Appetite is the desire to eat food, sometimes due to hunger. Appealing foods can stimulate appetite when hunger is absent, although appetite can be reduced by satiety. Appetite exists in all higher life-forms, serves to regulate adequate energy intake to maintain metabolic needs, it is regulated by a close interplay between adipose tissue and the brain. Appetite has a relationship with every individual's behavior. Appetitive behaviour known as approach behaviour, consummatory behaviours, are the only processes that involve energy intake, whereas all other behaviours affect the release of energy; when stressed, appetite levels may result in an increase of food intake. Decreased desire to eat is termed anorexia. Dysregulation of appetite contributes to anorexia nervosa, bulimia nervosa, cachexia and binge eating disorder. A limited or excessive appetite is not pathological. Abnormal appetite could be defined as eating habits causing malnutrition and related conditions such as obesity and its related problems.
Both genetic and environmental factors may regulate appetite, abnormalities in either may lead to abnormal appetite. Poor appetite may be a result of physical or psychological factors. Hyperphagia may be a result of hormonal imbalances, mental disorders and others. Dyspepsia known as indigestion, can affect appetite as one of its symptoms is feeling "overly full" soon after beginning a meal. Taste and smell or the lack thereof may effect appetite. Abnormal appetite may be linked to genetics on a chromosomal scale, shown by the 1950s discovery of Prader–Willi syndrome, a type of obesity caused by chromosome alterations. Additionally, anorexia nervosa and bulimia nervosa are more found in females than males – thus hinting at a possibility of a linkage to the X-chromosome. Dysregulation of appetite lies at the root of anorexia nervosa, bulimia nervosa, binge eating disorder. Anorexia nervosa is a mental disorder characterized as severe dietary restriction and intense fear of weight gain. Furthermore, persons with anorexia nervosa may exercise ritualistically.
Individuals who have anorexia have high levels of ghrelin, a hormone that stimulates appetite, so the body is trying to cause hunger, but the urge to eat is being suppressed by the person. Binge eating disorder is described as eating excessively between periodic time intervals; the risk for BED can be present in children and most manifests during adulthood. Studies suggest that the heritability of BED in adults is 50%. To bulimia some people may be involved in purging and binging, they might take purgatives. However, the person may still believe. Various hereditary forms of obesity have been traced to defects in hypothalamic signaling or are still awaiting characterization – Prader-Willi syndrome – in addition, decreased response to satiety may promote development of obesity, it has been found. Other than genetically-stimulated appetite abnormalities, there are physiological ones that do not require genes for activation. For example and leptin are released from the stomach and adipose cells into the blood stream.
Ghrelin stimulates feelings of hunger. Any changes in normal production levels of these two hormones can lead to obesity. Looking at leptin, the more cells present in a body, the more adipose tissues there are, thus, the more leptin would be produced; this overproduction of leptin will cause the hypothalamus to become resistant to leptin and so, although the adipose cells are producing leptin, the body will not understand that it should stop eating. This will produce a perpetual cycle for those. Eating issues such as "picky eating" affects about 25% of children, but among children with development disorders this number may be higher, which in some cases may be related to the sounds and tastes. Glycemic index has been thought to effect satiety. Mechanisms controlling appetite are a potential target for weight loss drugs. Appetite control mechanisms seem to counteract undereating, whereas they appear weak to control overeating. Early anorectics were phentermine. A more recent addition is sibutramine which increases serotonin and noradrenaline levels in the central nervous system, but had to be withdrawn from the market when it was shown to have an adverse cardiovascular risk profile.
The appetite suppressant rimonabant had to be withdrawn when it was linked with worsening depression and increased risk of suicide. Recent reports on recombinant PYY 3-36 suggest that this agent may contribute to weight loss by suppressing appetite. Given the epidemic proportions of obesity in the Western world and the fact that it is increasing in some poorer countries, observers expect developments in this area to snowball in the near future. Weight loss and loss of appetite is an effect of some diseases, a side effect of some drugs. Certain progestins such as medroxyprogesterone acetate and megestrol
Skin is the soft outer tissue covering of vertebrates with three main functions: protection and sensation. Other animal coverings, such as the arthropod exoskeleton, have different developmental origin and chemical composition; the adjective cutaneous means "of the skin". In mammals, the skin is an organ of the integumentary system made up of multiple layers of ectodermal tissue, guards the underlying muscles, bones and internal organs. Skin of a different nature exists in amphibians and birds. All mammals have some hair on their skin marine mammals like whales and porpoises which appear to be hairless; the skin is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogens and excessive water loss, its other functions are insulation, temperature regulation and the production of vitamin D folates. Damaged skin may heal by forming scar tissue; this is sometimes depigmented. The thickness of skin varies from location to location on an organism.
In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is the thickest skin on the body; the speed and quality of wound healing in skin is promoted by the reception of estrogen. Fur is dense hair. Fur augments the insulation the skin provides but can serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is hard and thick, can be processed to create leather. Reptiles and fish have hard protective scales on their skin for protection, birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier regarding the passage of chemicals via skin and is subject to osmosis and diffusive forces. For example, a frog sitting in an anesthetic solution would be sedated as the chemical diffuses through its skin. Amphibian skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.
Mammalian skin is composed of two primary layers: the epidermis, which provides waterproofing and serves as a barrier to infection. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing pathogens from entering, is a stratified squamous epithelium, composed of proliferating basal and differentiated suprabasal keratinocytes. Keratinocytes are the major cells, constituting 95% of the epidermis, while Merkel cells and Langerhans cells are present; the epidermis can be further subdivided into the following strata or layers: Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Stratum germinativum Keratinocytes in the stratum basale proliferate through mitosis and the daughter cells move up the strata changing shape and composition as they undergo multiple stages of cell differentiation to become anucleated. During that process, keratinocytes will become organized, forming cellular junctions between each other and secreting keratin proteins and lipids which contribute to the formation of an extracellular matrix and provide mechanical strength to the skin.
Keratinocytes from the stratum corneum are shed from the surface. The epidermis contains no blood vessels, cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis; the epidermis and dermis are separated by a thin sheet of fibers called the basement membrane, made through the action of both tissues. The basement membrane controls the traffic of the cells and molecules between the dermis and epidermis but serves, through the binding of a variety of cytokines and growth factors, as a reservoir for their controlled release during physiological remodeling or repair processes; the dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils and elastic fibers, embedded in hyaluronan and proteoglycans. Skin proteoglycans are varied and have specific locations.
For example, hyaluronan and decorin are present throughout the dermis and epidermis extracellular matrix, whereas biglycan and perlecan are only found in the epidermis. It harbors many mechanoreceptors that provide the sense of touch and heat through nociceptors and thermoreceptors, it contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as for the epidermis; the dermis is connected to the epidermis through a basement membrane and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, a deep thicker area known as the reticular region. The papillary region is composed of loose areolar connective tissue; this is named for its fingerlike projections called papillae. The papillae provide the dermis with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the tw
Phage therapy or viral phage therapy is the therapeutic use of bacteriophages to treat pathogenic bacterial infections. Phage therapy has many potential applications in human medicine as well as dentistry, veterinary science, agriculture. If the target host of a phage therapy treatment is not an animal, the term "biocontrol" is employed, rather than "phage therapy". Bacteriophages are much more specific than antibiotics, they are harmless not only to the host organism, but to other beneficial bacteria, such as the gut flora, reducing the chances of opportunistic infections. They have a high therapeutic index, that is, phage therapy would be expected to give rise to few side effects; because phages replicate in vivo, a smaller effective dose can be used. On the other hand, this specificity is a disadvantage: a phage will only kill a bacterium if it is a match to the specific strain. Phage mixtures are applied to improve the chances of success, or samples can be taken and appropriate phages identified and grown.
Phages tend to be more successful than antibiotics where there is a biofilm covered by a polysaccharide layer, which antibiotics cannot penetrate. In the West, no therapies are authorized for use on humans. Phages are being used therapeutically to treat bacterial infections that do not respond to conventional antibiotics in Russia and Georgia. There is a phage therapy unit in Wrocław, established 2005, the only such centre in a European Union country; the discovery of bacteriophages was reported by the Englishman Frederick Twort in 1915 and the French-Canadian Felix d'Hérelle in 1917. D'Hérelle said that the phages always appeared in the stools of Shigella dysentery patients shortly before they began to recover, he "quickly learned that bacteriophages are found wherever bacteria thrive: in sewers, in rivers that catch waste runoff from pipes, in the stools of convalescent patients". Phage therapy was recognized by many to be a key way forward for the eradication of pathogenic bacterial infections.
A Georgian, George Eliava, was making similar discoveries. He travelled to the Pasteur Institute in Paris where he met d'Hérelle, in 1923 he founded the Eliava Institute in Tbilisi, devoted to the development of phage therapy. Phage therapy is used in Russia and Poland. In Russia, extensive research and development soon began in this field. In the United States during the 1940s commercialization of phage therapy was undertaken by Eli Lilly and Company. While knowledge was being accumulated regarding the biology of phages and how to use phage cocktails early uses of phage therapy were unreliable. Since the early 20th century, research into the development of viable therapeutic antibiotics had been underway, by 1942 the antibiotic penicillin G had been purified and saw use during the Second World War; the drug proved to be extraordinarily effective in the treatment of injured Allied soldiers whose wounds had become infected. By 1944, large-scale production of Penicillin had been made possible, in 1945 it became publicly available in pharmacies.
Due to the drug's success, it was marketed in the U. S. and Europe, leading Western scientists to lose interest in further use and study of phage therapy for some time. Isolated from Western advances in antibiotic production in the 1940s, Russian scientists continued to develop successful phage therapy to treat the wounds of soldiers in field hospitals. During World War II, the Soviet Union used bacteriophages to treat many soldiers infected with various bacterial diseases e.g. dysentery and gangrene. Russian researchers continued to develop and to refine their treatments and to publish their research and results. However, due to the scientific barriers of the Cold War, this knowledge was not translated and did not proliferate across the world. A summary of these publications was published in English in 2009 in "A Literature Review of the Practical Application of Bacteriophage Research". There is an extensive library and research center at the George Eliava Institute in Tbilisi, Georgia. Phage therapy is today a widespread form of treatment in that region.
As a result of the development of antibiotic resistance since the 1950s and an advancement of scientific knowledge, there has been renewed interest worldwide in the ability of phage therapy to eradicate bacterial infections and chronic polymicrobial biofilm. Phages have been investigated as a potential means to eliminate pathogens like Campylobacter in raw food and Listeria in fresh food or to reduce food spoilage bacteria. In agricultural practice phages were used to fight pathogens like Campylobacter and Salmonella in farm animals and Vibrio pathogens in fish from aquaculture and Erwinia and Xanthomonas in plants of agricultural importance; the oldest use was, however, in human medicine. Phages have been used against diarrheal diseases caused by E. coli, Shigella or Vibrio and against wound infections caused by facultative pathogens of the skin like staphylococci and streptococci. The phage therapy approach has been applied to systemic and intracellular infections and the addition of non-replicating phage and isolated phage enzymes like lysins to the antimicrobial arsenal.
However, actual proof for the efficacy of these phage approaches in the field or the hospital is not available. Some of the interest in the West can be traced back to 1994, when Soothill demonstrated that the use of phages could improve the success of skin grafts by reducing the underlying Pseudomonas aeruginosa infection. Recent studies have provided add
In epidemiology, a disease vector is any agent who carries and transmits an infectious pathogen into another living organism. Arthropods form a major group of pathogen vectors with mosquitoes, sand flies, fleas and mites transmitting a huge number of pathogens. Many such vectors are haematophagous, which feed on blood at all stages of their lives; when the insects blood feed, the pathogen enters the blood stream of the host. This can happen in different ways; the Anopheles mosquito, a vector for malaria and various arthropod-borne-viruses, inserts its delicate mouthpart under the skin and feeds on its host's blood. The parasites the mosquito carries are located in its salivary glands. Therefore, the parasites are transmitted directly into the host's blood stream. Pool feeders such as the sand fly and black fly, vectors for pathogens causing leishmaniasis and onchocerciasis will chew a well in the host's skin, forming a small pool of blood from which they feed. Leishmania parasites infect the host through the saliva of the sand fly.
Onchocerca force their own way out of the insect's head into the pool of blood. Triatomine bugs are responsible for the transmission of a trypanosome, Trypanosoma cruzi, which causes Chagas Disease; the Triatomine bugs defecate during feeding and the excrement contains the parasites which are accidentally smeared into the open wound by the host responding to pain and irritation from the bite. Some plants and fungi act as vectors for various pathogens. For example, the big-vein disease of lettuce was long thought to be caused by a member of the fungal division Chytridiomycota, namely Olpidium brassicae. However, the disease was shown to be viral, it transpired that the virus was transmitted by the zoospores of the fungus and survived in the resting spores. Since many other fungi in the Chytridiomycota have been shown to vector plant viruses. Many plant pests that damage important crops depend on other plants weeds, to harbour or vector them. In the case of Puccinia graminis for example and related genera act as alternate hosts in a cycle of infection of grain.
More directly, when they twine from one plant to another, parasitic plants such as Cuscuta and Cassytha have been shown to convey phytoplasmal and viral diseases between plants. The World Health Organization states that control and prevention of vector-borne diseases are emphasizing "Integrated Vector Management", an approach that looks at the links between health and environment, optimizing benefits to both. In April 2014, WHO launched a campaign called “Small bite, big threat” to educate people about vector-borne illnesses. WHO issued reports indicating that vector-borne illnesses affect poor people people living in areas that do not have adequate levels of sanitation, drinking water and housing. Several articles, recent to early 2014, warn that human activities are spreading vector-borne zoonotic diseases. Several articles were published in the medical journal The Lancet, discuss how rapid changes in land use, trade globalization, "social upheaval" are causing a resurgence in zoonotic disease across the world.
Examples of vector-borne zoonotic diseases include: Lyme disease Plague West Nile virusMany factors affect the incidence of vector-borne diseases. These factors include animals hosting the disease and people. Airborne disease Asymptomatic carrier Fomite Globalization and disease Insect vectors of human pathogens Insect vectors of plant pathogens VectorBase: genomic database of invertebrate vectors of human pathogens List of diseases caused by insects Natural reservoir Waterborne disease 2007 Yap Islands Zika virus outbreak "Better environmental management for control of dengue"; the Health and Environment Linkages Initiative. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. "Division of Vector-Borne Diseases". Fort Collins, Colorado: Centers for Disease Control and Prevention. Retrieved 7 April 2014. "Issue Brief Series: Vector-borne Diseases". Healthy Environments for Children Alliance. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. "Malaria control: the power of integrated action".
The Health and Environment Linkages Initiative. Geneva, Switzerland: World Health Organization. Retrieved 7 April 2014. Pawan, J. L.. "Transmission of the Paralytic Rabies in Trinidad of the Vampire Bat: Desmodus rotundus murinus Wagner, 1840." Annual Tropical Medicine and Parasitol, 30, April 8, 1936:137–156. Pawan, J. L. "Rabies in the Vampire Bat of Trinidad with Special Reference to the Clinical Course and the Latency of Infection." Annals of Tropical Medicine and Parasitology. Vol. 30, No. 4. December 1936 Quammen, David. "Planet of the Ape. The New York Times. Retrieved 7 April 2014. "Vector-borne diseases". Articles about vector-borne disease. Vaccine News Daily. Chicago. WHO page on vector-borne diseasesBiological mosquito eradication in Monte Verde, Honduras
In evolutionary biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one". Parasites include protozoans such as the agents of malaria, sleeping sickness, amoebic dysentery. There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism, trophically transmitted parasitism, vector-transmitted parasitism and micropredation. Like predation, parasitism is a type of consumer-resource interaction, but unlike predators, with the exception of parasitoids, are much smaller than their hosts, do not kill them, live in or on their hosts for an extended period. Parasites of animals are specialised, reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, the malaria-causing Plasmodium species, fleas.
Parasites reduce host fitness by general or specialised pathology, from parasitic castration to modification of host behaviour. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate hosts to assist in their transmission from one definitive host to another. Although parasitism is unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, in some fungi, shading into being saprophytic. People have known about parasites such as roundworms and tapeworms since ancient Egypt and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations; these were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin".
In fiction, Bram Stoker's 1897 Gothic horror novel Dracula and its many adaptations featured a blood-drinking parasite. Ridley Scott's 1979 film Alien was one of many works of science fiction to feature a terrifying parasitic alien species. First used in English in 1539, the word parasite comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek παράσιτος, "one who eats at the table of another" and that from παρά, "beside, by" + σῖτος, "wheat", hence "food"; the related term parasitism appears in English from 1611. Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host. Unlike commensalism and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food; because parasites interact with other species, they can act as vectors of pathogens, causing disease. Predation is by definition not a symbiosis, as the interaction is brief, but the entomologist E. O. Wilson has characterised parasites as "predators that eat prey in units of less than one".
Within that scope are many possible strategies. Taxonomists classify parasites in a variety of overlapping schemes, based on their interactions with their hosts and on their life-cycles, which are sometimes complex. An obligate parasite depends on the host to complete its life cycle, while a facultative parasite does not. Parasite life-cycles involving only one host are called "direct". An endoparasite lives inside the host's body. Mesoparasites - like some copepods, for example - enter an opening in the host's body and remain embedded there; some parasites can be generalists, feeding on a wide range of hosts, but many parasites, the majority of protozoans and helminths that parasitise animals, are specialists and host-specific. An early basic, functional division of parasites distinguished macroparasites; these each had a mathematical model assigned in order to analyse the population movements of the host–parasite groupings. The microorganisms and viruses that can reproduce and complete their life cycle within the host are known as microparasites.
Macroparasites are the multicellular organisms that reproduce and complete their life cycle outside of the host or on the host's body. Much of the thinking on types of parasitism has focussed on terrestrial animal parasites of animals, such as helminths; those in other environments and with other hosts have analogous strategies. For example, the snubnosed eel is a facultative endoparasite that opportunistically burrows into and eats sick and dying fish. Plant-eating insects such as scale insects and caterpillars resemble ectoparasites, attacking much larger plants; as female scale-insects cannot move, they are obligate parasites, permanently attached to their hosts. There are six major parasitic strategies, namely parasitic castration, directly transmitted parasitism, trophically transmitted parasitism, vector-transmitted parasitism, parasitoid
A pet or companion animal is an animal kept for a person's company, entertainment, or as an act of compassion such as taking in and protecting a hungry stray cat, rather than as a working animal, livestock, or laboratory animal. Popular pets are noted for their attractive appearances and relatable personalities, or may just be accepted as they are because they need a home. Two of the most popular pets are cats; the technical term for a cat lover is an ailurophile, for a dog lover, a cynophile. Other animals kept include rabbits. Small pets may be grouped together as pocket pets, while the equine and bovine group include the largest companion animals. Pets provide their owners both emotional benefits. Walking a dog can provide both the human and the dog with exercise, fresh air, social interaction. Pets can give companionship to people who are living alone or elderly adults who do not have adequate social interaction with other people. There is a medically approved class of therapy animals dogs or cats, that are brought to visit confined humans, such as children in hospitals or elders in nursing homes.
Pet therapy utilizes trained animals and handlers to achieve specific physical, cognitive or emotional goals with patients. Some scholars and animal rights organizations have raised concerns over keeping pets because of the lack of autonomy and objectification of nonhuman animals. There are 86.4 million pet cats and 78.2 million pet dogs in the United States, a United States 2007–2008 survey showed that dog-owning households outnumbered those owning cats, but that the total number of pet cats was higher than that of dogs. The same was true for 2011. In 2013, pets outnumbered children four to one in the United States. For a small to medium-size dog, the total cost over a dog's lifetime is about $7,240 to $12,700. For an indoor cat, the total cost over a cat's lifetime is about $8,620 to $11,275. People most get pets for companionship, to protect a home or property, or because of the beauty or attractiveness of the animals; the most common reasons for not owning a pet are lack of time, lack of suitable housing, lack of ability to care for the pet when traveling.
According to the 2007-2008 Pet Owners survey: The latest survey done by Colin Siren of Ipsos Reid estimates that there are 7.9 million cats and 5.9 million dogs in Canada. The survey shows that 35% of Canadian households have a dog, while 38% have a cat, consistent with other surveys conducted around the world. In China, spending on domestic animals has grown from and estimated $3.12 billion in 2010 to $25 billion in 2018. The Chinese people own 51 million dogs and 41 million cats, with pet owners preferring to source pet food internationally. A 2007 survey by the University of Bristol found that 26% of UK households owned cats and 31% owned dogs, estimating total domestic populations of 10.3 million cats and 10.5 million dogs in 2006. The survey found that 47.2% of households with a cat had at least one person educated to degree level, compared with 38.4% of homes with dogs. According to a survey promoted by Italian family associations in 2009, it is estimated that there are 45 million pets in Italy.
This includes 7 million dogs, 7.5 million cats, 16 million fish, 12 million birds, 10 million snakes. Keeping animals as pets may be detrimental to their health if certain requirements are not met. An important issue is inappropriate feeding; the consumption of chocolate or grapes by dogs, for example, may prove fatal. Certain species of houseplants can prove toxic if consumed by pets. Examples include philodendrons and Easter lilies and poinsettias and aloe vera. Housepets dogs and cats in industrialized societies, are highly susceptible to obesity. Overweight pets have been shown to be at a higher risk of developing diabetes, liver problems, joint pain, kidney failure, cancer. Lack of exercise and high-caloric diets are considered to be the primary contributors to pet obesity, it is believed among the public, among many scientists, that pets bring mental and physical health benefits to their owners. A recent dissent comes from a 2017 RAND study, which found that at least in the case of children, having a pet per se failed to improve physical or mental health by a statistically significant amount.
Conducting long-term randomized trials to settle the issue would be costly or infeasible. Pets might have the ability to stimulate their caregivers, in particular the elderly, giving people someone to take care of, someone to exercise with, someone to help them heal from a physically or psychologically troubled past. Animal company can help people to preserve acceptable levels of happiness despite the presence of mood symptoms like anxiety or depression. Having a pet may help people achieve health goals, such as lowered blood pressure, or mental goals, such as decreased stress. Ther
In animal anatomy, the mouth known as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth; this cavity is known as the buccal cavity, from the Latin bucca. Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostomes and deuterostomes. In the first multicellular animals, there was no mouth or gut and food particles were engulfed by the cells on the exterior surface by a process known as endocytosis; the particles became enclosed in vacuoles into which enzymes were secreted and digestion took place intracellularly. The digestive products were diffused into other cells; this form of digestion is used nowadays by simple organisms such as Amoeba and Paramecium and by sponges which, despite their large size, have no mouth or gut and capture their food by endocytosis.
The vast majority of other multicellular organisms have a mouth and a gut, the lining of, continuous with the epithelial cells on the surface of the body. A few animals which live parasitically had guts but have secondarily lost these structures; the original gut of multicellular organisms consisted of a simple sac with a single opening, the mouth. Many modern invertebrates have such a system, food being ingested through the mouth broken down by enzymes secreted in the gut, the resulting particles engulfed by the other cells in the gut lining. Indigestible waste is ejected through the mouth. In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut. In deuterostomes, the blastopore becomes the anus while the gut tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth while the anus formed as an opening made by the other end of the gut.
More recent research, shows that in protostomes the edges of the slit-like blastopore close up in the middle, leaving openings at both ends that become the mouth and anus. Apart from sponges and placozoans all animals have an internal gut cavity, lined with gastrodermal cells. In less advanced invertebrates such as the sea anemone, the mouth acts as an anus. Circular muscles around the mouth are able to contract in order to open or close it. A fringe of tentacles thrusts food into the cavity and it can gape enough to accommodate large prey items. Food passes first into a pharynx and digestion occurs extracellularly in the gastrovascular cavity. Annelids have simple tube-like gets and the possession of an anus allows them to separate the digestion of their foodstuffs from the absorption of the nutrients. Many molluscs have a radula, used to scrape microscopic particles off surfaces. In invertebrates with hard exoskeletons, various mouthparts may be involved in feeding behaviour. Insects have a range of mouthparts suited to their mode of feeding.
These include mandibles and labium and can be modified into suitable appendages for chewing, piercing and sucking. Decapods have six pairs of mouth appendages, one pair of mandibles, two pairs of maxillae and three of maxillipeds. Sea urchins have a set of five sharp calcareous plates which are used as jaws and are known as Aristotle's lantern. In vertebrates, the first part of the digestive system is the buccal cavity known as the mouth; the buccal cavity of a fish is separated from the opercular cavity by the gills. Water flows in through passes over the gills and exits via the operculum or gill slits. Nearly all fish have jaws and may seize food with them but most feed by opening their jaws, expanding their pharynx and sucking in food items; the food may be held or chewed by teeth located in the jaws, on the roof of the mouth, on the pharynx or on the gill arches. Nearly all amphibians are carnivorous as adults. Many catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth where they hold the prey with their jaws.
They swallow their food whole without much chewing. They have many small hinged pedicellate teeth, the bases of which are attached to the jaws while the crowns break off at intervals and are replaced. Most amphibians have one or two rows of teeth in both jaws but some frogs lack teeth in the lower jaw. In many amphibians there are vomerine teeth attached to the bone in the roof of the mouth; the mouths of reptiles are similar to those of mammals. The crocodilians are the only reptiles to have teeth anchored in sockets in their jaws, they are able to replace each of their 80 teeth up to 50 times during their lives. Most reptiles are either carnivorous or insectivorous but turtles are herbivorous. Lacking teeth that are suitable for efficiently chewing of their food, turtles have gastroliths in their stomach to further grind the plant material. Snakes have a flexible lower jaw, the two halves of which are not rigidly attached, numerous other joints in their skull; these modifications allow them to open their mouths wide enough to swallow their prey whole if it is wider than they are.
Birds do not have teeth, macerating their food. Their beaks have a range of sizes and shapes according to their diet and are compose