Tuesday, January 26, 2010


Malacca (Malay: Melaka, dubbed The Historical State or Negeri Bersejarah amongst locals) is the third smallest Malaysian state, after Perlis and Penang. It is located in the southern region of the Malay Peninsula, on the Straits of Malacca. It borders Negeri Sembilan to the north and the state of Johor to the south. The capital is Malacca Town. This historical city centre has been listed as a UNESCO World Heritage Site since 7 July 2008.
Although one of the oldest Malay sultanates, the Yang di-Pertua Negeri or Governor, rather than a Sultan, acts as the head of state.

Sultanate of Malacca
Before the arrival of the first Sultan, Malacca was a simple fishing village inhabited by local Malays. Malacca was founded by Parameswara, a Srivijayan prince of Palembang who fled Sumatra following a Majapahit attack in 1377. He found his way to Malacca c. 1400 where he found a good port accessible in all seasons and on the strategically located narrowest point of the Malacca Straits.
According to a popular legend, Parameswara was resting under a gray tree near a river while hunting, when one of his dogs cornered a mouse deer. In self-defence, the mouse deer pushed the dog into the river. Impressed by the courage of the deer, and taking it as a propitious omen of the weak overcoming the powerful, Parameswara decided on the spot to found an empire on the very place that he was sitting. He named it 'Melaka' after the tree under which he had taken shelter. Another version of the story says that Parameswara chose the name 'Malacca' from the Tamil word 'mallakka' which means upside down or on ones back. Old illustrations of the scene where the mousedeer kicks the dog shows the dog falling on its back into the river, hence the inspiration. Parameswara converted to Islam in 1414 and changed his name to 'Raja Iskandar Shah'. In collaboration with allies from the sea-people (orang laut) the wandering proto-Malay privateers of the Straits, he established Malacca as major international port by compelling passing ships to call there, and establishing fair and reliable facilities for warehousing and trade. Mass settlement of Chinese, mostly from the imperial and merchant fleet occurred during the reign of Parameswara, occurred in the vicinity of the Bukit China ("Chinese Hill") area, which was perceived as having excellent Feng Shui (geomancy) in Malacca then. Sultan Iskandar Shah died in 1424, and was succeeded by his son, Sri Maharaja also called Sultan Muhammad Shah.
The prosperity of Malacca attracted the invasion of the Siamese. Attempts in 1446 and 1456, however, were warded off by Tun Perak, the then Bendahara (a position similar to Prime Minister). The development of relations between Malacca and China was then a strategic decision to ward off further Siamese attacks.
Because of its strategic location, Malacca was an important stopping point for Zheng He's fleet. To enhance relations, Hang Li Po, allegedly a princess of the Ming Emperor of China, arrived in Malacca, accompanied by 500 attendants, to marry Sultan Manshur Shah who reigned from 1456 until 1477. Her attendants married the locals and settled mostly in Bukit China (Bukit Cina).(See Zheng He in Malacca). Scholars have disputed Hang Li Po's status in China as because she was never recorded as a princess in the Chinese court of the Ming Dynasty in the Ming Chronicles. At the time of the arrival of the Sultan's envoy, the reigning Ming Emperor was Jingtai Emperor. Records of his reign was expunged following the ascension of Tianshun in 1457. It is likely that if she were a princess in the Ming court, records of her might not exist. In many historical text, she was said to have been a princess in the court of the Yongle Emperor(1402-1424).
A cultural result of the vibrant trade was the expansion of the Peranakan people, who spread to other major settlements in the region.
During its prime, Malacca was a powerful Sultanate which extended its rule over the southern Malay Peninsula and much of Sumatra. Its rise helped to hold off the Thai's southwards encroachment and arguably hasten the decline of the rival Majapahit Empire of Java which was in decline as Malacca was rising. Malacca was also central in the spread of Islam in the Malay Archipelago.

European colonization
In April 1511, Afonso de Albuquerque set sail from Goa to Malacca with a force of some 1200 men and seventeen or eighteen ships. They conquered the city on August 24, 1511. It became a strategic base for Portuguese expansion in the East Indies. Sultan Mahmud Shah, the last Sultan of Malacca took refuge in the hinterland, and made intermittent raids both by land and sea, causing considerable hardship for the Portuguese. In the meantime the Portuguese built the fort named A Famosa to defend Malacca (its gate is all that remains of the ruins at present). "In order to appease the King of Ayudhya" (Siam, whom had always intended in invading Malacca if not due to the latter's good relationship with the Ming Emperor, China) "the Portuguese sent up an ambassador, Duarte Fernandes, who was well received by Ramathibodi." in 1511.Finally in 1526, a large force of Portuguese ships, under the command of Pedro Mascarenhas, was sent to destroy Bintan, where Sultan Mahmud was based. Sultan Mahmud fled with his family across the Straits to Kampar in Sumatra, where he died two years later.
It soon became clear that Portuguese control of Malacca did not mean they now controlled Asian trade that centred around it. Their Malaccan rule was severely hampered by administrative and economic difficulties. Rather than achieving their ambition of dominating Asian trade, the Portuguese had fundamentally disrupted the organisation of the network. The centralised port of exchange of Asian wealth exchange had now gone, as was a Malay state to police the Straits of Malacca that made it safe for commercial traffic. Trade was now scattered over a number of ports amongst bitter warfare in the Straits.
The Jesuit missionary Francis Xavier spent several months in Malacca in 1545, 1546 and 1549. In 1641 the Dutch defeated the Portuguese to capture Malacca with the help of the Sultan of Johore. The Dutch ruled Malacca from 1641 to 1795 but they were not interested in developing it as a trading centre, placing greater importance to Batavia (Jakarta) in Indonesia as their administrative centre. However they still built their landmark better known as the Stadthuys or Red Building.
Malacca was ceded to the British in the Anglo-Dutch Treaty of 1824 in exchange for Bencoolen on Sumatra. From 1826 to 1946 Malacca was governed, first by the British East India Company and then as a Crown Colony. It formed part of the Straits Settlements, together with Singapore and Penang. After the dissolution of this crown colony, Malacca and Penang became part of the Malayan Union, which later became Malaysia.

Friday, January 22, 2010


Al-Khwārizmī (c. 780, Khwārizm – c. 850) was a Persian mathematician, astronomer and geographer, a scholar in the House of Wisdom in Baghdad.
His Kitab al-Jabr wa-l-Muqabala presented the first systematic solution of linear and quadratic equations. He is considered the founder of algebra, a credit he shares with Diophantus. In the twelfth century, Latin translations of his work on the Indian numerals, introduced the decimal positional number system to the Western world. He revised Ptolemy's Geography and wrote on astronomy and astrology.
His contributions had a great impact on language. "Algebra" is derived from al-jabr, one of the two operations he used to solve quadratic equations. Algorism and algorithm stem from Algoritmi, the Latin form of his name. His name is the origin of (Spanish) guarismo and of (Portuguese) algarismo, both meaning digit.

Al-Khwārizmī's contributions to mathematics, geography, astronomy, and cartography established the basis for innovation in algebra and trigonometry. His systematic approach to solving linear and quadratic equations led to algebra, a word derived from the title of his 830 book on the subject, "The Compendious Book on Calculation by Completion and Balancing" (al-Kitab al-mukhtasar fi hisab al-jabr wa'l-muqabalaالكتاب المختصر في حساب الجبر والمقابلة).
On the Calculation with Hindu Numerals written about 825, was principally responsible for spreading the Indian system of numeration throughout the Middle East and Europe. It was translated into Latin as Algoritmi de numero Indorum. Al-Khwārizmī, rendered as (Latin) Algoritmi, led to the term "algorithm".
Some of his work was based on Persian and Babylonian astronomy, Indian numbers, and Greek mathematics.
Al-Khwārizmī systematized and corrected Ptolemy's data for Africa and the Middle east. Another major book was Kitab surat al-ard ("The Image of the Earth"; translated as Geography), presenting the coordinates of places based on those in the Geography of Ptolemy but with improved values for the Mediterranean Sea, Asia, and Africa.
He also wrote on mechanical devices like the astrolabe and sundial.
He assisted a project to determine the circumference of the Earth and in making a world map for al-Ma'mun, the caliph, overseeing 70 geographers.
When, in the 12th century, his works spread to Europe through Latin translations, it had a profound impact on the advance of mathematics in Europe. He introduced Arabic numerals into the Latin West, based on a place-value decimal system developed from Indian sources.

Al-Kitāb al-mukhtasar fī isāb al-jabr wa-l-muqābala (Arabic: الكتاب المختصر في حساب الجبر والمقابلة “The Compendious Book on Calculation by Completion and Balancing”) is a mathematical book written approximately 830 CE. The book was written with the encouragement of the Caliph Al-Ma'mun as a popular work on calculation and is replete with examples and applications to a wide range of problems in trade, surveying and legal inheritance. The term algebra is derived from the name of one of the basic operations with equations (al-jabr) described in this book. The book was translated in Latin as Liber algebrae et almucabala by Robert of Chester (Segovia, 1145) hence "algebra", and also by Gerard of Cremona. A unique Arabic copy is kept at Oxford and was translated in 1831 by F. Rosen. A Latin translation is kept in Cambridge.
The al-jabr is considered the foundational text of modern algebra. It provided an exhaustive account of solving polynomial equations up to the second degree, and introduced the fundamental methods of "reduction" and "balancing", referring to the transposition of subtracted terms to the other side of an equation, that is, the cancellation of like terms on opposite sides of the equation.

Al-Khwārizmī's second major work was on the subject of arithmetic, which survived in a Latin translation but was lost in the original Arabic. The translation was most likely done in the twelfth century by Adelard of Bath, who had also translated the astronomical tables in 1126.
The Latin manuscripts are untitled, but are commonly referred to by the first two words with which they start: Dixit algorizmi ("So said al-Khwārizmī"), or Algoritmi de numero Indorum ("al-Khwārizmī on the Hindu Art of Reckoning"), a name given to the work by Baldassarre Boncompagni in 1857. The original Arabic title was possibly Kitāb al-Jam' wa-l-tafrīq bi-hisāb al-Hind ("The Book of Addition and Subtraction According to the Hindu Calculation")
Al-Khwarizmi's work on arithmetic was responsible for introducing the Arabic numerals, based on the Hindu-Arabic numeral system developed in Indian mathematics, to the Western world. The term "algorithm" is derived from the algorism, the technique of performing arithmetic with Hindu-Arabic numerals developed by al-Khwarizmi. Both "algorithm" and "algorism" are derived from the Latinized forms of al-Khwarizmi's name, Algoritmi and Algorismi, respectively.

Al-Khwārizmī's Zīj al-Sindhind (Arabic: زيج "astronomical tables of Sind and Hind") is a work consisting of approximately 37 chapters on calendrical and astronomical calculations and 116 tables with calendrical, astronomical and astrological data, as well as a table of sine values. This is the first of many Arabic Zijes based on the Indian astronomical methods known as the sindhind. The work contains tables for the movements of the sun, the moon and the five planets known at the time. This work marked the turning point in Islamic astronomy. Hitherto, Muslim astronomers had adopted a primarily research approach to the field, translating works of others and learning already discovered knowledge. Al-Khwarizmi's work marked the beginning of non-traditional methods of study and calculations.

Sunday, January 17, 2010


Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water. In the name, as in that of quicksilver (mercury), "quick" does not mean "fast," but "living" (cf. the expression the quick and the dead).
Water circulation underground can focus in an area with the optimal mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column of sand, and the sand remains a generally solid mass. This lubricates the sand particles and renders them unable to support significant weight, since they move with little friction, behaving more like a liquid when exposed to stress. Since water does not usually go up to the surface of the sand, the sand on top appears solid, and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.

Quicksand is a non-Newtonian fluid: when undisturbed it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of .01 m/s would require the same amount of force as "that needed to lift a medium-sized car."
Because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand itself is harmless on its own, but because it greatly impedes human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, or tides to harm a trapped person.

Quicksand may be found inland (on riverbanks, near lakes, or in marshes), or near the coast.
One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly.

Dry quicksand
Dry quicksand is loose sand whose bulk density is reduced by blowing air through it and which yields easily to weight or pressure. It acts similarly to normal quicksand, but it does not contain any water and does not operate on the same principle. Dry quicksand is an example of a granular material.
Until recently, the existence of dry quicksand was doubted, and the reports of humans and complete caravans being lost in dry quicksand were considered to be folklore.

Scientific research
Writing in Nature, physicist Detlef Lohse and coworkers of University of Twente in Enschede, Netherlands allowed air to flow through very fine sand (typical grain diameter was about 40 micrometers) in a container with a perforated base. They then turned the air stream off before the start of the experiment and allowed the sand to settle: the packing fraction of this sand was only 41% (compared to 55–60% for untreated sand).
Lohse found that a weighted table tennis ball (radius 2 cm, mass 133 g), when released from just above the surface of the sand, would sink to about five diameters. Lohse also observed a "straight jet of sand [shooting] violently into the air after about 100 ms". Objects are known to make a splash when they hit sand, but this type of jet has never been described before.

Lohse concluded that
In nature, dry quicksands may evolve from the sedimentation of very fine sand after it has been blown into the air and, if large enough, might be a threat to humans. Indeed, reports that travellers and whole vehicles have been swallowed instantly may even turn out to be credible in the light of our results.

During the planning of the Project Apollo moon missions, dry quicksand on the moon was considered as a potential danger to the missions. The successful landings of the unmanned Surveyor probes a few years earlier and their observations of a solid, rocky surface largely discounted this possibility, however. The large plates at the end of legs of the Apollo Lunar Module were designed to reduce this danger, but the astronauts did not encounter dry quicksand.

Monday, January 11, 2010


The housefly (also house fly, house-fly or common housefly), Musca domestica, is a Diptera of the Brachycera suborder. It is the most common of all domestic flies, accounting for about 90% of all flies in human habitations, and indeed one of the most widely distributed insects, found all over the world; it is considered a pest that can carry serious diseases.

Physical description
The adults are 8–12 mm long. Their thorax is gray, with four longitudinal dark lines on the back. The underside of their abdomen is yellow, and their whole body is covered with hair-like projections. The females are slightly larger than the males, and have a much larger space between their red compound eyes. The mass of pupae can range from about 8 to 20 mg under different conditions.
Like other Diptera (meaning "two-winged"), houseflies have only one pair of wings; the hind pair is reduced to small halteres that aid in flight stability. caterpillars (M1+2 or fourth long vein of the wing) shows a sharp upward bend.
Species that appear similar to the housefly include:
a) The lesser house fly, Fannia canicularis, is somewhat smaller, more slender, and the media vein is straight.
b) The stable fly, Stomoxys calcitrans, has piercing mouthparts and the media vein is only slightly curved.

Life cycle
Each female fly can lay approximately 500 eggs in several batches of about 75 to 150. The eggs are white and are about 1.2 mm in length. Within a day, larvae (maggots) hatch from the eggs; they live and feed in (usually dead and decaying) organic material, such as garbage or feces. They are pale-whitish, 3–9 mm long, thinner at the mouth end, and have no legs. They live at least one week. At the end of their third instar, the maggots crawl to a dry cool place and transform into pupae, colored reddish or brown and about 8 mm long. The adult flies then emerge from the pupae. (This whole cycle is known as complete metamorphosis.) The adults live from two weeks to a month in the wild, or longer in benign laboratory conditions. After having emerged from the pupae, the flies cease to grow; small flies are not young flies, but are indeed the result of getting insufficient food during the larval stage.
Some 36 hours after having emerged from the pupa, the female is receptive for mating. The male mounts her from behind to inject sperm. Copulation takes between a few seconds to a couple of minutes. Normally the female mates only once, storing the sperm to use it repeatedly for laying several sets of eggs. Males are territorial: they will defend a certain territory against other males and will attempt to mount any females that enter that territory.
The flies depend on warm temperatures; generally, the warmer the temperature the faster the flies will develop. In winter, most of them survive in the larval or the pupa stage in some protected warm location.

Some types of maggots found on corpses have been found to be of great use to forensic scientists; specifically Forensic Entomology. By their stage of development, these maggots (and other insects) can be used to give an indication of the time elapsed since death, as well as the place the organism died. The lack of maggot presence is also telling in an investigation.
Maggot species can be identified using their DNA. The size of the house fly maggot is 10–20 mm (⅜–¾ in). At the height of the summer season, a generation of flies (egg to adult) may be produced in 12–14 days. Some other families of Insecta, such as Histeridae, feed on maggots. Thus, the lack of maggots would increase the estimated time of death.
Other types of maggots are bred commercially, as a popular bait in angling, and a food for carnivorous pets such as reptiles or birds.
Maggots have been used in medicine to clean out necrotic wounds and in food production, particularly of cheeses designed to rot as part of their 'aging' process. (casu marzu).

Houseflies feed on feces, open sores, sputum, and moist decaying organic matter such as spoiled food, eggs and flesh. Houseflies can take in only liquid foods. They spit out saliva on solid foods to predigest it, and then suck it back in. They also regurgitate partly digested matter and pass it again to the abdomen.

Housefly as a vector of disease
Mechanical transmission of organisms on its hairs, mouthparts, vomitus and feces:
a) parasitic diseases: cysts of protozoa e.g. Entamoeba histolytica, Giardia lamblia and eggs of helminths e.g.: Ascaris lumbricoides, Trichuros trichura, Haemenolypes nana, Enterobius vermicularis.
b) bacterial diseases: typhoid, cholera, dysentery, pyogenic cocci...etc. House flies have been demonstrated to be vectors of Campylobacter and E. coli O157:H7 using PCR. House flies can be monitored for bacterial pathogens using filter paper spot cards and PCR
c) Viruses: enteroviruses: poliomyelitis, infective hepatitis (A & E)..etc
House flies feed on liquid or semi-liquid substances beside solid material which has been softened by saliva or vomit. Because of their high intake of food, they deposit feces constantly, one of the factors that makes the insect a dangerous carrier of pathogens. Although they are domestic flies, usually confined to the human habitations, they can fly for several miles from the breeding place. They are active only in daytime and rest at night e.g. at the corners of rooms, ceiling hangings, etc.

Thursday, January 7, 2010

True polar wander

True polar wander is a phenomenon in which a planet or moon undergoes a solid-body rotation with respect to its spin axis. As this occurs, the geographic locations of the North and South Poles change, or "wander". This can happen when the two larger moments of inertia are near equal.

Description in the context of Earth
The Earth is not a true sphere, and therefore has three orthogonal axes of inertia. The axis around which the moment of inertia is greatest is closely aligned with the rotation axis (the axis going through the North and South Poles). The other two axes are near the equator. This is similar to a brick rotating around an axis going through its shortest dimension (a vertical axis when the brick is lying flat). But if the moment of inertia around one of the two axes close to the equator becomes nearly equal to that around the polar axis, then the constraint on the orientation of the object (the Earth) is relaxed.
This situation is like an American football (or a Rugby ball) spinning around an axis running through its "equator". (Note that the "equator" of the ball does not correspond to the equator of the Earth.) Small perturbations can move the football so that it spins around another axis through this same "equator". In the same way, when the conditions are right, the Earth (both the crust and the mantle) can slowly reorient so that a new geographic point moves to the North Pole, while keeping the axis of low moment of inertia quite near the equator.
Such a reorientation changes the latitudes of most points on the Earth, by amounts that depend on how far they are from the axis near the equator that does not move.

Claimed examples
Cases of true polar wander have occurred several times in the course of the Earth's history.
The speed of rotation (around the axis of lowest inertia) is limited to about 1° per million years. Mars, Europa, and Enceladus are also believed to have undergone true pole wander, in the case of Europa by 80°, flipping over almost completely. The crust of Titan has also shifted, though pole wander has not been detected.

Distinctions and delimitations
Polar wander should not be confused with precession, which is where the axis of rotation moves, in other words the North Pole points toward a different star. Precession is caused by the gravitational attraction of the Moon and Sun, and occurs all the time and at a much faster rate than polar wander. It does not result in changes of latitude.
True polar wander has to be distinguished from continental drift, which is where different parts of the Earth's crust move in different directions because of circulation in the mantle.

Geomagnetic reversal
A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged. These events often involve an extended decline in field strength followed by a rapid recovery after the new orientation has been established. These events occur on a scale of tens of thousands of years or longer.
More generally, the term may refer to a reversal of the polarity of any magnetosphere.
Scientific opinion is divided on what causes geomagnetic reversals. One theory holds that they are due to events internal to the system that generates the Earth's magnetic field. The other holds that they are due to external events.

Internal events
Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core.
In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength.
Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.

External events
Others, such as Richard A. Muller, believe that geomagnetic reversals are not spontaneous processes but rather are triggered by external events which directly disrupt the flow in the Earth's core. Such processes may include the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones, the initiation of new mantle plumes from the core-mantle boundary, and possibly mantle-core shear forces resulting from very large impact events. Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that a recovery is seen as a reversal in about half of all cases.
Brief disruptions which do not result in reversal are also known and are called geomagnetic excursions.

Sunday, January 3, 2010


Jellyfish (also jellies or sea jellies) are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000-1500 species that make jellyfish and many more that do not). The jellyfish in these groups are also called, respectively, scyphomedusae, stauromedusae, cubomedusae, and hydromedusae; medusa is another word for jellyfish, but it also refers specifically to the adult stage of the life cycle.
Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water and are less than half an inch in size. They are partially white and clear and do not sting. Many of the best-known jellyfish, such as Aurelia, are scyphomedusae. These are the large, often colorful, jellyfish that are common in coastal zones worldwide.
In its broadest sense, the term jellyfish also generally refers to members of the phylum Ctenophora. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are generally transparent or translucent, and exist in shallow to deep portions of all the world's oceans.

Life cycle
Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage, when the animal takes the form of a small stalk with feeding tentacles; this polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish. Polyps generally have a mouth surrounded by upward-facing tentacles like miniatures of the closely-related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. Polyps may be solitary or colonial, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters.
In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the bell's border that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.
Jellyfish are dioecious; that is, they are either male or female. In most cases, both release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber.
After fertilization and initial growth, a larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, resembling a tiny sea anemone.
After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyrae may be formed. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few of species of hydromedusae reproduce by fission (splitting in half.)
Other species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles and at least one species of Pacific salmon. Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. One unusual species is reported to live as long as 30 years. Another species, Turritopsis dohrnii as T. nutricula, may be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death. Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. They feed continuously and grow to adult size fairly rapidly. After reaching adult size, jellyfish spawn daily if there is enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.

Toxicity to humans
Scyphozoan jellyfish stings are not generally deadly, though some species of the completely separate class Cubozoa (box jellyfish), such as the famous and especially toxic Irukandji, can be.
Jellyfish sting using microscopic cells called nematocysts, which are capsules full of poison expelled through a microscopic lance. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom.
Jellyfish sting prey and threatening humans using their nematocysts, but only some jellyfish species harm humans. Even beached and dying jellyfish can still sting when touched. Sting effects range from no effect to extreme pain to death.
Stings may cause anaphylaxis, which may result in death. Hence, victims should immediately get out of the water. Medical care may include administration of an antivenom.
The three goals of first aid for uncomplicated jellyfish stings are: prevent injury to rescuers, inactivate the nematocysts, and remove tentacles attached to the patient. Rescuers should wear barrier clothing, such as panty hose, wet suits or full-body sting-proof suits. Inactivating the nematocysts, or stinging cells, prevents further injection of venom.
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