that may have escaped primary digestion in the ileum.
Health-enhancing intestinal bacteria of the gut flora serve to prevent the overgrowth of potentially harmful bacteria in the gut. These two types of bacteria compete for space and "food," as there are limited resources within the intestinal tract. Enzymes such as CYP3A4 , along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics. There are many diseases and conditions that can affect the gastrointestinal system, including infections , inflammation and cancer.
Various pathogens can cause gastroenteritis an inflammation of the stomach and small intestine. These can include those organisms that cause foodborne illnesses. Gastroenteritis is the most common disease of the GI tract. Diverticular disease is a condition that is very common in older people in industrialized countries. It usually affects the large intestine but has been known to affect the small intestine as well. Diverticulosis occurs when pouches form on the intestinal wall. Once the pouches become inflamed it is known as diverticulitis.
Inflammatory bowel disease is an inflammatory condition affecting the bowel walls, and includes the subtypes Crohn's disease and ulcerative colitis. While Crohn's can affect the entire gastrointestinal tract, ulcerative colitis is limited to the large intestine.
Crohn's disease is widely regarded as an autoimmune disease. Although ulcerative colitis is often treated as though it were an autoimmune disease, there is no consensus that it actually is such. Functional gastrointestinal disorders the most common of which is irritable bowel syndrome.
Functional constipation and chronic functional abdominal pain are other functional disorders of the intestine that have physiological causes, but do not have identifiable structural, chemical, or infectious pathologies.