An organism is defined in a medical dictionary as any living thing that functions as an individual . Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is. Among the most common is that an organism has autonomous reproduction , growth , and metabolism . This would exclude viruses , despite the fact that they evolve like organisms. Other problematic cases include colonial organisms ; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation , with some insects reproducing, others not, like cells in an animal's body. The body of a siphonophore , a jelly-like marine animal, is composed of organism-like zooids , but the whole structure looks and functions much like an animal such as a jellyfish , the parts collaborating to provide the functions of the colonial organism.
79-497: A head is the part of an organism which usually includes the ears , brain , forehead , cheeks , chin , eyes , nose , and mouth , each of which aid in various sensory functions such as sight , hearing , smell , and taste . Some very simple animals may not have a head, but many bilaterally symmetric forms do, regardless of size. Heads develop in animals by an evolutionary trend known as cephalization . In bilaterally symmetrical animals, nervous tissue concentrate at
158-406: A sessile existence attached to the seabed, but others are colonial and a few are pelagic . Some are supported by a stalk, but most are attached directly to a substrate , which may be a rock, shell, coral, seaweed, mangrove root, dock, piling, or ship's hull. They are found in a range of solid or translucent colours and may resemble seeds, grapes, peaches, barrels, or bottles. One of the largest
237-437: A statocyst . When sufficiently developed, the larva of the sessile species finds a suitable rock and cements itself in place. The larval form is not capable of feeding, though it may have a rudimentary digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body during metamorphosis , one of the most significant being the reduction of the cerebral ganglion, which controls movement and
316-409: A tadpole . Tunicates are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits. However, doliolids still display segmentation of the muscle bands. Some tunicates live as solitary individuals, but others replicate by budding and become colonies , each unit being known as a zooid . They are marine filter feeders with
395-522: A bacterium. When, in 1845, Carl Schmidt first announced the presence in the test of some ascidians of a substance very similar to cellulose, he called it "tunicine", but it is now recognized as cellulose rather than any alternative substance. Nearly all adult tunicates are suspension feeders (the larval form usually does not feed), capturing planktonic particles by filtering sea water through their bodies. Ascidians are typical in their digestive processes, but other tunicates have similar systems. Water
474-569: A few enzymes and molecules like those in living organisms, they have no metabolism of their own; they cannot synthesize the organic compounds from which they are formed. In this sense, they are similar to inanimate matter. Viruses have their own genes , and they evolve . Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, viruses are evolved by their host cells, meaning that there
553-400: A part of the primitive coelom, and its cells extract nitrogenous waste matter from circulating blood. They accumulate the wastes inside the vesicles as urate crystals , and do not have any obvious means of disposing of the material during their lifetimes. Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland. Both originate from
632-669: A particular race or nationality (such as Moors' heads, Saxons' heads, Egyptians' heads or Turks' heads), or specifically identified (such as the head of Moses in the crest of Hilton, or the head of St. John the Baptist in the crest of the London Company of Tallowchandlers). Several varieties of women's heads also occur, including maidens' heads (often couped under the bust, with hair disheveled), ladies' heads, nuns' heads (often veiled), and occasionally queens' heads. The arms of Devaney of Norfolk include "three nun's heads veiled couped at
711-557: A pharyngeal mucous net to catch their prey. The pyrosomes are bioluminous colonial tunicates with a hollow cylindrical structure. The buccal siphons are on the outside and the atrial siphons inside. About 10 species are known, and all are found in the tropics. The 23 species of doliolids are small, mostly under 2 cm (0.79 in) long. They are solitary, have the two siphons at opposite ends of their barrel-shaped bodies, and swim by jet propulsion. The 40 species of salps are also small, under 4 cm (1.6 in) long, and found in
790-517: A process of recombination (a primitive form of sexual interaction ). Tunicate Urochordata Lankester, 1877 A tunicate is an exclusively marine invertebrate animal, a member of the subphylum Tunicata ( / ˌ tj uː n ɪ ˈ k eɪ t ə / TEW -nih- KAY -tə ). This grouping is part of the Chordata , a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates ). The subphylum
869-429: A range of forms, and vary in the degree to which individual organisms, known as zooids , integrate with one another. In the simplest systems, the individual animals are widely separated, but linked together by horizontal connections called stolons , which grow along the seabed. Other species have the zooids growing closer together in a tuft or clustered together and sharing a common base. The most advanced colonies involve
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#1732765685061948-414: A rudimentary tailed tadpole stage, which is never free-living and lacks a brain. Tunicates have a well-developed heart and circulatory system . The heart is a double U-shaped tube situated just below the gut. The blood vessels are simple connective tissue tubes, and their blood has several types of corpuscle . The blood may appear pale green, but this is not due to any respiratory pigments, and oxygen
1027-486: A suitable surface, later developing into a barrel-like and usually sedentary adult form. The species in the class Appendicularia are pelagic , and the general larval form is kept throughout life. Also the class Thaliacea is pelagic throughout their lives and may have complex lifecycles. In this class a free living larval stage is absent: Doliolids and pyrosomatids are viviparous–lecithotrophic, and salpids are viviparous–matrotrophic. Only some species of doliolids still have
1106-411: A water-filled, sac-like body structure and two tubular openings, known as siphons, through which they draw in and expel water. During their respiration and feeding, they take in water through the incurrent (or inhalant) siphon and expel the filtered water through the excurrent (or exhalant) siphon. Adult ascidian tunicates are sessile , immobile and permanently attached to rocks or other hard surfaces on
1185-670: Is Shankouclava shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming ( South China ). There is also a common bioimmuration , ( Catellocaula vallata ), of a possible tunicate found in Upper Ordovician bryozoan skeletons of the upper midwestern United States. A well-preserved Cambrian fossil, Megasiphon thylakos , shows that the tunicate basic body design had already been established 500 million years ago. Three enigmatic species were also found from
1264-445: Is a microorganism such as a protist , bacterium , or archaean , composed of a single cell , which may contain functional structures called organelles . A multicellular organism such as an animal , plant , fungus , or alga is composed of many cells, often specialised. A colonial organism such as a siphonophore is a being which functions as an individual but is composed of communicating individuals. A superorganism
1343-401: Is a teleonomic or goal-seeking behaviour that enables them to correct errors of many kinds so as to achieve whatever result they are designed for. Such behaviour is reminiscent of intelligent action by organisms; intelligence is seen as an embodied form of cognition . All organisms that exist today possess a self-replicating informational molecule (genome), and such an informational molecule
1422-741: Is a colony, such as of ants , consisting of many individuals working together as a single functional or social unit . A mutualism is a partnership of two or more species which each provide some of the needs of the other. A lichen consists of fungi and algae or cyanobacteria , with a bacterial microbiome ; together, they are able to flourish as a kind of organism, the components having different functions, in habitats such as dry rocks where neither could grow alone. The evolutionary biologists David Queller and Joan Strassmann state that "organismality" has evolved socially, as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as
1501-426: Is a stalked sea tulip, Pyura pachydermatina , which can grow to be over 1 metre (3.3 ft) tall. The Tunicata were established by Jean-Baptiste Lamarck in 1816. In 1881, Francis Maitland Balfour introduced another name for the same group, "Urochorda", to emphasize the affinity of the group to other chordates. No doubt largely because of his influence, various authors supported the term, either as such, or as
1580-535: Is an argument for viewing viruses as cellular organisms. Some researchers perceive viruses not as virions alone, which they believe are just spores of an organism, but as a virocell - an ontologically mature viral organism that has cellular structure. Such virus is a result of infection of a cell and shows all major physiological properties of other organisms: metabolism , growth, and reproduction , therefore, life in its effective presence. The philosopher Jack A. Wilson examines some boundary cases to demonstrate that
1659-568: Is called Olfactores . The Tunicata contain roughly 3,051 described species, traditionally divided into these classes: Members of the Sorberacea were included in Ascidiacea in 2011 as a result of rDNA sequencing studies. Although the traditional classification is provisionally accepted, newer evidence suggests the Ascidiacea are an artificial group of paraphyletic status. A close relationship between Thaliacea and Ascidiacea, with
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#17327656850611738-443: Is drawn into the body through the buccal siphon by the action of cilia lining the gill slits. To obtain enough food, an average ascidian needs to process one body-volume of water per second. This is drawn through a net lining the pharynx which is being continuously secreted by the endostyle. The net is made of sticky mucus threads with holes about 0.5 μm in diameter which can trap planktonic particles including bacteria . The net
1817-442: Is formed from proteins and carbohydrates, and acts as an exoskeleton . In some species, it is thin, translucent, and gelatinous, while in others it is thick, tough, and stiff. About 3,000 species of tunicate exist in the world's oceans, living mostly in shallow water. The most numerous group is the ascidians ; fewer than 100 species of these are found at depths greater than 200 m (660 ft). Some are solitary animals leading
1896-472: Is likely intrinsic to life. Thus, the earliest organisms also presumably possessed a self-replicating informational molecule ( genome ), perhaps RNA or an informational molecule more primitive than RNA. The specific nucleotide sequences in all currently extant organisms contain information that functions to promote survival, reproduction , and the ability to acquire resources necessary for reproduction, and sequences with such functions probably emerged early in
1975-503: Is lost by the time they have completed their metamorphosis. As members of the Chordata, they are true Coelomata with endoderm , ectoderm , and mesoderm , but they do not develop very clear coelomic body cavities, if any at all. Whether they do or not, by the end of their larval development, all that remain are the pericardial , renal, and gonadal cavities of the adults. Except for the heart , gonads, and pharynx (or branchial sac),
2054-461: Is rolled up on the dorsal side of the pharynx, and it and the trapped particles are drawn into the esophagus . The gut is U-shaped and also ciliated to move the contents along. The stomach is an enlarged region at the lowest part of the U-bend. Here, digestive enzymes are secreted and a pyloric gland (absent in appendicularians) adds further secretions. After digestion, the food is moved on through
2133-436: Is surrounded by a test or tunic, from which the subphylum derives its name. This varies in thickness between species but may be tough, resembling cartilage, thin and delicate, or transparent and gelatinous. The tunic is composed of proteins, crosslinked by phenoloxidase reaction, and complex carbohydrates, and includes tunicin , a variety of cellulose. The tunic is unique among invertebrate exoskeletons in that it can grow as
2212-408: Is the "upper lip" which is in the front area of the head and is the most exterior part. A pair of mandibles is found on the backside of the labrum flanking the side of the mouth, succeeded by a pair of maxillae each of which is known as maxilliary palp . At the back side of the mouth is the labium or lower lip. There is also an extra mouth part in some insects which is termed as hypopharynx which
2291-490: Is the equivalent of the vertebrate brain. From this comes the common saying that the sea squirt "eats its own brain". However, the adult does possess a cerebral ganglion adapted to lack of self-locomotion. In the Thaliacea, the larval stage is rudimentary or suppressed, and the adults are pelagic (swimming or drifting in the open sea). Colonial forms also increase the size of the colony by budding off new individuals to share
2370-475: Is transported dissolved in the plasma . Exact details of the circulatory system are unclear, but the gut, pharynx, gills, gonads, and nervous system seem to be arranged in series rather than in parallel, as happens in most other animals. Every few minutes, the heart stops beating and then restarts, pumping fluid in the reverse direction. Tunicate blood has some unusual features. In some species of Ascidiidae and Perophoridae , it contains high concentrations of
2449-411: Is unique to tunicates. Excess photosynthetic products are assumed to be available to the host . Ascidians are almost all hermaphrodites and each has a single ovary and testis, either near the gut or on the body wall. In some solitary species, sperm and eggs are shed into the sea and the larvae are planktonic . In others, especially colonial species, sperm is released into the water and drawn into
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2528-422: Is usually located between the maxillac . Though invertebrate chordates – such as the tunicate larvae or the lancelets – have heads, there has been a question of how the vertebrate head, characterized by a bony skull clearly separated from the main body, might have evolved from the head structures of these animals. According to Hyman (1979), the evolution of the head in the vertebrates has occurred by
2607-548: The Ancient Greek ὀργανισμός , derived from órganon , meaning instrument, implement, tool, organ of sense or apprehension) first appeared in the English language in the 1660s with the now-obsolete meaning of an organic structure or organization. It is related to the verb "organize". In his 1790 Critique of Judgment , Immanuel Kant defined an organism as "both an organized and a self-organizing being". Among
2686-644: The Ediacaran period – Ausia fenestrata from the Nama Group of Namibia , the sac-like Yarnemia ascidiformis , and one from a second new Ausia -like genus from the Onega Peninsula of northern Russia , Burykhia hunti . Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians. Ausia and Burykhia lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be
2765-410: The anterior region, forming structures responsible for information processing. Through biological evolution, sense organs and feeding structures also concentrate into the anterior region; these collectively form the head. The human head is an anatomical unit that consists of the skull , hyoid bone and cervical vertebrae . The term "skull" collectively denotes the mandible (lower jaw bone) and
2844-443: The cephalon , or cephalic region, is the region of the head which is a collective of "fused segments". A typical insect head is composed of eyes, antennae , and components of mouth. As these components differ substantially from insect to insect, they form important identification links. Eyes in the head found, in several types of insects, are in the form of a pair of compound eyes with multiple faces. In many other types of insects,
2923-405: The cranium (upper portion of the skull that houses the brain). Sculptures of human heads are generally based on a skeletal structure that consists of a cranium, jawbone , and cheekbone . Though the number of muscles making up the face is generally consistent between sculptures, the shape of the muscles varies widely based on the function, development, and expressions reflected on the faces of
3002-404: The intestine , where absorption takes place, and the rectum , where undigested remains are formed into faecal pellets or strings. The anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial siphon. Here, the faeces are caught up by the constant stream of water which carries the waste to the exterior. The animal orientates itself to the current in such a way that
3081-642: The "defining trait" of an organism. Samuel Díaz‐Muñoz and colleagues (2016) accept Queller and Strassmann's view that organismality can be measured wholly by degrees of cooperation and of conflict. They state that this situates organisms in evolutionary time, so that organismality is context dependent. They suggest that highly integrated life forms, which are not context dependent, may evolve through context-dependent stages towards complete unification. Viruses are not typically considered to be organisms, because they are incapable of autonomous reproduction , growth , metabolism , or homeostasis . Although viruses have
3160-766: The Integrated Taxonomic Information System. Various common names are used for different species. Sea tulips are tunicates with colourful bodies supported on slender stalks. Sea squirts are so named because of their habit of contracting their bodies sharply and squirting out water when disturbed. Sea liver and sea pork get their names from the resemblance of their dead colonies to pieces of meat. Tunicates are more closely related to craniates (including hagfish , lampreys , and jawed vertebrates ) than to lancelets , echinoderms , hemichordates , Xenoturbella or other invertebrates . The clade consisting of tunicates and vertebrates
3239-604: The Permian and the Triassic, there were also forms with a calcareous exoskeleton. At first, they were mistaken for corals. A multi-taxon molecular study in 2010 proposed that sea squirts are descended from a hybrid between a chordate and a protostome ancestor (before the divergence of panarthropods and nematodes ). This study was based on a quartet partitioning approach designed to reveal horizontal gene transfer events among metazoan phyla. Colonies of tunicates occur in
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3318-407: The animal enlarges and does not need to be periodically shed. Inside the tunic is the body wall or mantle composed of connective tissue , muscle fibres, blood vessels , and nerves . Two openings are found in the body wall: the buccal siphon at the top through which water flows into the interior, and the atrial siphon on the ventral side through which it is expelled. A large pharynx occupies most of
3397-407: The atria of other individuals with the incoming water current. Fertilization takes place here and the eggs are brooded through their early developmental stages. Some larval forms appear very much like primitive chordates with a notochord (stiffening rod) and superficially resemble small tadpoles . These swim by undulations of the tail and may have a simple eye, an ocellus , and a balancing organ,
3476-420: The atrium. Tunicates are unusual among animals in that they produce a large fraction of their tunic and some other structures in the form of cellulose . The production in animals of cellulose is so unusual that at first some researchers denied its presence outside of plants, but the tunicates were later found to possess a functional cellulose synthesizing enzyme , encoded by a gene horizontally transferred from
3555-519: The buccal siphon is always upstream and does not draw in contaminated water. Some ascidians that live on soft sediments are detritivores . A few deepwater species, such as Megalodicopia hians , are sit-and-wait predators , trapping tiny crustacea, nematodes, and other small invertebrates with the muscular lobes which surround their buccal siphons. Certain tropical species in the family Didemnidae have symbiotic green algae or cyanobacteria in their tunics, and one of these symbionts, Prochloron ,
3634-543: The compound eyes are seen in a "single facet or group of single facets". In some cases, the eyes may be seen as marks on the dorsal or located near or toward the head, two or three ocelli (single faceted organs). Antennae on the insect's head is found in the form of segmented attachments, in pairs, that are usually located between the eyes. These are in varying shapes and sizes, in the form of filaments or in different enlarged or clubbed form. Insects have mouth parts in various shapes depending on their feeding habits. Labrum
3713-678: The concept of organism is not sharply defined. In his view, sponges , lichens , siphonophores , slime moulds , and eusocial colonies such as those of ants or naked molerats , all lie in the boundary zone between being definite colonies and definite organisms (or superorganisms). Scientists and bio-engineers are experimenting with different types of synthetic organism , from chimaeras composed of cells from two or more species, cyborgs including electromechanical limbs, hybrots containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed. An evolved organism takes its form by
3792-476: The criteria that have been proposed for being an organism are: Other scientists think that the concept of the organism is inadequate in biology; that the concept of individuality is problematic; and from a philosophical point of view, question whether such a definition is necessary. Problematic cases include colonial organisms : for instance, a colony of eusocial insects fulfills criteria such as adaptive organisation and germ-soma specialisation. If so,
3871-399: The embryonic neural tube and are located between the two siphons. Nerves arise from the two ends of the ganglion; those from the anterior end innervate the buccal siphon and those from the posterior end supply the rest of the body, the atrial siphon, organs, gut and the musculature of the body wall. There are no sense organs but there are sensory cells on the siphons, the buccal tentacles and in
3950-422: The evolution of life. It is also likely that survival sequences present early in the evolution of organisms included sequences that facilitate the avoidance of damage to the self-replicating molecule and promote the capability to repair such damages that do occur. Repair of some of the genome damages in these early organisms may have involved the capacity to use undamaged information from another similar genome by
4029-435: The exhalent siphon for the new, four-zooid colony. Doliolids have a very complex life cycle that includes various zooids with different functions. The sexually reproducing members of the colony are known as gonozooids. Each one is a hermaphrodite with the eggs being fertilised by sperm from another individual. The gonozooid is viviparous , and at first, the developing embryo feeds on its yolk sac before being released into
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#17327656850614108-681: The former possibly emerging from the latter, had already been proposed since the early 20th century under the name of Acopa. The following cladogram is based on the 2018 phylogenomic study of Delsuc and colleagues. Oikopleuridae [REDACTED] Kowalevskiidae Fritillariidae [REDACTED] Pyrosomida [REDACTED] Salpida [REDACTED] Doliolida [REDACTED] Phlebobranchia [REDACTED] Aplousobranchia [REDACTED] Molgulidae [REDACTED] Styelidae [REDACTED] Pyuridae [REDACTED] Undisputed fossils of tunicates are rare. The best known and earliest unequivocally identified species
4187-435: The fusion of a fixed number of anterior segments, in the same manner as in other "heteronomously segmented animals". In some cases, segments or a portion of the segments disappear. The head segments also lose most of their systems, except for the nervous system. With the progressive development of cephalization, "the head incorporates more and more of the adjacent segments into its structure, so that in general it may be said that
4266-437: The higher the degree of cephalization the greater is the number of segments composing the head". In the 1980s, the "new head hypothesis" was proposed, suggesting that the vertebrate head is an evolutionary novelty resulting from the emergence of neural crest and cranial placodes . In 2014, a transient larva tissue of the lancelet was found to be virtually indistinguishable from the neural crest-derived cartilage which forms
4345-499: The integration of the zooids into a common structure surrounded by the tunic. These may have separate buccal siphons and a single central atrial siphon and may be organized into larger systems, with hundreds of star-shaped units. Often, the zooids in a colony are tiny but very numerous, and the colonies can form large encrusting or mat-like patches. By far the largest class of tunicates is the Ascidiacea . The body of an ascidiacean
4424-410: The interior of the body. It is a muscular tube linking the buccal opening with the rest of the gut. It has a ciliated groove known as an endostyle on its ventral surface, and this secretes a mucous net which collects food particles and is wound up on the dorsal side of the pharynx. The gullet, at the lower end of the pharynx, links it to a loop of gut which terminates near the atrial siphon. The walls of
4503-404: The kidney-like metanephridial organs typical of deuterostomes . Most have no excretory structures, but rely on the diffusion of ammonia across their tissues to rid themselves of nitrogenous waste, though some have a simple excretory system. The typical renal organ is a mass of large clear-walled vesicles that occupy the rectal loop, and the structure has no duct. Each vesicle is a remnant of
4582-405: The lower end at the fourth stage. Eyes and chins are fitted in various shapes to form the head. Leonardo da Vinci , considered one of the world's greatest artists, drew sketches of human anatomy using grid structures. His image of the face drawn on the grid structure principle is in perfect proportion. In this genre, using the technique of pen and ink, Leonardo created a sketch which is a "Study on
4661-476: The ocean floor. Thaliaceans (pyrosomes, doliolids, and salps) and larvaceans on the other hand, swim in the pelagic zone of the sea as adults. Various species of ascidians , the most well-known class of tunicates, are commonly known as sea squirts , sea pork, sea livers, or sea tulips . The earliest probable species of tunicate appears in the fossil record in the early Cambrian period . Their name derives from their unique outer covering or "tunic", which
4740-704: The oldest evidence of the chordate lineage of metazoans. The Russian Precambrian fossil Yarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia , so this identification has been questioned. Fossils of tunicates are rare because their bodies decay soon after death, but in some tunicate families, microscopic spicules are present, which may be preserved as microfossils. These spicules have occasionally been found in Jurassic and later rocks, but, as few palaeontologists are familiar with them, they may have been mistaken for sponge spicules . In
4819-466: The organs are enclosed in a membrane called an epicardium , which is surrounded by the jelly-like mesenchyme . Ascidian tunicates begin life as a lecithotrophic (non-feeding) mobile larva that resembles a tadpole, with the exception of some members of the families Styelidae and Molgulidae which has direct development. The latter also have several species with tail-less larval forms. The ascidian larvae very rapidly settle down and attach themselves to
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#17327656850614898-414: The outer surface of the tunic, where their presence is thought to deter predation , although it is unclear whether this is due to the presence of the metal or low pH. Other species of tunicates concentrate lithium , iron , niobium , and tantalum , which may serve a similar function. Other tunicate species produce distasteful organic compounds as chemical defenses against predators. Tunicates lack
4977-501: The partially understood mechanisms of evolutionary developmental biology , in which the genome directs an elaborated series of interactions to produce successively more elaborate structures. The existence of chimaeras and hybrids demonstrates that these mechanisms are "intelligently" robust in the face of radically altered circumstances at all levels from molecular to organismal. Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common
5056-488: The pharynx are perforated by several bands of slits, known as stigmata, through which water escapes into the surrounding water-filled cavity, the atrium. This is criss-crossed by various rope-like mesenteries which extend from the mantle and provide support for the pharynx, preventing it from collapsing, and also hold up the other organs. The Thaliacea , the other main class of tunicates, is characterised by free-swimming, pelagic individuals. They are all filter feeders using
5135-434: The proportions of head and eyes" (pictured). An idiom is a phrase or a fixed expression that has a figurative, or sometimes literal, meaning. The head's function and appearance play an analogous role in the etymology of many technical terms. Cylinder head , pothead , and weatherhead are three such examples. Organism The evolutionary biologists David Queller and Joan Strassmann state that "organismality",
5214-480: The qualities or attributes that define an entity as an organism, has evolved socially as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as the "defining trait" of an organism. This would treat many types of collaboration, including the fungus / alga partnership of different species in a lichen , or the permanent sexual partnership of an anglerfish , as an organism. The term "organism" (from
5293-612: The same argument, or a criterion of high co-operation and low conflict, would include some mutualistic (e.g. lichens) and sexual partnerships (e.g. anglerfish ) as organisms. If group selection occurs, then a group could be viewed as a superorganism , optimized by group adaptation . Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately rather than demanding that an organism should have all of them; if so, there are multiple dimensions to biological individuality, resulting in several types of organism. A unicellular organism
5372-417: The same tunic. Pyrosome colonies grow by budding off new zooids near the posterior end of the colony. Sexual reproduction starts within a zooid with an internally fertilized egg. This develops directly into an oozooid without any intervening larval form. This buds precociously to form four blastozooids which become detached in a single unit when the oozoid disintegrates. The atrial siphon of the oozoid becomes
5451-425: The sea as a free-swimming, tadpole-like larva. This undergoes metamorphosis in the water column into an oozooid. This is known as a "nurse" as it develops a tail of zooids produced by budding asexually . Some of these are known as trophozooids, have a nutritional function, and are arranged in lateral rows. Others are phorozooids, have a transport function, and are arranged in a single central row. Other zooids link to
5530-419: The shape of the head in the shape of an egg. The female head, in particular, is sketched in a double circle design procedure with proportions considered as an ideal of a female head. In the first circle, the division is made of five sections on the diameter , each section of five eyes width. It is then developed over a series of ten defined steps, with the smaller circle imposed partially over the larger circle at
5609-450: The shoulders proper," and the bust of a queen occurs in the arms of Queenborough, Kent . Infants' or children's heads are often couped at the shoulders with a snake wrapped around the neck (e.g. "Argent, a boy's head proper, crined or, couped below the shoulders, vested gules, tarnished gold," in the arms of Boyman). One of the ways of drawing sketches of heads—as Jack Hamm advises—is to develop it in six well-defined steps, starting with
5688-496: The slightly older "Urochordata", but this usage is invalid because "Tunicata" has precedence, and grounds for superseding the name never existed. Accordingly, the current (formally correct) trend is to abandon the name Urochorda or Urochordata in favour of the original Tunicata, and the name Tunicata is almost invariably used in modern scientific works. It is accepted as valid by the World Register of Marine Species but not by
5767-563: The subjects. Proponents of identism believe that the mind is identical to the brain. Philosopher John Searle asserts his identist beliefs, stating "the brain is the only thing in the human head". Similarly, Dr. Henry Bennet-Clark has stated that the head encloses billions of "miniagents and microagents (with no single Boss)". The evolution of a head is associated with the cephalization that occurred in Bilateria some 555 million years ago. In some arthropods , especially trilobites ,
5846-469: The surface waters of both warm and cold seas. They also move by jet propulsion, and often form long chains by budding off new individuals. A third class, the Larvacea (or Appendicularia), is the only group of tunicates to retain their chordate characteristics in the adult state, a product of extensive neoteny . The 70 species of larvaceans superficially resemble the tadpole larvae of amphibians, although
5925-401: The tail is at right angles to the body. The notochord is retained, and the animals, mostly under 1 cm long, are propelled by undulations of the tail. They secrete an external mucous net known as a house, which may completely surround them and is very efficient at trapping planktonic particles. Like all other chordates , tunicates have a notochord during their early development, but it
6004-431: The transitional metal vanadium and vanadium-associated proteins in vacuoles in blood cells known as vanadocytes . Some tunicates can concentrate vanadium up to a level ten million times that of the surrounding seawater. It is stored in a +3 oxidation form that requires a pH of less than 2 for stability, and this is achieved by the vacuoles also containing sulfuric acid . The vanadocytes are later deposited just below
6083-413: The vertebrate skull, suggesting that persistence of this tissue and expansion into the entire headspace could be a viable evolutionary route to formation of the vertebrate head. The heads of humans and other animals are commonly recurring charges in heraldry . Heads of humans are sometimes blazoned simply as a "man's head", but are far more frequently described in greater detail, either characteristic of
6162-454: Was at one time called Urochordata , and the term urochordates is still sometimes used for these animals. Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord , a hollow dorsal nerve cord , Pharyngeal slits , post-anal tail, and an endostyle . They resemble
6241-459: Was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. As for reproduction, viruses rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms, but the genes have a cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts. There
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