Sombrero known as Hat Island, is part of the British overseas territory of Anguilla and is the northernmost island of the Lesser Antilles. It lies 54 km north-west of Anguilla across Prickly Pear Passage; the distance to Dog Island, the next nearest island of Anguilla, is 38 kilometres. Sombrero is 1.67 kilometres long north-south, 0.38 km wide. The land area is 0.38 km2. When viewed from the sea, the island had the shape of a sombrero hat but guano mining operations have left the island with precipitous sides and a flat top, 12 m above sea level; the surface of the island is rough, vegetation is sparse. The guano-mining operation yielded some 3000 tons of phosphate a year by 1870. By 1890, the phosphate reserves had been exhausted; the lighthouse marks the Anegada Passage, the route from Europe into the Caribbean. The first lighthouse was erected in 1868. In 1960, Hurricane Donna damaged this lighthouse, with the result that a new lighthouse replaced it in 1962; the lighthouses were manned from 1868 to 2001.
In that year, Trinity House donated and installed the current, automated tower, a 15 m round tower, painted white. Anguilla's Department of Fisheries and Marine Resources is responsible for the maintenance of the navigational aids; as a result of the Treaty of Utrecht in 1714, Sombrero passed into the hands of the British. Captain Warwick Lake of Recruit marooned an impressed seaman, Robert Jeffrey, there on 13 December 1807; as it turned out, Jeffrey survived. A passing American vessel, the schooner Adams from Marblehead, had rescued him. Still, a court martial dismissed Lake from the Royal Navy. In 1814, again in 1825, a British geologist surveyed the island and found that it abounded in guano and reported this to the British government. In 1856 the Americans claimed the island, in a short period of time quarried 100,000 tons of phosphate that served as fertilizer for the exhausted lands of the Southern States. Uniquely, an important insurrection occurred when West Indian black workers revolted against the “slavery proclivities” of a white American superintendent vis-à-vis wage-earning free men.
Four of the 200 workers "fatally injured" Superintendent Snow and commandeered the island and company money and stores. The British intervened and demanded compensation from the United States for the occupation; the conflicting claims to the island were settled in Britain's favour in 1867. Sombrero, lying in the route of shipping from England to South and Central America, lay in an area with many hazards and in 1848 the Admiralty was asked to install a light on it. On 30 June 1859, the Royal Mail Steam Packet Company's ship Paramatta was wrecked on her maiden voyage on Horseshoe Reef, which resulted in another request to the Admiralty; the lighthouse was built and first exhibited its light on the evening of 1 January 1868. Paramatta and the Lighthouse built following her demise were both constructed by the same shipyard, the Thames Ironworks and Shipbuilding Company, on the banks of the River Lea in Blackwall, London. Paramatta was the last ship launched from their Middlesex bank, while the lighthouse was built on the Essex side, the yard occupying premises on either side of the river.
In 1871, the lease of the island was sold for £55,000 and sold again for £110,000 to the New Sombrero Phosphate Company, which led to litigation in Erlanger v. New Sombrero Phosphate Co 3 App Cas 1218. From the early 1870s until 1885, a Cornish mining engineer, Thomas Corfield, was Superintendent of Sombrero, his duties included organizing the conveying of the guano to a spot, convenient for loading the lighters to take the guano to the ships lying off the island, overseeing the construction of derricks and engine houses, arranging for the laying of the tram lines for the wagons, which were loaded at the quarries. The guano was just piled in dumps near the engine derricks. There was no semblance of no beach; the black workers were recruited from various islands and lived in wooden huts during their term of service. Stores and various supplies were obtained from a merchant at Sint Maarten, a Mr Nesbit; the company's schooner Logos brought supplies and took the black labourers to and from their homes on the other islands.
The superintendent's house was a wooden bungalow near the middle of the island, around it were grouped the quarters of the technicians, store keepers, lighthouse keepers, other wooden buildings. On the side opposite to the main buildings was a small building for the superintendent. There was a wide veranda round the house and he used to live there with his family, except near the hurricane season. In 1890, the phosphate works on the island were abandoned and by 1893 the lighthouse had come under the authority of the British Board of Trade the Department of Transport. Administration of the light was carried out by Trinity House. In 1931, the old light system was changed and improved to 200,000 candle power and the tower received its first major repair when the basement was encased in concrete. On 20 July 1962, after the destruction caused by Hurricane Donna in 1960, the present lighthouse was put into operation and the old tower demolished on 28 July 1962; the lighthouse is located near the centre of the island, reaches a height of 166 feet above sea level.
It protects ships passing from the Atlantic Ocean to the Caribbean Sea through the Anegada Passage. Full responsibility for the light passed from Trinity House to the Anguillan government on 1 December 2001; until the only inhabitants were the staff of the lighthouse but the light was automated in 2002 and now the island is uninhabit
Washington, D. C. formally the District of Columbia and referred to as Washington or D. C. is the capital of the United States. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, first President of the United States and Founding Father; as the seat of the United States federal government and several international organizations, Washington is an important world political capital. The city is one of the most visited cities in the world, with more than 20 million tourists annually; the signing of the Residence Act on July 16, 1790, approved the creation of a capital district located along the Potomac River on the country's East Coast. The U. S. Constitution provided for a federal district under the exclusive jurisdiction of the U. S. Congress, the District is therefore not a part of any state; the states of Maryland and Virginia each donated land to form the federal district, which included the pre-existing settlements of Georgetown and Alexandria.
The City of Washington was founded in 1791 to serve as the new national capital. In 1846, Congress returned the land ceded by Virginia. Washington had an estimated population of 702,455 as of July 2018, making it the 20th most populous city in the United States. Commuters from the surrounding Maryland and Virginia suburbs raise the city's daytime population to more than one million during the workweek. Washington's metropolitan area, the country's sixth largest, had a 2017 estimated population of 6.2 million residents. All three branches of the U. S. federal government are centered in the District: Congress and the U. S. Supreme Court. Washington is home to many national monuments, museums situated on or around the National Mall; the city hosts 177 foreign embassies as well as the headquarters of many international organizations, trade unions, non-profit, lobbying groups, professional associations, including the World Bank Group, the International Monetary Fund, the Organization of American States, AARP, the National Geographic Society, the Human Rights Campaign, the International Finance Corporation, the American Red Cross.
A locally elected mayor and a 13‑member council have governed the District since 1973. However, Congress may overturn local laws. D. C. residents elect a non-voting, at-large congressional delegate to the House of Representatives, but the District has no representation in the Senate. The District receives three electoral votes in presidential elections as permitted by the Twenty-third Amendment to the United States Constitution, ratified in 1961. Various tribes of the Algonquian-speaking Piscataway people inhabited the lands around the Potomac River when Europeans first visited the area in the early 17th century. One group known as the Nacotchtank maintained settlements around the Anacostia River within the present-day District of Columbia. Conflicts with European colonists and neighboring tribes forced the relocation of the Piscataway people, some of whom established a new settlement in 1699 near Point of Rocks, Maryland. In his Federalist No. 43, published January 23, 1788, James Madison argued that the new federal government would need authority over a national capital to provide for its own maintenance and safety.
Five years earlier, a band of unpaid soldiers besieged Congress while its members were meeting in Philadelphia. Known as the Pennsylvania Mutiny of 1783, the event emphasized the need for the national government not to rely on any state for its own security. Article One, Section Eight, of the Constitution permits the establishment of a "District as may, by cession of particular states, the acceptance of Congress, become the seat of the government of the United States". However, the Constitution does not specify a location for the capital. In what is now known as the Compromise of 1790, Alexander Hamilton, Thomas Jefferson came to an agreement that the federal government would pay each state's remaining Revolutionary War debts in exchange for establishing the new national capital in the southern United States. On July 9, 1790, Congress passed the Residence Act, which approved the creation of a national capital on the Potomac River; the exact location was to be selected by President George Washington, who signed the bill into law on July 16.
Formed from land donated by the states of Maryland and Virginia, the initial shape of the federal district was a square measuring 10 miles on each side, totaling 100 square miles. Two pre-existing settlements were included in the territory: the port of Georgetown, founded in 1751, the city of Alexandria, founded in 1749. During 1791–92, Andrew Ellicott and several assistants, including a free African American astronomer named Benjamin Banneker, surveyed the borders of the federal district and placed boundary stones at every mile point. Many of the stones are still standing. A new federal city was constructed on the north bank of the Potomac, to the east of Georgetown. On September 9, 1791, the three commissioners overseeing the capital's construction named the city in honor of President Washington; the federal district was named Columbia, a poetic name for the United States in use at that time. Congress held its first session in Washington on November 17, 1800. Congress passed the District of Columbia Organic Act of 1801 that organized the District and placed the entire territory under the exclusive control of the federal
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
British Virgin Islands
The British Virgin Islands simply the Virgin Islands, are a British Overseas Territory in the Caribbean, to the east of Puerto Rico. The islands are geographically part of the Virgin Islands archipelago and are located in the Leeward Islands of the Lesser Antilles; the British Virgin Islands consist of the main islands of Tortola, Virgin Gorda and Jost Van Dyke, along with over 50 other smaller islands and cays. About 15 of the islands are inhabited; the capital, Road Town, is on Tortola, the largest island, about 20 km long and 5 km wide. The islands had a population of about 28,000 at the 2010 Census, of whom 23,500 lived on Tortola. For the islands, the latest United Nations estimate is 30,661. British Virgin Islanders are British Overseas Territories citizens and since 2002 are British citizens as well. Although the territory is not part of the European Union and not directly subject to EU law, British Virgin Islanders are deemed to be citizens of the EU by virtue of their British citizenship.
The official name of the territory is still the "Virgin Islands", but the prefix "British" is used. This is believed to distinguish it from the neighbouring American territory which changed its name from the "Danish West Indies" to "Virgin Islands of the United States" in 1917. However, local historians have disputed this, pointing to a variety of publications and public records dating from between 21 February 1857 and 12 September 1919 where the Territory is referred to as the British Virgin Islands. British Virgin Islands government publications continue to begin with the name "The territory of the Virgin Islands", the territory's passports refer to the "Virgin Islands", all laws begin with the words "Virgin Islands". Moreover, the territory's Constitutional Commission has expressed the view that "every effort should be made" to encourage the use of the name "Virgin Islands", but various public and quasi-public bodies continue to use the name "British Virgin Islands" or "BVI", including BVI Finance, BVI Electricity Corporation, BVI Tourist Board, BVI Athletic Association, BVI Bar Association and others.
In 1968 the British Government issued a memorandum requiring that the postage stamps in the territory should say "British Virgin Islands", a practice, still followed today. This was to prevent confusion following on from the adoption of US currency in the Territory in 1959, the references to US currency on the stamps of the Territory; the Virgin Islands were first settled by the Arawak from South America around 100 BC. The Arawaks inhabited the islands until the 15th century when they were displaced by the more aggressive Caribs, a tribe from the Lesser Antilles islands, after whom the Caribbean Sea is named; the first European sighting of the Virgin Islands was by the Spanish expedition of Christopher Columbus in 1493 on his second voyage to the Americas. Columbus gave them the fanciful name Santa Ursula y las Once Mil Vírgenes, shortened to Las Vírgenes, after the legend of Saint Ursula; the Spanish Empire claimed the islands by discovery in the early 16th century, but never settled them, subsequent years saw the English, French and Danish all jostling for control of the region, which became a notorious haunt for pirates.
There is no record of any native Amerindian population in the British Virgin Islands during this period, although most of the native population on nearby Saint Croix was killed or dispersed. The Dutch established a permanent settlement on the island of Tortola by 1648. In 1672, the English captured Tortola from the Dutch, the English annexation of Anegada and Virgin Gorda followed in 1680. Meanwhile, over the period 1672–1733, the Danish gained control of the nearby islands of Saint Thomas, Saint John and Saint Croix; the British islands were considered principally a strategic possession, but were planted when economic conditions were favourable. The British introduced sugar cane, to become the main crop and source of foreign trade, slaves were brought from Africa to work on the sugar cane plantations; the islands prospered economically until the middle of the nineteenth century, when a combination of the abolition of slavery in the territory, a series of disastrous hurricanes, the growth in the sugar beet crop in Europe and the United States reduced sugar cane production and led to a period of economic decline.
In 1917, the United States purchased St. John, St. Thomas, St. Croix from Denmark for US$25 million, renaming them the United States Virgin Islands; the British Virgin Islands were administered variously as part of the British Leeward Islands or with St. Kitts and Nevis, with an administrator representing the British Government on the islands; the islands gained separate colony status in 1960 and became autonomous in 1967. Since the 1960s, the islands have diversified away from their traditionally agriculture-based economy towards tourism and financial services, becoming one of the wealthiest areas in the Caribbean; the British Virgin Islands comprise around 60 tropical Caribbean islands, ranging in size from the largest, being 20 km long and 5 km wide, to tiny uninhabited islets, altogether about 150 square kilometres in extent. They are located in the Virgin Islands archipelago, a few miles east of the US Virgin Islands, about 95 km from the Puerto Rican mainland. About 150 km east south-east lies Anguilla.
The North Atlantic Ocean lies to the east of the islands, the Caribbean Sea lies to the
Lobsters are a family of large marine crustaceans. Lobsters have long bodies with muscular tails, live in crevices or burrows on the sea floor. Three of their five pairs of legs have claws, including the first pair, which are much larger than the others. Prized as seafood, lobsters are economically important, are one of the most profitable commodities in coastal areas they populate. Commercially important species include two species of Homarus from the northern Atlantic Ocean, scampi – the Northern Hemisphere genus Nephrops and the Southern Hemisphere genus Metanephrops. Although several other groups of crustaceans have the word "lobster" in their names, the unqualified term "lobster" refers to the clawed lobsters of the family Nephropidae. Clawed lobsters are not related to spiny lobsters or slipper lobsters, which have no claws, or to squat lobsters; the closest living relatives of clawed lobsters are the reef lobsters and the three families of freshwater crayfish. Lobsters are invertebrates with a hard protective exoskeleton.
Like most arthropods, lobsters must moult to grow. During the moulting process, several species change color. Lobsters have eight walking legs. Although lobsters are bilaterally symmetrical like most other arthropods, some genera possess unequal, specialized claws. Lobster anatomy includes two main body parts: the abdomen; the cephalothorax fuses the thorax, both of which are covered by a chitinous carapace. The lobster's head bears antennae, mandibles, the first and second maxillae; the head bears the compound eyes. Because lobsters live in murky environments at the bottom of the ocean, they use their antennae as sensors; the lobster eye has a reflective structure above a convex retina. In contrast, most complex eyes use a concave retina; the lobster's thorax is composed of maxillipeds, appendages that function as mouthparts, pereiopods, appendages that serve for walking and for gathering food. The abdomen includes pleopods, used for swimming as well as the tail fan, composed of uropods and the telson.
Lobsters, like snails and spiders, have blue blood due to the presence of hemocyanin, which contains copper. In contrast and many other animals have red blood from iron-rich hemoglobin. Lobsters possess a green hepatopancreas, called the tomalley by chefs, which functions as the animal's liver and pancreas. Lobsters of the family Nephropidae are similar in overall form to a number of other related groups, they differ from freshwater crayfish in lacking the joint between the last two segments of the thorax, they differ from the reef lobsters of the family Enoplometopidae in having full claws on the first three pairs of legs, rather than just one. The distinctions from fossil families such as the Chilenophoberidae are based on the pattern of grooves on the carapace. Lobsters are dark colored, either bluish green or greenish brown as to blend in with the ocean floor, but they can be found in a multitude of colors. Lobsters with atypical coloring are rare, accounting for only a few of the millions caught every year, due to their rarity, they aren't eaten, instead released back into the wild or donated to aquariums.
In cases of atypical coloring, there is a genetic factor, such as albinism or hermaphroditism. Notably, the New England Aquarium has a collection of such lobsters, called the Lobster Rainbow, on public display. Special coloring doesn't appear to have an effect on the lobster's taste once cooked. Lobsters live up to an estimated 45 to 50 years in the wild. In 2012, a report was published describing how growth bands in calcified regions of the eyestalk or gastric mill in shrimps and lobsters could be used to measure growth and mortality in decapod crustaceans. Without such a technique, a lobster's age is estimated by size and other variables. Research suggests that lobsters may not slow down, weaken or lose fertility with age, that older lobsters may be more fertile than younger lobsters; this longevity may be due to telomerase, an enzyme that repairs long repetitive sections of DNA sequences at the ends of chromosomes, referred to as telomeres. Telomerase is expressed by most vertebrates during embryonic stages, but is absent from adult stages of life.
However, unlike most vertebrates, lobsters express telomerase as adults through most tissue, suggested to be related to their longevity. Telomerase is present in'Green Spotted' lobsters - whose markings are thought to be produced by the enzyme interacting with their shell pigmentation Lobster longevity is limited by their size. Moulting requires metabolic energy and the larger the lobster, the more energy is needed. Lobsters, like many other decapod crustaceans, grow throughout life and are able to add new muscle cells at each moult. Lobster longevity allows them to reach impressive sizes. According to Guinness World Records, the largest lobster caught was in Nov
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They live in compact colonies of many identical individual polyps. Corals species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton. A coral "group" is a colony of myriad genetically identical polyps; each polyp is a sac-like animal only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals breed sexually by spawning: polyps of the same species release gametes over a period of one to several nights around a full moon. Although some corals are able to catch small fish and plankton using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates in the genus Symbiodinium that live within their tissues.
These are known as zooxanthellae. Such corals require sunlight and grow in clear, shallow water at depths less than 60 metres. Corals are major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not rely on zooxanthellae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,300 metres; some have been found on the Darwin Mounds, northwest of Cape Wrath and others as far north as off the coast of Washington State and the Aleutian Islands. Aristotle's pupil Theophrastus described the red coral, korallion, in his book on stones, implying it was a mineral, but he described it as a deep-sea plant in his Enquiries on Plants, where he mentions large stony plants that reveal bright flowers when under water in the Gulf of Heroes. Pliny the Elder stated boldly that several sea creatures including sea nettles and sponges "are neither animals nor plants, but are possessed of a third nature".
Petrus Gyllius copied Pliny, introducing the term zoophyta for this third group in his 1535 book On the French and Latin Names of the Fishes of the Marseilles Region. Gyllius further noted, following Aristotle, how hard it was to define what was a plant and what was an animal; the Persian polymath Al-Biruni classified sponges and corals as animals, arguing that they respond to touch. People believed corals to be plants until the eighteenth century, when William Herschel used a microscope to establish that coral had the characteristic thin cell membranes of an animal. Presently, corals are classified as certain species of animals within the sub-classes Hexacorallia and Octocorallia of the class Anthozoa in the phylum Cnidaria. Hexacorallia includes the stony corals and these groups have polyps that have a 6-fold symmetry. Octocorallia includes blue coral and soft corals and species of Octocorallia have polyps with an eightfold symmetry, each polyp having eight tentacles and eight mesenteries.
Fire corals are not true corals. Corals are sessile animals and differ from most other cnidarians in not having a medusa stage in their life cycle; the body unit of the animal is a polyp. Most corals are colonial, the initial polyp budding to produce another and the colony developing from this small start. In stony corals known as hard corals, the polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism; this is deposited by the coenosarc, the living tissue that connects them. The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are variable in appearance. In soft corals, there is no stony skeleton but the tissues are toughened by the presence of tiny skeletal elements known as sclerites, which are made from calcium carbonate. Soft corals are variable in form and most are colonial. A few soft corals are stolonate. In some species this is thick and the polyps are embedded; some soft corals are form lobes. Others have a central axial skeleton embedded in the tissue matrix.
This is composed either of a fibrous protein called gorgonin or of a calcified material. In both stony and soft corals, the polyps can be retracted, with stony corals relying on their hard skeleton and cnidocytes for defence against predators, soft corals relying on chemical defences in the form of toxic substances present in the tissues known as terpenoids; the polyps of stony corals have six-fold symmetry. The mouth of each polyp is surrounded by a ring of tentacles. In stony corals these are cylindrical and taper to a point, but in soft corals they are pinnate with side branches known as pinnules. In some tropical species these are reduced to mere stubs and in some they are fused to give a paddle-like appearance. In most corals, the tentacles are retracted by day and spread out at night to catch plankton and other small organisms. Shallow water species of both stony and soft corals can be zooxanthellate, the corals supplementing their plankton diet with t
A continent is one of several large landmasses of the world. Identified by convention rather than any strict criteria, up to seven regions are regarded as continents. Ordered from largest in area to smallest, they are: Asia, North America, South America, Antarctica and Australia. Geologically, the continents correspond to areas of continental crust that are found on the continental plates. However, some areas of continental crust are regions covered with water not included in the list of continents. Zealandia is one such area. Islands are grouped with a neighbouring continent to divide all the world's land into geopolitical regions. Under this scheme, most of the island countries and territories in the Pacific Ocean are grouped together with the continent of Australia to form a geopolitical region called Oceania. By convention, "continents are understood to be large, discrete masses of land, ideally separated by expanses of water." Several of the seven conventionally recognized continents are not discrete landmasses separated by water.
The criterion "large" leads to arbitrary classification: Greenland, with a surface area of 2,166,086 square kilometres is considered the world's largest island, while Australia, at 7,617,930 square kilometres is deemed the smallest continent. Earth's major landmasses all have coasts on a single, continuous World Ocean, divided into a number of principal oceanic components by the continents and various geographic criteria; the most restricted meaning of continent is that of a continuous area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense the term continental Europe is used to refer to mainland Europe, excluding islands such as Great Britain, Ireland and Iceland, the term continent of Australia may refer to the mainland of Australia, excluding Tasmania and New Guinea; the continental United States refers to the 48 contiguous states and the District of Columbia in central North America and may include Alaska in the northwest of the continent, while excluding Hawaii, Puerto Rico, Guam in the oceans.
From the perspective of geology or physical geography, continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area and the islands on the shelf, as they are structurally part of the continent. From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level. In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent; as a cultural construct, the concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered Madagascar part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental plate with other islands in the Pacific into one continent called Oceania; this divides the entire land surface of Earth into quasi-continents. The ideal criterion that each continent is a discrete landmass is relaxed due to historical conventions.
Of the seven most globally recognized continents, only Antarctica and Australia are separated from other continents by the ocean. Several continents are defined not as distinct bodies but as "more or less discrete masses of land". Asia and Africa are joined by the Isthmus of Suez, North and South America by the Isthmus of Panama. In both cases, there is no complete separation of these landmasses by water. Both these isthmuses are narrow compared to the bulk of the landmasses they unite. North America and South America are treated as separate continents in the seven-continent model. However, they may be viewed as a single continent known as America or the Americas; this viewpoint was common in the United States until World War II, remains prevalent in some Asian six-continent models. This remains the more common vision in Latin American countries, Portugal, Italy and Hungary where they are taught as a single continent; the criterion of a discrete landmass is disregarded if the continuous landmass of Eurasia is classified as two separate continents: Europe and Asia.
Physiographically and South Asia are peninsulas of the Eurasian landmass. However, Europe is considered a continent with its comparatively large land area of 10,180,000 square kilometres, while South Asia, with less than half that area, is considered a subcontinent; the alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view separation of Eurasia into Asia and Europe as a residue of Eurocentrism: "In physical and historical diversity and India are comparable to the entire European landmass, not to a single European country.." However, for historical and cultural reasons, the view of Europe as a separate continent continues in several categorizations. If continents are defined as discrete landmasses, embracing all the contiguous land of a body Africa and Europe form a single continent which may be referred to as Afro-Eurasia; this produces a four-continent model consisting of Afro-Eurasia, America and Australia. When sea levels were lower during the Pleistocene ice ages, gre