Fish

Fish

Fish, any of the approximately 34,000 species of vertebrate animals (worldwide) are found in fresh and saltwater. Living species range from primitive jaw lampreys and hagfish through cartilaginous sharks, skates and rays to abundant and diverse bony fishes. Most fish species are cold-headed; However, one species, Opah (Lamprice guttatus), is rife with warmth.

The term fish is applied to a variety of vertebrates of many evolutionary lines. It describes a life-form rather than a classification group. As members of Felium chordata, the fish share some characteristics with other vertebrates. These features are gill slits, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail at some point of the life cycle.

Living fish represent some five classes, which are indistinguishable from each other, such as the four classes of familiar air-breathing animals - amphibians, reptiles, birds, and mammals. For example, jawed fishes (Agnatha) have gills in the sac and lack limbs. The extant Agnathan are lampreys and hagfish. As is clear from the name itself, the skeletons of the fishes of the class chondrichchiates (chondr, "cartilage," and ichthys, "fish") are made entirely of cartilage.

Distribution and abundance

Fish life occurs in almost all natural bodies of water, exceptions for very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The current distribution of fishes is a consequence of the geological history and development of the earth as well as the evolution of fishes to undergo evolutionary changes and adapt to available habitats. Fishes can be distributed according to habitat and according to geographical area. The major habitat differences are marine and freshwater.

For the most part, fishes in a marine habitat are isolated from freshwater habitats, even in nearby areas, but some, such as salmon, move from one to another. Many types of freshwater habitats can be seen. Fishes found in mountain ridges, arctic lakes, tropical lakes, temperate rivers, and tropical rivers will all differ from each other, both in markedly macro-structure and physical characteristics.

Even in nearby habitats, where, for example, a tropical mountain stream enters a lowland stream, fish fauna will be different. Marine habitats can be subdivided into deep sea floors (benthic), mid-oceanic (bathypegic), surface oceanic (pelagic), rocky shores, sandy shores, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal coasts in tropical and temperate regions will have different fish fauna, even when there are such habitats along the same coastline.

Although much is known about the current geographic distribution of fishes, little is known about this distribution yet. Many parts of the freshwater fish fauna of North America and Eurasia are related and undoubtedly have a common origin. The fauna of Africa and South America are related, much older, and probably an expression of drifting apart from two continents. The fauna of Southern Asia belongs to Central Asia, and some of it enters Africa.

The very large coast-fish fauna of the Indian and tropical Pacific Oceans includes a related complex, but the tropical shore fauna of the Atlantic, although consisting of Indo-Pacific components, is relatively limited and probably young. Arctic and Antarctic sea creatures are quite different from each other. The coastal region of the North Pacific is quite different, and it is more limited and perhaps smaller in the North Atlantic.

Pelagic marine fishes, especially those living in deep waters, are similar worldwide, showing little geographic isolation in terms of family groups. Intensive oceanic habitat is very similar worldwide, but species differences exist, reflecting geographic areas and geographical areas determined by water mass.

Natural History
Life history

All aspects of fish life are closely linked with adaptation to the total environment, physical, chemical and biological. In the study, all the interdependent aspects of the fish, such as behavior, movement, reproduction and physiological and physiological characteristics should be taken into consideration.

There is a very wide variety of life cycles correlated with their adaptation to an extremely wide variety of habitats that fish exhibit. The eggs hatch with a large majority hatch from relatively small eggs to a few days to several weeks or longer after shattering in water. Newly married young are still partially underdeveloped and are called larvae until body structures such as wings, skeletons, and some organs are fully formed.

The life of the larva is often very short, usually less than a few weeks, but it can be very long, with some lampreys continuing as larvae for at least five years. Young and larval fishes must grow significantly before reaching sexual maturity, and their small size and other factors often determine that they live in different habitats than adults. For example, pelvic larvae occur in most tropical seashore fishes. Larval feeding is also different, and larval fishes often live in shallow waters, where they may be exposed to fewer predators.

After a fish reaches adult size, its length of life is subject to several factors, such as the rate of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium has nothing to do with how long it lives in that wild. Many small fish live for one to three years at most. However, in some species, individuals may live up to 10 or 20 or even 100 years.

The behavior

The behavior of fish is a complex and varied subject. In almost all animals with a central nervous system, the nature of an individual fish's response to stimuli from its environment depends on the inherited characteristics of its nervous system, from what it has learned from previous experience, and on nature. Stimuli. Compared with the diversity of human responses, however, is the stereotype of a fish, not subject to much modification from "thought" or learning, and investigators must guard against anthropological interpretations of fish behavior.

Fishes experience the world around them with normal senses of sight, smell, hearing, touch and taste, and specialized lateral line water-current detectors. In some fish generating electric fields, a process that can be called electroallocation aids in perception. One or the other of these senses is often emphasized by the other's adaptation to the fish, at the expense of the others. In fishes with large eyes, the sense of smell may be reduced; Others, with small eyes, hunt and feed primarily by smell (such as some eels).

Special behavior is mainly concerned with the three most important activities in the life of fish: feeding, breeding, and escape from enemies. The schooling behavior of sardines on the high seas, for example, is largely a protective tool to avoid enemies, but is also linked to and modified by their breeding and feeding requirements. Predatory fishes are often solitary, suddenly waiting for a dart after their prey, a type of movement impossible for beak parrot fishes, which feed on coral, small groups from one coral head to the next. Swims in In addition, some predatory fishes, such as tunas, often live in pelvic environments at school.

Sleep in fish, all of which lack true eyelids, is in a properly streamlined state in which the fish maintain their balance and move slowly. If attacked or harassed, most people can get away. Some types of fish lay on the floor to sleep. Most catfish, some lochs, and some eel and electric fish are strictly nocturnal, active and hunting for food during the night and retiring during the day in holes, thick vegetation, or other protective parts of the environment.

Communication between members of a species or between members of two or more species is often extremely important, especially in reproductive behavior (see below breeding). The mode of communication can be visual, such as small so-called cleaner fish and a larger fish of very different species. Larger fish often allow cleaners to enter their mouths to remove gill parasites. The cleaner is identified by its specific color and functions and is therefore not eaten, even if the larger fish are normally predatory. Communication is often chemical, the signals sent by specific chemicals known as pheromones.

Activity

Many fish have a streamlined body and swim freely in open water. Is closely associated with the habitat and ecological niche of the fish (the animal's general condition for its environment). Many fish swim both on the surface in both marine and freshwater and are best suited to feeding (and sometimes only) on the surface.

Often such fish are long and thin, which are able to dart on surface insects or other surface fish and in turn move away from predators; Needlefish, halfback and topaminoses (such as marishfish and mosquito fish) are good examples. Oceanic fly fishes escape their predators by gathering momentum above the water's surface, pushing the lower lobe of the tail into the water. They then divide hundreds of yards on elongated, winged pectoral and pelvic fins. South American freshwater fishes jump and jump their enemies out of the water and rescue their enemies.

So-called mid-water swimmers, the most common type of fish, are of many types and live in many habitats. Powerful fusiform tuna and trout, for example, are adapted for strong, fast swimming, trout to cope with the fast currents of dunes and streams and rivers for rapid capture in the open sea. The trout body form is well adapted to many habitats. Fishes that live in relatively calm waters such as bays or lakes or slow-moving rivers are generally not strong, fast, but capable of short, quick bursts to avoid a predator.

The arms of many of these fish are flattened, for example the freshwater angles of sunflowers and aquarists. Fish attached to the bottom or substrate are usually slow swimmers. Open-water plankton-eating fishes are almost always tasteful and are capable of fast, strong movements (for example, sardines and herds of the open ocean and many small towers of streams and lakes).

There are many types of downstream fish and there are many types of modifications in their body shape and swimming habits. The rays, which develop from strong-swimming mid-water sharks, usually stay close to the bottom and move with their large pectoral fins removed. The flounders live in a similar habitat and move downwards, lowering the entire body. Many bottomed fish dart from place to place, resting downward between movements, a motion common in gobies.

A gobby relative, the mudskipper, has taken to the edge of the pool on the edge of the muddy mangrove swamps. It escapes its enemies by rapidly flowing into the mud out of the water. Some catfish, synchranched eels, so-called climbing perch, and some other fish are out on moist land, finding more promising waters than the water they release. They move forward, ripping their bodies, sometimes using strong pectoral fins; Most have minor air-breathing organs.

Many downstream fish live in mud holes or rocky crevices. Marine eels and gobies are commonly found in such habitats and for the most part enterprises are beyond their cavelike houses. Some downstream habitats, such as clingfish (Gobiosokida), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky shores, where the action of the waves is great.

Reproduction

Breeding methods in fish are diverse, but most fish lay large numbers of small eggs, fertilized and scattered outside the body. The eggs of pelvic fishes are usually suspended in open water. Many shore and freshwater fish lay eggs below or between plants. Some have cling eggs. The mortality rate of young and especially eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs.

Males produce sperm, usually in the form of a milky white substance called milt, tested in two (sometimes) within the body cavity. In bony fishes, a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in a cyclone the duct leads to a cloaca. Pelvic fins are sometimes modified to help spread eggs on the woman's vent or on the substrate where the woman has placed them. Sometimes used to internally fertilize minor organs - for example, many sharks and ray clashers.

In females, eggs are formed in two ovaries (sometimes only one) and pass from the ovary to the urogenital opening and exit. In some fish the eggs are fertilized internally but are shed before they develop. Members of about a dozen families live each young of bony fishes (telests) and sharks. Many skates and rays also remain young. In some bony fishes, eggs develop only within the female, young when incubated (Dimbavahini).

Others develop within the ovary and are nourished by the tissues of the ovary after hatching (viviparous). Other methods are used by fish to nourish the young within the female. In all living individuals, young are born in relatively large sizes and are few in number. Mainly in a family of marine fishes, surfers from the Pacific coast of North America, Japan, and Korea, males of at least one species are sexually mature, although they are not fully developed.

Some fish are hermaphroditic - a person that produces both sperm and eggs, usually at different stages of its life. Self-fertilization, however, rarely occurs.

Successful breeding and, in many cases, eggs and young are protected not only by stereotyping but often courtship and parental behavior, either by man or woman or both. Some fish build nests by hollowing out sediments (eg, for example) in the sand floor, building nests with plant material and sticky threads emitted by kidneys (stickbacks), or mucus on the water surface (mucus). A group of bubbles). Eggs are laid in these structures. Cichlids and some varieties of catfish incubate eggs in their mouths.

Some fish, such as salmon, migrate longitudinally from the sea and raise large rivers in gravel beds, where they sit on their own. Some, such as freshwater eels (family Anguillidae), survive to maturity in freshwater and thrive and migrate to the sea (catadromous fishes). Other fish make short migrations from lakes to streams, within the ocean, or to habitats where they do not live organically by other means.

Form and function
Body plan

The basic structure and function of the body of the fish are similar to all other vertebrates. Four types of normal tissue exist: surface or epithelium, connective (bone, cartilage, and fibrous tissue, as well as their derivatives, blood), nerve and muscle tissue. In addition, fish organs and organ systems are parallel to other vertebrates.

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter in the midline just below the gill chamber. The body cavity, consisting of vital organs, is located behind the head in the lower anterior part of the body. The anus usually marks the posterior end of the body cavity and is most often in front of the base of the anal fin.

The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is made up of muscle tissue, a high proportion of which is required by swimming. In order of development, this basic body plan has been revised repeatedly in the shape of many types of fish present today.

The skeleton is an integral part of the fish's movement system, as well as serving to protect vital parts. The inner skeleton consists of skull bones (except for the roof bones of the head, which are actually part of the outer skeleton), vertebral column, and fin supports (fin ray). The fin supports are derived from the outer skeleton, but will be treated here due to their close functional relationship with the inner skeleton.

The cyclostome is the internal skeletal cartilage of sharks and rays; Many of the fossil groups and some of the primitive living fishes belong to cartilage, but may include some bone. In place of the vertebral column, the earliest vertex was a fully developed nochord, a flexible rigid rod of viscous cells surrounded by a strong fibrous sheath. During the development of modern fishes, the rod was replaced in part by cartilage and then by bone cartilage.

Sharks and rays maintain a cartilaginous vertebral column; Bony fishes have spool-shaped vertebrae that partially replace the notochord in more primitive living forms. The skull is completely, or at least partially, ossified, including the jaws of gill arches and bony fishes. These sharks and rays remain cartilaginous, sometimes partially altered by calcium deposition but never by true bone.

The supporting elements of the wings (basal or radial bones, or both) have changed greatly during fish development. Some of these changes are described in the section below (Evolution and Paleontology). Most fish have a single dorsal fin on the midline of the back. Many have two and some have three dorsal fins. Other feathers are single tail and anal fins and paired pelvic and pectoral fins. A small fin, the fat fin, which has hyalaric fin rays, occurs in many of the relatively primitive telestomes (such as trout) on the back near the base of the caudal fin.

The skin

The skin of a fish must serve many functions. It is helpful in maintaining osmotic balance, provides physical protection for the body, is a site of coloration, has sensory receptors, and, in some fish, functions in respiration. Mucous glands, which help maintain water balance and provide protection from bacteria, are very high in fish skin, particularly in cyclostomes and telosts. Since mucous glands are present in modern lampreys, it is reasonable to assume that they were present in primitive fishes such as the ancient Silurian and Devonian Aganathan.

Protection from friction and predation is another function of fish skin, and the dermal (skin) bone arose early in fish development in response to this requirement. It is believed that the bone first developed in the skin and later invaded the cartilaginous areas of the fish's body, to provide additional support and protection. There is some argument that came before, cartilage or bone, and fossil evidence do not dispose of the question. In any event, dermal bone has played an important role in the development of fish and different groups of fish have different characteristics. Many groups are characterized, at least in part, by the bony scales that they possess.

Scales have played an important role in the development of fish. Primitive fishes typically had thick bony plates or thick scales in several layers of bone, enamel and related substances. Modern teleost fishes have bone scales, which are still protective, allowing greater freedom of motion in the body. Some modern telosts (some catfish, sticklebacks and others) have acquired bony plates for a second time in the skin.

Modern and early sharks had placoid scales, a relatively primitive type with a tooth structure, consisting of an outer layer of enamel-like substance (vitroidentantine), an inner layer of dentin, and a pulp cavity of veins and blood vessels. Primitive bony fishes had thicker scales of ganoid or kosmoid type. Cosmod scales consist of a rigid, enamel-like outer layer, an inner layer of cosmin (a form of dentine), and then a layer of vascular bone (isopedine).

The rigid outer layer in the ganoid scales is chemically differentiated and is called ganoin. It consists of a cosmic layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloids and catenoids (the latter distinguished by distinctions at the edges), lack enamel and dentine layers.

Skin has many other functions in fishes. It is well supplied with nerve endings and possibly elicits tactile, thermal and pain stimuli. The skin is also well supplied with blood vessels. The exchange of oxygen and carbon dioxide between the surrounding water and several small blood vessels near the surface of the skin causes some fish to run through the skin.

The skin performs protection through the control of dyeing. Fish exhibit an almost limitless range of colors. The colors often blend closely with the surroundings, effectively hiding the animal.

Many fish use bright colors for territorial advertising or as an identification mark for other members of their species or sometimes for members of other species. The movement of pigments within pigment cells (chromatophores) can cause many fish to change their color to a greater or lesser degree. Black pigment cells (melanophores) of almost universal occurrence in fishes are often combined with other pigment cells.

When placed under iridocytes or leucophores (bearing silvery or white pigment guanine), melanophores produce blue and green structural colors. These colors are often extremely intense, as they are formed by refraction of light through Guanin's needle-like crystals. The refracted colors of blue and green are often relatively pure, lacking red and yellow rays, which are absorbed by the black pigment (melanin) of melanophores. The yellow, orange and red colors are produced by erythrophorus, cells with appropriate carotenoid pigments. Other dyes are formed by combining melanophores, erythrophores and iridocytes.

Muscle system

Most fish have muscles in the major part of their body. Most of the mass is trunk musculature, the wing muscles are usually relatively small. The caudal fin is usually the most powerful feather, transferred by trunk musculature. The musculature of the body is usually arranged in rows of chevron-shaped segments on each side. The contractions of these segments, each associated with adjacent vertebral and vertebral processes, tilt the body at the vertebral joint, causing gradual undesirableness of the body, passing from head to tail, and producing a driving stroke of the tail. It is the latter that provides strong anterograde movement for most fish.

Digestive System

The digestive system, in a functional sense, begins with the mouth, which is used to catch prey or collect plant foods. The size of the mouth and the structure of the teeth vary greatly in fishes, depending on the food they usually eat. Most fishes are anterior, feeding on small invertebrates or other fishes and have simple conical teeth on at least some bones on the jaw, and on the jaw, in front of the jaw roof, especially on gill structures.

The latter are throat teeth. Most predatory fishes swallow their prey completely, and the teeth are used to catch the grasping and prey, to swallow prey (head first) and to prey work toward the esophagus. Fishes have different types of teeth. Some fish, such as sharks and piranhas, bite teeth to bite the chicks that emerge from their prey.

A shark's tooth, although superficially like a piranha's, appears in many ways on a modified scale, while piranhas resemble other bony fishes consisting of teeth and enamel. Parrot fishes have small incision teeth to break the moong and have heavy teeth around their throat to crush the coral. Some catfish have small brushing teeth, arranged in rows on the jaw, to scrape plants and grow animals from rocks. Many fish (such as Cyprinidae or Minnows) do not have jaw teeth at all, but their throats are very strong.

Some fish gather plankton food from their gill bunches with several long hard rods (gill rakes) anchored from one end to the end of the gill. The food collected on these rods goes to the throat, where it is swallowed. Most fish only have gill gillers which help prevent food particles from leaking out into the gill chamber.

Once reaching the throat, the food enters a small, often very condensed esophagus, a simple tube containing a muscular wall in the stomach. Stomach in fish varies greatly depending on diet. In most anterior fishes, it is a simple straight or curved tube or sac with a muscular wall and a glandular layer. Food is digested to a great extent and leaves the stomach in liquid form.

The ducts enter the digestive tract from the liver and pancreas, between the stomach and intestine. The liver is a large, clearly defined organ. The pancreas may become embedded in it, spread through it, or break up into smaller parts that are spread over parts of the intestine. The junction between the stomach and intestine is marked by a muscular valve. Some fish have pyloric seca (blind sax) at this junction and have digestive or absorption functions or both.

The gut itself is quite variable in length, depending on the diet of the fish. It is shorter in anterior forms, sometimes not longer than in the body cavity, but in longer vein forms, being coiled and in some species of South American catfish several times longer than the full length of the fish is. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorption capacity and the spiral valve is a method of increasing its absorption surface.

Sharks, rays, chimeras, lamefish, live chondrostines, holosteins, and even some of the more primitive teleosts have at least traces in a spiral valve or intestine. Most modern teleosts have increased the area of the intestinal walls manifold and villi (projections of the fingers) somewhat like humans. Most telost fishes do not pass external substances through the anus. In lung fishes, sharks and rays, it is first passed through the cloaca, a normal cavity that receives the intestinal opening and ducts from the urogenital system.

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The respiratory system

Oxygen and carbon dioxide dissolve in water, and most fish dissolve in water and exchange oxygen and carbon dioxide. The gills are at the back and at the back of the cavity of the mouth and consist of fleshy fibers supported by gill arches and filled with blood vessels, giving the gills a bright red color. Water continuously taken through the mouth passes backwards between the gill bar and the gill filaments, where the gases exchange.

Gills are protected by gill cover in tails and many other fish, but by skin flaps in sharks, rays, and some older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take oxygen from the water and release excess carbon dioxide into the water.

Most modern fishes have a hydrostatic (ballast) organ, called a swim bladder, located in the body cavity just below the kidneys and stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially asanthoprotectives, the bladder has lost its association with the digestive system, a condition known as physioclastic. The connection has been maintained by several relatively primitive telestomes.

In many unrelated lines of fish, the bladder has become specialized as a lung or, at least, as a highly vascular accessory breathing organ. Some fish with such accessory organs measure air vents and are denied access to the surface, even in oxygen-rich waters. Fishes with a hydrostatic form of the swim bladder can control their depth by regulating the amount of gas in the bladder.

Gas, mostly oxygen, is secreted into the bladder by specialized glands, which makes the fish more happy; The gas is absorbed into the bloodstream by another specialized organ, reducing overall buoyancy and allowing the fish to sink. Some deep sea fish bladder may contain oil instead of gas. Other deep seas and some downstream forms have very few floating bladder or have lost limbs completely.

The swim bladder of fish follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive in more primitive types of fishes, such as lamefish lepidosiran and protopteras.

Communication Systems

The circulatory, or blood vessel, system includes the heart, arteries, capillaries, and veins. It is in capillaries that oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products are exchanged. The capillaries carry to the veins, which along with their waste products, bring the venous blood back to the heart, kidneys and gills. There are two types of capillary beds: in gills and in the rest of the body.

The heart, a folded continuous muscular tube with three or four sacral growths, undergoes rhythmic contractions and receives venous blood in a sinus venus. It passes blood into the ankle and then a thick muscle pump, the ventricle. Blood from the ventricle goes into a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arch and then to the gill capillaries. There the waste gases are delivered to the environment, and oxygen is absorbed.

Oxygen-rich blood enters the efferent arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The prevalence of fish is different in that reptiles, birds, and mammals do not return to the heart prior to distribution in that oxygen-rich blood to other parts of the body.

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Excretory organ

Like other vertebrates, the primary excretory organ in fish is the kidney. In fish, there is some excretion from the digestive system, skin, and especially from gills (where ammonia is released). Compared to land vertebrates, fishes have a particular problem in maintaining their internal environment with constant amounts of water and dissolved substances such as salts. The proper balance of the fish's internal environment (homeostasis) is a great part maintained by the excretory system, especially the kidneys.

Kidneys, gills, and skin play an important role in maintaining the fish's internal environment and examining the effects of osmosis. Sea fishes live in an environment that has a high amount of salts in the water around them, which can be inside their body and still maintain life. On the other hand, freshwater fish live in water with a much lower amount of salts inside their bodies than necessary.

Osmosis promotes water loss from the body of a marine fish and absorption of water by a freshwater fish. Mucus in the skin slows down the process, but there is not enough barrier to stop the circulation of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, the water passes from the membrane into a more concentrated solution, while the dissolved chemicals move to a region of lower concentration (diffusion).

The kidney of freshwater fish is often larger in relation to body weight than in marine fish. Kidney excretion in both groups originates from the body, but the kidney of freshwater fish also emits large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes lose salt to the environment and should be replaced.

They get some salt from their food, but gills and skin inside the mouth actively absorb salt from the water passed through the mouth. This absorption is done by specialized cells that are able to move salts against diffusion gradients. Freshwater fish drink very little water and take very little water with their food.

Sea fishes must conserve water, and so their kidneys release little water. To maintain their water balance, marine fishes drink large amounts of seawater, retain most of the water and destroy salt. Most of the nitrogenous waste in marine fish is secreted by gills as ammonia. Sea fishes can eject salt by groups of specialized cells (chloride cells) in gills.

There are several telosts — for example, salmon — that travel between freshwater and seawater and must adjust to the reversal of osmotic gradients. They adjust their physical processes by spending time (often surprisingly little time) in intermediate saline environments.

Sea hagfishes, sharks, and rays have osmotic concentrations in their blood that are equal to that of seawater, and so they neither have to drink water nor do much physical exertion to maintain their osmotic balance. Osmotic concentrations are kept high by the retention of urea in the blood in sharks and rays. Freshwater sharks have low concentrations of urea in their blood.

Endocrine glands

The endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, regulate and regulate many types of body functions. The cyclostome has a well-developed endocrine system, and was probably well developed in the ancestral agnus of modern fishes. Although the endocrine system in fishes is similar to that in higher vertebrates, there are many differences in detail.

The pituitary, thyroid, supernaturals, adrenal, pancreatic islets, the sex glands (ovaries and testes), the internal wall of the intestine and the body of the ultimobical gland form the endocrine system in fish. There are some others whose function is not well understood. These organs regulate sexual function and reproduction, development, osmotic pressure, normal metabolic activities such as fat storage, and the use of foods, blood pressure and some aspects of skin color. Many of these activities are also regulated by the central nervous system, which works with the endocrine system in sustaining fish life. Parts of the endocrine system are developmentally, and undoubtedly evolutionarily, generated from the nervous system.

Nervous system and sensory organs

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body movements, as well as properly integrating these activities with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrated system. The peripheral nervous system, which connects the brain and spinal cord to the various organs of the body, carries sensory information to integrate specialized receptor organs such as the eye, inner ear, nasal (sense of smell), taste glands, and others. is.

Centers of the brain and spinal cord. The peripheral nervous system also receives information from the integrated centers of the brain and spinal cord through various nerve cells.

This coded information leads various organs and body systems, such as skeletal muscle systems, to take appropriate action in response to basic external or internal stimuli. Another branch of the nervous system, the autonomic nervous system, helps coordinate the activities of many glands and organs and is itself closely connected to the integrated centers of the brain.

The brain of a fish is divided into several physiological and functional parts, all closely linked to each other, but each serves as the primary focus of integrating particular types of reactions and activities. Many of these centers or parts are primarily associated with a type of sensory perception, such as sight, hearing or smell (olfactory).

Smelling

The sense of smell is important in almost all fish. Some eels with small eyes mostly depend on smell for the location of food. The olfactory, or nasal, body of the fish is located on the dorsal surface of the snout. The lining of the nasal organ consists of specialized sensory cells that send chemicals that dissolve in water, such as substances from food materials, and sensory information to the brain via the first cranial nerve. Odor also acts as an alarm system. Many fish, especially various species of freshwater, react with alarm to a chemical released from the skin of an injured member of their species.

The taste

Many fish have a well-developed sense of taste, and small pit-like taste buds or organs are located not only within the cavities of their mouths, but also in their heads and parts of their bodies. Catfish, which often have poor eyesight, have barbells ("whiskers") that serve as complementary flavor organs, actively used around the mouth to search for food on the floor. Some species of fish that live naturally in the blind cave are specifically supplied with taste buds, often covering the surface of their bodies.

Vision

Vision is extremely important in most fish. A fish's eye is basically like all other vertebrates, but fish eyes are extremely diverse in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they are somewhat compensatory, so that another emotion (such as smell) is predominant, in which case the eyes will often shrink.

Fishes living in brightly lit shallow waters often have relatively small but efficient eyes. Cyclostomes have somewhat less eyes than other fish, with eyesight perhaps slightly less effective than skin drawn on the eyeball. Most fish have a spherical lens and adjust their vision to distant or nearby subjects by rotating the lens within the eyeball. Some sharks adjust by changing the shape of the lens, as in land vertebrates. Fishes that are very dependent on the eyes have particularly strong muscles for accommodation. Most fish are well sighted, despite the restrictions imposed by the constant turbidity of the water and light refraction.

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Fossil evidence suggests that color vision evolved in fishes 300 million years ago, but not all living fishes retain this capability. Experimental evidence indicates that many shallow-water fishes, if not all, have color vision and see some colors particularly well, but some low-shore fishes live in those areas, Where the water is deep enough to filter all the colors, if not all the colors, and these fish never see the color clearly. When tested in shallow water, they are clearly unable to respond to color differences.

The hearing

Sound perception and balance are intimately connected in a fish. The hearing organs are completely internal, located within the skull, on each side of the brain and to some extent behind the eyes. Sound waves, especially those with low frequencies, travel easily through water and directly impact the head and body bones and fluids, transmitted to the auditory organs.

Fish respond to sound easily; For example, a trout will also take flight considering the pedestrians in a stream bank to avoid the fishermen's viewpoint, even if it cannot see the fisherman. Compared to humans, however, the range of sound frequencies heard by fishes is very limited. Many fish communicate with each other by producing sounds in their swim bladder, rubbing their teeth, and in other ways.

Other senses (touch, pain and special senses)

A fish or other vertebrate rarely has to rely on one type of sensory information to determine the nature of its surrounding environment. A catfish uses taste and touch when examining a food item with its oral barbels. Like most other animals, fish have many tactile receptors on the surface of the body. Pain and temperature receptors are also present in fish and probably produce the same information for a fish as humans. Fish respond in a negative fashion to stimuli that would be painful to humans, suggesting that they feel pain.

An important sensory system in fishes that is absent in other vertebrates (except for some amphibians) is the lateral line system. It consists of a series of heavy canals located under the skin and in the bones, along the lower jaw, above the head, and below the central part of the body, where it attaches to the scales. Finally, small sensory organs (pit organs) are located along these canals which clearly detect changes in pressure. The system allows the fish to feel changes in water currents and pressures, enabling the fish to orient themselves to various changes occurring in the physical environment.

Development and paleontology

Although a great many fossil fishes have been found and described, they represent a small part of the long and complex development of fishes, and knowledge of fish development remains relatively fragrant. In the taxonomy presented in this article, fish-like vertebrates are divided into seven categories, each with a different basic structural organization and different physical and physiological adaptations to the problems presented by the environment.

The broad basic pattern has been one of gradual replacement of old groups by new, better-adapted groups. A fundamentally more efficient means of feeding, breathing, or swimming or living life to one or a few members of a group evolved. These better-adapted groups then forced the extinction of older group members, with whom they competed for available food, breeding places, or other necessities of life.

As the new fishes became well established, some of them developed further and adapted to other habitats, where they continued to replace the members of the already old group. The process was repeated until all or almost all members of the old group were replaced by members of the new evolutionary line in different types of dwellings.

Agnath: Early Jaw Fishes

The earliest vertebrate fossils of some relationships date from the Upper Ordovian period in North America at approximately 450 million years of dermal armor fragments of jaw fishes (superclass Agnetha, order heterostracy). Early Ordovian toothwat fragments from the former Soviet Union are certainly remnants of Agnayathana. It is uncertain whether North American young fish live in shallow coastal seawater, where their remains have been fossilized, or have been washed into freshwater vertebrate coastal deposits by stream action.

Jaw fishes probably originate like ancient, small, soft-baited filter-feed organisms, and are probably also ancestral to cephalochordata (Amphaxus and its relatives) for filter feeders living in modern sands. In ancestral animals, the body was probably hardened by a notch. Although a vertebrate origin in freshwater is discussed by paleontologists, it is possible that the dynamics of the body and the protection provided by the dermal shell need to flow in freshwater environments and avoid and resist claw invertebrate eurypterids Arising due to Lived in the same water. Due to the marine distribution of living primitive bacteria, however, many paleontologists suspect that vertebrates originated in freshwater.

Heterostracan remains appear as delta deposits in two North American areas of the Siluri era. Until the close of the Silurian, about 416 million years ago, European heterostracan remains are found that appear as deltas or coastal deposits. In the Late Silurian of the Baltic region, the deposition of lagoon or freshwater yields fishes of the order of osteostracy. Sometime later in the Silurian of the same region, the layers consist of jaw teeth, the initial group of jaw vertebrae and jaw fishes. These layers lie between the sea beds but are washed with fresh water from the coastal area.

Therefore, it is clear that by the end of the Silurian, both the jaw and jaw vertebrae were well established and would have already had a long history of development. Yet paleontologists remain only those particular forms that cannot be ancestors of placoderms and bony fishes that appear in the next period, the Devonian. No fossil is known of the more primitive ancestors of the Aganathas and the Acanthodians. The extensive sea beds of the Silurian and those of the Ordovian are essentially void of vertebrate history. It is believed that the ancestors of fish-like vertebrates develop in freshwater, where a few more relatively small fossil beds were created that have probably been faded away for a long time. The remains of the earliest vertebrates can never be found.

By the close of the Silurian, all known orders had developed without vertebrates, except perhaps modern cyclostomes, which are without rigid parts that are usually preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamp of modern body structure was reported in a more than 300 million year old assemblage from central Xfilville, Illinois. Fossil evidence of the four orders of armored jeweler vertebrates is absent from later deposits than the Devonian. Presumably, these vertebrates became extinct at the time, being replaced by more efficient and perhaps more invasive placeoderms, asanthodians, Selachians (sharks and relatives), and early bony fishes.

The cyclostomes probably survived because they soon evolved from the Anaspeed Agnathan and developed a rasping tangulike structure and a sucker mouth, enabling them to hunt other fish. With this way of life, they clearly had no competition from other fish groups. Cyclostomes, hagfish and lampreys, were sometimes thought to be intimately related due to similarities in their suctionial mouth, but it is now understood that hagfish, the order myciniformis, are the most primitive living organisms, and are classified separately from laparisse. is. , Order petromyzioniforms.

Early jeweler vertebrates probably fed on small organisms by filter feeding, as did their descendants, the larvae of modern lampreys. The initial cavity had a large cavity. It is believed that small organisms are taken from the bottom by mouth action, or of course by sucking action through the mouth to enter the gill cavity with water to breathe. The smaller organisms were then sieved by the gill apparatus and directed to the food canal. The gill system thus evolved into a feeding, as well as a breathing, structure. At Agnathan, the head and gills were protected by a heavy dermal shield; The tail area was independent, allowing speed for swimming.

Most important for the development of fishes and vertebrates in general was the early appearance of a substance such as bone, cartilage, and enamel. These materials were modified in later fish, making them adaptable to many aquatic environments and eventually even to land. Other basic organs and tissues of vertebrates — such as the central nervous system, heart, liver, digestive system, kidneys, and circulatory system — were undoubtedly present in the ancestors of the agnostic. In many ways, bone, both external and internal, was the key to vertebral development.

Acanthodi: Early jaw fishes

The next class of fishes that appeared was anthodody, with the earliest known jaw vertebrae, which originated in the Late Silurian more than 416 million years ago. After the Devonian, the recluse declined, but moved to the early Permian 280 million years ago. The first complete specimens appear in the Lower Devonian freshwater deposits, but later some members in the Devonian and Permian appear marine. Most were small fish, whose length did not exceed 75 cm (about 30 in).

We do not know anything about the ancestors of seclists. They may have originated from some jaw vertebrae, perhaps in fresh water. They appear to be active swimmers with almost no head armor, but with large eyes, indicating that they relied heavily on vision. Perhaps they hunted invertebrates. The lines of spines and spinelike fins between the pectoral and pelvic fins somewhat recognize the idea that the pair's fins originated from "fin folds" along the edges of the body.

The relationship of other Javed vertebrates is unclear. They have characteristics found in both shark and bony fishes. They are similar to early bony fishes in keeping the gonadoeal scale and partially skeletal internal skeletons. Some aspects of the jaw appear more like bony fishes than sharks, but some aspects of the bony fin spine and gill system favor relationships with early sharks. Acanthodians do not seem particularly close to placodermi, although, like the placeoderm, they apparently possessed less efficient tooth replacement and tooth structure than sharks and bony fishes, possibly a reason for their subsequent extinction.

Placodody: Plate-skin fishes

The first record of Javed placeodermy is of early Devonian about 400 million years ago. The placoderm flourished for nearly 60 million years and almost passed away at the end of Devioni. Nothing is known of his ancestors, who may have been present in the Silurian.

The development of several other, better adapted fish groups soon followed the presence of placoderms, and this apparently led to their early extinction. The greatest period of his success was almost in the midst of the Devonian, when some of them became maritime.

As their name indicates (placoderm means "plate skin"), most of these fish had heavy coats of bony armor, particularly about the anterior part of the head and body. The tail remained independent and heterosercle (ie the upper lobe long, the lower one short or lacking). Most placoderms remained small, 30 cm (12 in) or less in length, but one group, the arthrodire, had few marine members that reached 10 m (about 33 ft) in length.

Significant evolutionaries of the placeoderm were in the jaws (which were usually amphistial - ie, involving the hypoid and quadrate bones) and the development of wings, particularly wings paired with well-constructed coral or radial elements. The jaws are of single elements with strongly attached toothpaste structures. These are considered ancestral to the more adaptable jaws of later bony fish groups. It has been proposed that sharks originated from some group of plasoderms near Stensiolariformes and the chimere line (orbit holocephaly) from some arthrodorses; This suggestion, however, is inconclusive.

Paleospondylus, a peculiar 5-cm (2 in) fossil fish from Middle Devonian rocks in Scotland, is probably not a placoderm, although it is sometimes classified with placoderm. The various suggestions that it has relationships with agnostics, placoderms, acanthodians, sharks and even lamefish and amphibians are unrelated and its relationships are completely unknown.

In the year 2019 review

Chondrichthis: Sharks and Rays

The earliest sharks (squares Chondrichthy) first appeared in the Early Devonian about 400 million years ago, which became quite famous by the end of the Devonian, and are still successful today. About 251 million years ago, by the end of the Permian, two early Devonian orders of primitive shark-like fishes, Cladocellachiformes and Cladodontiformes, became extinct, while the freshwater order Xenacrantiformes ran until the end of the Tricic about 200 million years ago. Heterodontiforms, the last Devonian order, are still living members.

Modern sharks and rays originated during the Jurassic period, about 200 million to 145.5 million years ago, probably from an older group, the highbodont shark. Possibly the marine cladocellacians gave rise to the Hybodont heterodontiform while staying close to the Devonian. These had placoderm amphistic jaws but feathers were of the more efficient type. Hybodots in turn are believed to have given birth to the living but archaic mollusk-eating Port Jackson shark (Heterodones).

The relationship of the remaining (but archaic) hexanchiform sharks is unknown. Three main orders of modern Selachi — Carchariniformes (ground sharks) and Lamniformes (mackerel sharks) and Rajiformes (skates and rays) — appeared during the Jurassic period. They are characterized by a hyalistic jaw (which includes only the hypoid bone in articulation), an improved feature in the greater mobility of the jaw and the methods of prediction used by modern celiacs.

During the Jurassic, skates and rays evolved from some lower-surviving shark-like ancestors. The primary development and diversification of modern sharks, skates, and rays occurred in the Cretaceous period and the Cenozoic era. Thus, with telost fishes (discussed below), most living sharks, skates, and rays are of relatively recent origin, with their main evolutionary radiation occurring from Jurassic times.

Holocephali

The class Holocephaly- chymas or ratfish, as their modern living people are called - first appeared in the Late Devonian, but were the most common and diverse during the Mesozoic Era.

Although many modern species of chimers are not known, they are sometimes relatively abundant in their deep sea habitats.

The relationships of these fish are under question. It has been proposed that they belong to the Devonian pterodontant arthrodire, which had a chimera-like shape and pelvic flakes. It has also been suggested that they are closely related to Celachi because both Celachians and Holocephalians have many characters in general, such as the absence of placoid scales, pelvic clasper, and true bone. It has been suggested that both Holocephalian and Selechian gills belong to the Acanthodians based on arch structures. Further evidence is needed to solve their classification and relationship problem.

Sarcopterygii: fleshy-winged fishes

The fishes of the class Sarcopritagii are originally very ancient, their first remains visible in the Lower Devonian Strata of Germany. Some authorities argue that the Ripepedistians, one of the three groups of sarcasm, gave birth to amphibians by the end of the Devonian; However, other authorities believe that tetrapods evolved from two other groups, coelacanths and dipnoans (lamefish). Ripedistians became extinct around 120 million years after the beginning of the Permian, but coalcants and dipnoons have survived, although in small numbers. Primitive satirism shows many similarities, supporting the view that he had a common ancestor. The ancestor's nature remains a mystery. Sarcasm has probably evolved from unknown Silurian freshwater fishes that may also be ancestral to actinopterygins.

Some authorities support the idea that Rhipdistian crossoptryption thrives in the fresh waters of the Middle Devonian, where some developed pectoral and pelvic apples are quite strong and resilient, adapting to a habitat for seasonal drought. Enable the pool to leave. From the ponds to which the water was retained. Paradoxically, terrestrial amphibians first arose through the need to survive in water.

The early Kolacanths of the Late Devonian were small freshwater and inshore fishes, and it was not until the Late Permian and Tricic that they became marine and became larger and more diverse. They are not known to be fossils later than Cretaceous, and so it was very surprising when in 1938 a 160 cm (63 in) specimen was taken 120 meters (about 390 ft) deep off the coast of eastern East Africa. I went . In 1997, another surviving coelacanth species was discovered on the island of Sulawesi, Indonesia.

Dipnoan first appeared in the early Devonian and was completely different at the time. They continued to flourish until the Trisic came closer, when their numbers were greatly reduced. Modern Australian lungfish differ slightly from one of the Triassic forms. Living South American and especially African lamefish have proliferated, with special fish adapted to live and survive in more or less annual ponds.

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Actinopteryg: Ray-finned Fishes

Actinopterygii, or ray-finned fishes, are the largest class of fishes. In existence for nearly 400 million years, since the early Devonian, it contains over 42 orders with more than 480 families, of which at least 80 are known only from fossils. The class consists of the majority of living and fossil fishes, with about 26,900 living species. The history of actinoprecepts can be divided into three basic stages or evolutionary radiations, each representing a different level of structural organization and efficiency.

Chondrosteae may have first arisen as early Devonian, increased in number and complexity about the Permian, and then nearly became extinct by the middle of the Cretaceous 100 million years ago.

The chondrosterian order Palaeonyciformes is the basal actinopristies stock from which all other chondrosthene and holostein have evolved. They were the most common fishes of their time, relatively small and usually similar in appearance to later fishes. Compared to today's fish, they had odd-looking jaws and tails. His tail was heterosexual.

They had thick ganoid scales on their bodies that cut each other, rather than most modern fishes. Palaeonisciformes often have large eyes, long mouths with upper jaws that are completely tied to fully armored cheeks, and relatively weak jaw muscles. They gave rise to a wide variety of types, with enlarged bodies and jaws, feeding on microorganisms, deep-bodied sea reef fishes, and coral-eating reef fishes. Almost all of these were replaced by modern teleists.

Chondrosti are the surviving marine and freshwater sturgeon, the strange plankton-feeding paddlefish of the Mississippi River in North America and the Yangtze River (Chang Jiang) of China, and Africa's freshwater biscuits and reedfish (family Polyperidae). The relation of polyperids is in some doubt, and that group is sometimes placed in the class sarcopritagii.

Many of Chondrostein's orders have developed features that approach the overall level of anatomy and are sometimes called subholstein. One of these orders, parasimoniotiforms, evolved from the Palaeoeniformes in the early quarter and may have given rise to at least some Holocene. This evolutionary line leads to Follidiformis, which gave rise to modern bony fishes, or Teleosto.

Holstein is thought to be of mixed origin and represents a stage in the development of Chondrostein's group of orders. Therefore, the infraclass or division holostei does not represent a dynasty. Important Holstein features are the approach of the homocerecal position and the tail toward the same number of fin rays and the same number of base rays. Both of these conditions make Holosteen a more efficient swimmer than Chondrostein, as is Holstein's body scales that are thinner. Another important advance of Holstein was the release of the upper jaw from the protruding bone of the cheek, leading to greater movement of the gill chamber and jaw with more powerful development of the muscles of the lower jaw.

Five of Holstein's orders are known, with their greatest evolutionary radiation during the Tricic, Jurassic, and Cretaceous periods, when chondrostine was declining and Teleosto began to expand. Two Holocene groups survive today: Boffin, Amia calva, and several species of Gars, Lepisosteus, all found in North America. The current understanding of the evolution of bony fish identifies amiforms as Telest's closest living relatives.

Modern bony fishes, infraclasses or division telostei, include a large number of live fishes. They appear in the fossil record some 200 million years ago (as the family Leptolepidae), with their homoserial caudal fins and caudal skeletons already fully developed. They are now extinct by a sequence of Holocene, Follidophoriformes. This group was intermediate in character between chondrosterians and telosts. Telests reached their full extent within the last 50 million years and represent a distinct functional advance over their Holstein ancestors. They have a greater swimming ability due to improved tail structure, and still more efficient feeding and gill-ventilated systems.

Bony fishes represent the culmination of long development towards a body plan with maximum swimming efficiency. Particularly important in this development is the change in wings and tail. Some authorities believe that the paired wings originated from a single continuous tail and anal fins that split at the vent and proceeded to the head of each side.

The segments between the pectoral, pelvic, anus and the caudal fin were subsequently lost. The last rays of sharks and rays are of one horn material, but many primitive fossils are of fish. The bony fin rays of sarcasm and actinopastrifience possibly originated from scales lying in the folds. Modern telost fishes have flexible wing rays (called soft rays) or solid rays of joint segments of bone. The first dorsal fin of Asanthoprotegian fishes is of the spiny type.

The original tail fin of primitive fishes was not an effective swimming organ, because of its asymmetry. Continuous improvement in tail size over 400 million years is one of the major features of fish development. In primitive fishes, the tail (vertebral) axis turns upward (heteroceral) or downward (hypocarrel), and a lobe of flesh emanates from it. This form of tail cannot provide a powerful driving mechanism, because the driving force is unevenly distributed relative to the axis of the body.

With an asymmetrical tail, the fish swim at an indistinct pace of body and tail. Developed in both modern and ancient fishes, with a bipolar tail (extending into the middle of the fin lobe along the axis of the vertebrae) in some fish, the tail remains relatively ineffective as it remains rigid for proper propulsion action. The development of a true homoserial tail fin, in which powerful muscles transfer strong fin rays with a very flexible basal joint and in which the upper and lower lobes are nearly equal, is a special development for teleost fishes.

As the existence of more than 400 families has been suggested, telestomes are very diverse structurally and in habitat habitat. They can be divided into about 12 super-borders or subdivisions, each with different evolutionary significance. Leptolepidimorpha, an extinct, relatively primitive group, has uncertain relationships with other telosts and is poorly understood yet.

The second group, the superorder Osteoglossomorpha, consists of relatively primitive telosts, most of which are now extinct. Some of the remaining members are mostly tropical and worldwide in distribution but adapted to restricted habitats. The third group, the allopomorph, holds some relatively primitive living members, such as tarpon, but is mostly represented by a large variety of specialized true eels.

Cluopomorpha includes herds and anchovies, relatively primitive fishes, mostly specialized for survival near the surface of the open ocean. Some species that breed in freshwater environments are anachronistic, but spend most of their lives in the sea. Protacanthopterygii is a diverse collection of freshwater in relatively primitive orders, marine, deep sea, and distribution; Examples are trout, smelts, and argentine. Ostariophysi is an important group of mainly freshwater fishes, including charikin, carp, minnose, lochia, suckers, and catfish.

The remaining groups have a complex fossil history and are not yet fully understood, but all possess similar evolutionary trends. Each group shows a tendency to develop dorsal and anal fins (fewer in some) and a shelf of bone below the eye. Pelvic fins have a tendency to move over the body, with minor gains in restructuring and maneuvering of swimming methods. All three groups are probably related and possibly originated from some early Protacanthropelazian-ancestor.

Scopelomorpha includes deep-sea open-sea plankton feeders and a wide variety of predators, some of which are light limbs. Paracanthopterygii are rather diverse collections of fishes, most important to humans. The last superorder, Acanthopterygii, is the result of the great radiation of modern spiny-raced fishes and includes seaside habitats, major fishes in the tropics, temperate and arctic. They also live in freshwater environments, especially in lakes, slow-moving rivers and ponds. Superorders have some important open-ocean members, such as tuna. The key to successful acanthopterygian radiation is probably their mobile, long mouth.

Classification
Special Classification Features

In formulating hypotheses about the evolution of fishes and establishing taxonomy based on these hypotheses, ichthyologists lay special emphasis on comparative studies of skeletons. This approach has two primary benefits. First, direct comparisons between extinct and fossil groups are possible, with the latter usually represented only by bony remains. A second advantage is that the bones of living fish are relatively easy to observe and study compared to other body structures. Proper preservation and specialized preparation of the nervous system, for example, are difficult and expensive when the fish being compared are from the far end of the earth. In the study of species relationships within a group, major uses have been made of the dimensions and differences of external characteristics, such as head and body length, and the count of external characters, such as teeth, fin rays, and scales. The color pattern is also important. In recent years, valuable data on the classification of fish have been obtained from studies of comparative behavior, physiology, genetics and functional anatomy.

Annotated Classification

The following classification is mainly the British ichthologist c. Patterson, R. Miles, P.H. Greenwood, and K.S. Thomson and American ichthyologist D.E. Rosen, American ichthyologists G.D. Johnson, W. N. Eschmeyer, M.L.J. With extensive modifications from. Stacini, L.R. Parent, s.v. Frank, and W.L. Fink and Canadian ichthologist J.S. Nelson, among others. Fishes are generally divided into three groups: the superclass Agnatha (jaw fishes), the class Chondrichthys (cartilaginous fishes), and the superclass Ostichites (bony fishes). The latter two groups are included within the infantileum Gnathostomata, a category that consists of all jawed vertebrae.

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Other senses (touch, pain and special senses)

Development and paleontology

Agnath: Early Jaw Fishes

Acanthodi: Early jaw fishes

Placodody: Plate-skin fishes

In the year 2019 review

Chondrichthis: Sharks and Rays

Holocephali

Sarcopterygii: fleshy-winged fishes

In the year 2019 review

Actinopteryg: Ray-finned Fishes

Annotated Classification

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