The Embryophyta, or land plants, are the most familiar group of green plants that form vegetation on earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that includes several green algae groups, within this large clade the embryophytes are sister to the Zygnematophyceae/Mesotaeniaceae and consist of the bryophytes plus the polysporangiophytes. Living embryophytes therefore include hornworts, mosses, lycophytes and flowering plants; the Embryophyta are informally called land plants because they live in terrestrial habitats, while the related green algae are aquatic. All are complex multicellular eukaryotes with specialized reproductive organs; the name derives from their innovative characteristic of nurturing the young embryo sporophyte during the early stages of its multicellular development within the tissues of the parent gametophyte. With few exceptions, embryophytes obtain their energy by photosynthesis, by using the energy of sunlight to synthesize their food from carbon dioxide and water.
The evolutionary origins of the embryophytes are discussed further below, but they are believed to have evolved from within a group of complex green algae during the Paleozoic era from terrestrial unicellular charophytes, similar to extant Klebsormidiophyceae. Embryophytes are adapted for life on land, although some are secondarily aquatic. Accordingly, they are called land plants or terrestrial plants. On a microscopic level, the cells of embryophytes are broadly similar to those of green algae, but differ in that in cell division the daughter nuclei are separated by a phragmoplast, they are eukaryotic, with a cell wall composed of cellulose and plastids surrounded by two membranes. The latter include chloroplasts, which conduct photosynthesis and store food in the form of starch, are characteristically pigmented with chlorophylls a and b giving them a bright green color. Embryophyte cells generally have an enlarged central vacuole enclosed by a vacuolar membrane or tonoplast, which maintains cell turgor and keeps the plant rigid.
In common with all groups of multicellular algae they have a life cycle which involves'alternation of generations'. A multicellular generation with a single set of chromosomes – the haploid gametophyte – produces sperm and eggs which fuse and grow into a multicellular generation with twice the number of chromosomes – the diploid sporophyte; the mature sporophyte produces haploid spores which grow into a gametophyte, thus completing the cycle. Embryophytes have two features related to their reproductive cycles which distinguish them from all other plant lineages. Firstly, their gametophytes produce sperm and eggs in multicellular structures, fertilization of the ovum takes place within the archegonium rather than in the external environment. Secondly, most the initial stage of development of the fertilized egg into a diploid multicellular sporophyte, take place within the archegonium where it is both protected and provided with nutrition; this second feature is the origin of the term'embryophyte' – the fertilized egg develops into a protected embryo, rather than dispersing as a single cell.
In the bryophytes the sporophyte remains dependent on the gametophyte, while in all other embryophytes the sporophyte generation is dominant and capable of independent existence. Embryophytes differ from algae by having metamers. Metamers are repeated units of development, in which each unit derives from a single cell, but the resulting product tissue or part is the same for each cell; the whole organism is thus constructed from repeating parts or metamers. Accordingly, these plants are sometimes termed'metaphytes' and classified as the group Metaphyta. In all land plants a disc-like structure called a phragmoplast forms where the cell will divide, a trait only found in the land plants in the streptophyte lineage, some species within their relatives Coleochaetales and Zygnematales, as well as within subaerial species of the algae order Trentepohliales, appears to be essential in the adaptation towards a terrestrial life style. All green algae and land plants are now known to form a single evolutionary lineage or clade, one name for, Viridiplantae.
According to several molecular clock estimates the Viridiplantae split 1,200 million years ago to 725 million years ago into two clades: chlorophytes and streptophytes. The chlorophytes are more diverse and were marine, although some groups have since spread into fresh water; the streptophyte algae are less diverse and adapted to fresh water early in their evolutionary history. They have not spread into marine environments; some time during the Ordovician period one or more streptophytes invaded the land and began the evolution of the embryophyte land plants. Present day embryophytes form. Becker and Marin speculate that land plants evolved from streptophytes rather than any other group of algae because streptophytes were adapted to living in fresh water; this prepared them to tolerate a range of environmental conditions found on land. Fresh water living made.
Earliest known life forms
The earliest known life forms on Earth are putative fossilized microorganisms found in hydrothermal vent precipitates. The earliest time that life forms first appeared on Earth is unknown, they could have lived earlier than 3.77 billion years ago as early as 4.28 billion years ago, or nearly 4.5 billion years ago according to some. The earliest direct evidence of life on Earth are microfossils of microorganisms permineralized in 3.465-billion-year-old Australian Apex chert rocks. A life form, or lifeform, is an entity or being, living. Earth remains the only place in the universe known to harbor life forms. More than 99% of all species of life forms, amounting to over five billion species, that lived on Earth are estimated to be extinct; some estimates on the number of Earth's current species of life forms range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. However, a May 2016 scientific report estimates that 1 trillion species are on Earth, with only one-thousandth of one percent described.
The total number of DNA base pairs on Earth is estimated at 5.0 x 1037 with a weight of 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4 trillion tons of carbon. In July 2016, scientists reported identifying a set of 355 genes from the Last Universal Common Ancestor of all organisms living on Earth; the Earth's biosphere can be considered sort of a shell around the earth, extending down to at least 19 km below the surface of the earth, extending up to at least 64 km into the atmosphere. At and below the surface of the earth, the biosphere includes soil, hydrothermal vents, rock, it includes the deepest parts of the ocean. Under certain test conditions, life forms have been observed to thrive in the near-weightlessness of space and to survive in the vacuum of outer space. Life forms appear to thrive in the Mariana Trench, the deepest spot in the Earth's oceans, reaching a depth of 11,034 m. Other researchers reported related studies that life forms thrive inside rocks up to 580 m below the sea floor under 2,590 m of ocean, off the coast of the northwestern United States, as well as 2,400 m beneath the seabed off Japan.
In August 2014, scientists confirmed the existence of life forms living 800 m below the ice of Antarctica. In December 2018, researchers announced that considerable amounts of life forms, including 70% of bacteria and archea on Earth, comprising up to 23 billion tonnes of carbon, live at least 4.8 km deep underground, including 2.5 km below the seabed, according to a ten-year Deep Carbon Observatory project. According to one researcher, "You can find microbes everywhere — adaptable to conditions, survive wherever they are." Fossil evidence informs most studies of the origin of life. The age of the Earth is about 4.54 billion years. There is evidence. In 2017, fossilized microorganisms, or microfossils, were announced to have been discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that may be as old as 4.28 billion years old, the oldest record of life on Earth, suggesting "an instantaneous emergence of life", after ocean formation 4.41 billion years ago, not long after the formation of the Earth 4.54 billion years ago.
Nonetheless, life may have started earlier, at nearly 4.5 billion years ago, as claimed by some researchers. "Remains of life" have been found in 4.1 billion-year-old rocks in Western Australia. Evidence of biogenic graphite, stromatolites, were discovered in 3.7 billion-year-old metasedimentary rocks in southwestern Greenland. In May 2017, evidence of life on land may have been found in 3.48 billion-year-old geyserite, found around hot springs and geysers, other related mineral deposits, uncovered in the Pilbara Craton of Western Australia. This complements the November 2013 publication that microbial mat fossils had been found in 3.48 billion-year-old sandstone in Western Australia. A December 2017 report stated that 3.465-billion-year-old Australian Apex chert rocks once contained microorganisms, the earliest direct evidence of life on Earth. In January 2018, a study found that 4.5 billion-year-old meteorites found on Earth contained liquid water along with prebiotic complex organic substances that may be ingredients for life.
According to biologist Stephen Blair Hedges, "If life arose quickly on Earth … it could be common in the universe." Categories Terminology of biology Biota Life Vitae Wikispecies – a free directory of life Google Images: Earliest known life forms
The Cambrian explosion or Cambrian radiation was an event 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record. It lasted for about 20–25 million years and resulted in the divergence of most modern metazoan phyla; the event was accompanied by major diversification of other organisms. Before the Cambrian explosion, most organisms were simple, composed of individual cells organized into colonies; as the rate of diversification subsequently accelerated, the variety of life began to resemble that of today. All present animal phyla appeared during this period; the rapid appearance of fossils in the "Primordial Strata" was noted by William Buckland in the 1840s, in his 1859 book On the Origin of Species, Charles Darwin discussed the inexplicable lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection. The long-running puzzlement about the appearance of the Cambrian fauna abruptly, without precursor, centers on three key points: whether there was a mass diversification of complex organisms over a short period of time during the early Cambrian.
Interpretation is difficult due to a limited supply of evidence, based on an incomplete fossil record and chemical signatures remaining in Cambrian rocks. The first discovered Cambrian fossils were trilobites, described by Edward Lhuyd, the curator of Oxford Museum, in 1698. Although their evolutionary importance was not known, on the basis of their old age, William Buckland realised that a dramatic step-change in the fossil record had occurred around the base of what we now call the Cambrian. Nineteenth-century geologists such as Adam Sedgwick and Roderick Murchison used the fossils for dating rock strata for establishing the Cambrian and Silurian periods. By 1859, leading geologists including Roderick Murchison, were convinced that what was called the lowest Silurian stratum showed the origin of life on Earth, though others, including Charles Lyell, differed. In On the Origin of Species, Charles Darwin considered this sudden appearance of a solitary group of trilobites, with no apparent antecedents, absence of other fossils, to be "undoubtedly of the gravest nature" among the difficulties in his theory of natural selection.
He reasoned that earlier seas had swarmed with living creatures, but that their fossils had not been found due to the imperfections of the fossil record. In the sixth edition of his book, he stressed his problem further as: To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. American paleontologist Charles Walcott, who studied the Burgess Shale fauna, proposed that an interval of time, the "Lipalian", was not represented in the fossil record or did not preserve fossils, that the ancestors of the Cambrian animals evolved during this time. Earlier fossil evidence has since been found; the earliest claim is that the history of life on earth goes back 3,850 million years: Rocks of that age at Warrawoona, were claimed to contain fossil stromatolites, stubby pillars formed by colonies of microorganisms. Fossils of more complex eukaryotic cells, from which all animals and fungi are built, have been found in rocks from 1,400 million years ago, in China and Montana.
Rocks dating from 580 to 543 million years ago contain fossils of the Ediacara biota, organisms so large that they are multicelled, but unlike any modern organism. In 1948, Preston Cloud argued that a period of "eruptive" evolution occurred in the Early Cambrian, but as as the 1970s, no sign was seen of how the'relatively' modern-looking organisms of the Middle and Late Cambrian arose; the intense modern interest in this "Cambrian explosion" was sparked by the work of Harry B. Whittington and colleagues, who, in the 1970s, reanalysed many fossils from the Burgess Shale and concluded that several were as complex as, but different from, any living animals; the most common organism, was an arthropod, but not a member of any known arthropod class. Organisms such as the five-eyed Opabinia and spiny slug-like Wiwaxia were so different from anything else known that Whittington's team assumed they must represent different phyla unrelated to anything known today. Stephen Jay Gould's popular 1989 account of this work, Wonderful Life, brought the matter into the public eye and raised questions about what the explosion represented.
While differing in details, both Whittington and Gould proposed that all modern animal phyla had appeared simultaneously in a rather short span of geological period. This view led to the modernization of Darwin's tree of life and the theory of punctuated equilibrium, which Eldredge and Gould developed in the early 1970s and which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. Other analyses, some more recent and some dating back to the 1970s, argue that complex animals similar to modern types evolved well before the start of the Cambrian. Radiometric dates for much of the Cambrian, obtained by analysis of radioactive elements contained within rocks, have only become available, for only a few regions. Relative dating is assumed sufficient for studying processes of evolution, but this, has been difficult, because of the problems involved in matching up rocks of the same age across different continents. Therefore, dates or descriptions of sequences of events should be regarded with some caution until better data become
Late Paleozoic icehouse
The late Paleozoic icehouse known as the Karoo ice age, was the climate state 360–260 million years ago in which large land-based ice-sheets were present on Earth's surface. It was the second major glacial period of the Phanerozoic, it is named after the tillite found in the Karoo Basin of South Africa, where evidence for this ice age was first identified in the 19th century. The tectonic assembly of the continents of Euramerica and Gondwana into Pangaea, in the Hercynian-Alleghany Orogeny, made a major continental land mass within the Antarctic region, the closure of the Rheic Ocean and Iapetus Ocean saw disruption of warm-water currents in the Panthalassa Ocean and Paleotethys Sea, which led to progressive cooling of summers, the snowfields accumulating in winters, causing mountainous alpine glaciers to grow, spread out of highland areas, making continental glaciers which spread to cover much of Gondwana. At least two major periods of glaciation have been discovered: The first glacial period was associated with the Mississippian subperiod: ice sheets expanded from a core in southern Africa and South America.
The second glacial period was associated with the Pennsylvanian subperiod. According to Eyles and Young, "Renewed Late Devonian glaciation is well documented in three large intracratonic basins in Brazil and in Bolivia. By the Early Carboniferous glacial strata were beginning to accumulate in sub-andean basins of Bolivia and Paraguay. By the mid-Carboniferous glaciation had spread to Antarctica, southern Africa, the Indian Subcontinent and the Arabian Peninsula. During the Late Carboniferous glacial accumulation a large area of Gondwana land mass was experiencing glacial conditions; the thickest glacial deposits of Permo-Carboniferous age are the Dwyka Formation in the Karoo Basin in southern Africa, the Itarare Group of the Parana Basin and the Carnarvon Basin in eastern Australia. The Permo-Carboniferous glaciations are significant because of the marked glacio-eustatic changes in sea level that resulted and which are recorded in non-glacial basins. Late Paleozoic glaciation of Gondwana could be explained by the migration of the supercontinent across the South Pole."In northern Ethiopia glacial landforms like striations, rôche moutonnées and chatter marks can be found buried beneath Late Carboniferous-Early Permian glacial deposits.
The evolution of land plants with the onset of the Devonian Period, began a long-term increase in planetary oxygen levels. Large tree ferns, growing to 20 m high, were secondarily dominant to the large arborescent lycopods of the Carboniferous coal forests that flourished in equatorial swamps stretching from Appalachia to Poland, on the flanks of the Urals. Oxygen levels reached up to 35%, global carbon dioxide got below the 300 parts per million level, today associated with glacial periods; this reduction in the greenhouse effect was coupled with lignin and cellulose accumulating and being buried in the great Carboniferous Coal Measures. The reduction of carbon dioxide levels in the atmosphere would be enough to begin the process of changing polar climates, leading to cooler summers which could not melt the previous winter's snow accumulations; the growth in snowfields to 6 m deep would create sufficient pressure to convert the lower levels to ice. Earth's increased planetary albedo produced by the expanding ice sheets would lead to positive feedback loops, spreading the ice sheets still further, until the process hit limit.
Falling global temperatures would limit plant growth, the rising levels of oxygen would increase the frequency of fire-storms because damp plant matter could burn. Both these effects return carbon dioxide to the atmosphere, reversing the "snowball" effect and forcing greenhouse warming, with CO2 levels rising to 300 ppm in the following Permian period. Over a longer period the evolution of termites, whose stomachs provided an anoxic environment for methanogenic lignin-digesting bacteria, prevented further burial of carbon, returning carbon to the air as the greenhouse gas methane. Once these factors brought a halt and a small reversal in the spread of ice sheets, the lower planetary albedo resulting from the fall in size of the glaciated areas would have been enough for warmer summers and winters and thus limit the depth of snowfields in areas from which the glaciers expanded. Rising sea levels produced by global warming drowned the large areas of flatland where anoxic swamps assisted in burial and removal of carbon.
With a smaller area for deposition of carbon, more carbon dioxide was returned to the atmosphere, further warming the planet. By 250 Mya, planet Earth had returned to a percentage of oxygen similar to that found today; the rising levels of oxygen during the late Paleozoic icehouse had major effects upon evolution of plants and animals. Higher oxygen concentration enabled energetic metabolic processes which encouraged evolution of large land-dwelling vertebrates and flight, with the dragonfly-like Meganeura, an aerial predator, with a wingspan of 60 to 75 cm; the herbivorous stocky-bodied and armoured millipede-like Arthropleura was 1.8 m long, the semiterrestrial Hibbertopterid eurypterids were as large, some scorpions reached 50 or 70 cm. The rising levels of oxygen led to the evolution of greater fire resistance in vegetation and to the evolution of flowering plants. During this time, unique se
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal, that belongs to group 8 of the periodic table, it is by mass the most common element on Earth, forming much of Earth's inner core. It is the fourth most common element in the Earth's crust. Pure iron is rare on the Earth's crust being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE; that event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost. Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts with oxygen and water to give brown to black hydrated iron oxides known as rust.
Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion. The body of an adult human contains about 3 to 5 grams of elemental iron in hemoglobin and myoglobin; these two proteins play essential roles in vertebrate metabolism oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. Chemically, the most common oxidation states of iron are +2 and +3. Iron shares many properties of other transition metals, including the other group 8 elements and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron forms many coordination compounds. At least four allotropes of iron are known, conventionally denoted α, γ, δ, ε; the first three forms are observed at ordinary pressures.
As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic crystal structure. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a face-centered cubic crystal structure, or austenite. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope; the physical properties of iron at high pressures and temperatures have been studied extensively, because of their relevance to theories about the cores of the Earth and other planets. Above 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another hexagonal close-packed structure, known as ε-iron; the higher-temperature γ-phase changes into ε-iron, but does so at higher pressure. Some controversial experimental evidence exists for a stable β phase at pressures above 50 GPa and temperatures of at least 1500 K, it is supposed to have a double hcp structure. The inner core of the Earth is presumed to consist of an iron-nickel alloy with ε structure.
The melting and boiling points of iron, along with its enthalpy of atomization, are lower than those of the earlier 3d elements from scandium to chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus. This same trend appears for ruthenium but not osmium; the melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data still varies by tens of gigapascals and over a thousand kelvin. Below its Curie point of 770 °C, α-iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom align with the spins of its neighbors, creating an overall magnetic field; this happens because the orbitals of those two electrons do not point toward neighboring atoms in the lattice, therefore are not involved in metallic bonding. In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometres across, such that the atoms in each domain have parallel spins, but different domains have other orientations.
Thus a macroscopic piece of iron will have a nearly zero overall magnetic field. Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field; this effect is exploited in devices that needs to channel magnetic fields, such as electrical transformers, magnetic recording heads, electric motors. Impurities, lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists after the external field is removed -- thus turning the iron object into a magnet. Similar behavior is exhibited by some iron compounds, such as the fer
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-