Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases. Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates.

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration, some for the purpose of camouflage, others for the enhancement of behavioral signals.

Fishes range in adult length from less than 10 millimetres (²/₅ inches) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than ¹/₁₆ ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42° C (100° F), others in cold Arctic seas a few degrees below 0° C (32° F) or in cold deep waters more than 10,000 metres (3,500 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes such as the Dead Sea and Great Salt Lake in Utah. The present distribution of fishes is a result of the geological history and development of the Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and fresh waters. For the most part the fishes in them, even in adjacent areas, are different, but some, such as the salmon, migrate from one to the other. The freshwater habitat may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. Marine habitats can be divided into deep ocean floors (benthic), midwater oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have a different fish fauna, even when such habitats occur along the same coastline.

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of central Asia and some of it appears to have entered Africa. The extremely large shore fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents.

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies of fish life, all the interdependent aspects of their life, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, some tropical marine shore fishes have pelagic larvae. Larval food also is different and they often live in shallow waters, where they may be less exposed to predators.

After the fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity (life span) of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In a few large species some individuals may live as long as 10 or 20 years or even longer.

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral-line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others depending upon the fish's other adaptations. In fishes with large eyes the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (e.g., some eels).

Specialized behaviour is primarily concerned with the three most important activities in the fish's life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines (sardine) on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are most often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next.

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction (fish)). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator.

Many fishes have a streamlined body and swim freely in the open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows are good examples. Oceanic flying fishes (flying fish) escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water with their pectoral fins, which function as flapping wings.

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes (bony fish) a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female's vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species appear to be born sexually mature, although they are not fully grown.

Some fishes are hermaphroditic (hermaphroditism), an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

Successful reproduction and in many cases defense of the eggs and young is assured by rather stereotyped but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in gravel beds where they themselves hatched (anadromous fishes). Others undertake shorter migrations from lakes into streams or in other ways enter for spawning habitats that they do not ordinarily occupy.

The basic structure and function of the fish body is similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the organs and organ systems parallel those of other vertebrates.

The typical fish body is streamlined and spindle-shaped, with an anterior head, gill apparatus, and heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just 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 of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

The skeleton forms an integral part of the fish's locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the sections below (Evolution and paleontology (fish)). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, almost always without fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnaths. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish's body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales (scale) they possess.

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosmine-like layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid scales (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by expansion and contraction of the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed near iridocytes or leucophores (bearing the silvery or white pigment guanine) melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, with the largest amount of direct musculature. Its musculature is really a structural and functional continuation of the main musculature of the body. The body musculature is usually arranged in two rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some, such as sharks and the piranhas, have cutting teeth for biting chunks out of their victims. A shark's tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrotfishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavement-like throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (e.g., the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers), anchored by one end to the gill bars. The food collected on these rods is passed to the throat where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested here and leaves the stomach in liquid form.

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be imbedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or an absorptive function, or both.

The intestine itself is quite variable in length depending upon the diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in man. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the uro-genital system.

Oxygen and carbon dioxide dissolve in water and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills (gill). The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes, but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; it is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oil in the bladder, rather than gas. Other deep-sea and some bottom-living forms have much reduced swim bladders or have lost the organ entirely.

The swim bladder of fishes 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 among the more primitive types of fishes such as the lungfishes Lepidosiren and Protopterus.

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins: it is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries in turn lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds, those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four sacklike enlargements, undergoes rhythmic contractions, and receives venous blood in a sinus venosus. It then passes the blood to an auricle and then into a thick, muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood then passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment and oxygen is absorbed. From there the oxygenated blood enters efferent (exuant) arteries of the gill arches and then into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals, in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment ( homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

The kidney, gills, and skin play an important role in maintaining a fish's internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration ( diffusion).

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but that of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable (like those of the kidney) of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water in their food.

Marine fishes must conserve water, therefore their kidneys excrete little water. To maintain their water balance marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. By reabsorption of needed water in the kidney tubules, they discharge a more concentrated urine than do freshwater fishes. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Some marine fishes, at least, can excrete salt by clusters of special cells in the gills and intestine.

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

Marine lampreys, hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes (cyclostome) have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The endocrine glands of fishes are the pituitary, thyroid, suprarenals, adrenals, pancreatic islets, sex glands (ovaries and testes), the inner wall of the intestine, and the ultimobranchial bodies. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities also are controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, the brain, and spinal cord, are the primary integrating mechanisms. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception such as sight, hearing, or smell (olfaction).

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water such as substances from food material and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to the body fluids produced by an injured member of their own species.

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. The barbels (“whiskers”) of catfishes, which often have poor vision, serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, these often covering most of their body's surface.

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

Sound (sound reception) perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see the fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. It is thought that many fishes communicate with each other in a crude way by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head and down the midside of the body where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

Although a great many fossil fishes have been found and described, they represent a tiny portion of the long and complex evolution of fishes and knowledge of fish evolution remains relatively fragmentary. In the classification presented in this article fishlike vertebrates are divided into seven classes, the members of each having a different basic structural organization and different physical and physiological adaptations for the problems presented by the environment. The broad basic pattern has been one of successive replacement of older groups by newer, better adapted groups. One or a few members of a group evolved a basically more efficient means of feeding, breathing, swimming, or several better ways of living. These better adapted groups then forced the extinction of members of the older group with which they competed for available food, breeding places, or other necessities of life. As the new fishes became well established, some of them evolved further and adapted to other habitats, where they continued to replace members of the old group already there. The process was repeated until all or almost all members of the old group in a variety of habitats had been replaced by members of the newer evolutionary line.

The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes (class Agnatha, order Heterostraci) from the Middle Ordovician Period in North America, about 450,000,000 years in age. Early Ordovician toothlike fragments from the U.S.S.R. are less certainly remains of the class Agnatha. It is uncertain whether the North American jawless fishes inhabited shallow coastal marine waters, where their remains became fossilized, or were freshwater vertebrates washed into coastal deposits by stream action.

Jawless fishes probably arose from ancient small, softbodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata (Amphioxus and its relatives). The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters. Because of the marine distribution of the surviving primitive chordates, many paleontologists doubt that the vertebrates arose in fresh water.

Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 400,000,000 years ago, European heterostracan remains are found in what appear to be delta or coastal deposits. In the Late Silurian of the Baltic area, lagoon or freshwater deposits yield jawless fishes of the order Osteostraci. Somewhat later in the Silurian from the same region, layers contain fragments of jawed acanthodians, the earliest group of jawed vertebrates, and of jawless fishes. These layers lie between marine beds but appear to be washed out from fresh waters of a coastal region.

It is evident, therefore, that by the end of the Silurian both jawed and jawless vertebrates were well established and already must have had a long history of development. Yet paleontologists have remains only of specialized forms that cannot have been the ancestors of the placoderms and bony fishes that appear in the next period, the Devonian. No fossils are known of the more primitive ancestors of the agnaths and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away. Remains of the earliest vertebrates may never be found.

By the close of the Silurian, all five known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes (cyclostome), which are without the hard parts that ordinarily are preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits almost 300,000,000 years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably they became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms, acanthodians, selachians (sharks and relatives), and by early bony fishes. Cyclostomes survived probably because they early evolved from anaspid agnaths and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups.

Early jawless vertebrates probably fed on tiny organisms by filter feeding, as do the larvae of their descendants, the modern lampreys. The gill cavity of the early agnaths was large. It is thought that small organisms taken from the bottom by a nibbling action of the mouth, or more certainly by a sucking action through the mouth, were passed into the gill cavity along with water for breathing. Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnaths were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming.

Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates such as the central nervous system, heart, liver, digestive tract, kidney, and circulatory system undoubtedly were present in the ancestors of the Agnatha. In many ways, bone, both external and internal, was the key to vertebrate evolution.

The next class of fishes to appear was the Acanthodii, containing the earliest known jawed vertebrates, which arose in the Upper Silurian, over 400,000,000 years ago. The acanthodians declined after the Devonian but lasted into the Lower Permian, a little less than 280,000,000 years ago. The first complete specimens appear in Lower Devonian freshwater deposits, but later in the Devonian and Permian some members appear to have been marine. Most were small fishes, not over 75 centimetres (approximately 30 inches) in length.

We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water. They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. Perhaps they preyed on invertebrates. The rows of spines and spinelike fins between the pectoral and pelvic fins give some credence to the idea that paired fins arose from “fin folds” along the body sides.

The relationships of the acanthodians to other jawed vertebrates are obscure. They possess features found in both sharks and bony fishes. They are like early bony fishes in possessing ganoid-like scales and a partially ossified internal skeleton. Certain aspects of the jaw appear to be more like those of bony fishes than sharks, but the bony fin spines and certain aspects of the gill apparatus would seem to favour relationships with early sharks. Acanthodians do not seem particularly close to the Placodermi although, like the placoderms, they apparently possessed less efficient tooth replacement and tooth structure than the sharks and the bony fishes, possibly one reason for their subsequent extinction.

The first record of the jawed Placodermi is from the Early Devonian, about 390,000,000 years ago. The placoderms flourished for about 60,000,000 years and were almost gone at the end of the Devonian. Nothing is known of their ancestors, who must have existed in the Silurian. The evolution of several other, better adapted, fish groups soon followed the appearance of the placoderms and this apparently led to their early extinction. Their greatest period of success was approximately during the middle of the Devonian, when some of them became marine. As their name indicates (placoderm means “plate skin”), most of these fishes had heavy coats of bony armour, especially about the head and anterior part of the body. The tail remained free and heterocercal (i.e., the upper lobe long, the lower one small or lacking). Most placoderms remained small, 30 centimetres or less in length, but one group, the arthrodires (arthrodire), had a few marine members that reached 10 metres in length. Important evolutionary advances of the placoderms were in the jaws (jaw) (which usually were amphistylic—i.e., involving the hyoid and quadrate bones) and development of fins, especially the paired fins with well-formed basal or radial elements. The jaws tended to be of single elements with strongly attached toothlike structures. These were too specialized to be considered ancestral to the more adaptable jaws of subsequent bony fish groups. It has been proposed that sharks (shark) arose from some group of placoderms near the Stensioelliformes and that the chimaera line (class Holocephali) arose from certain arthrodires; this suggestion, however, is uncertain.

A peculiar, five-centimetre fish, Palaeospondylus, from Middle Devonian rocks in Scotland, is probably not a placoderm, although classed with them here. Various suggestions that its relationships are with the agnaths, placoderms, acanthodians, sharks, and even lungfishes and amphibians are unconvincing and its relationships remain completely unknown.

Sharks (class Selachii) first appear in the Middle Devonian about 375,000,000 years ago, became quite prominent by the end of the Devonian, and are still successful today. Two Early Devonian orders of primitive sharklike fishes, the Cladoselachiformes and the Cladodontiformes, became extinct by the end of the Permian, about 230,000,000 years ago, while the freshwater order Xenacanthiformes (Xenacanthus) lasted until the Middle Triassic, about 200,000,000 years ago. The final Devonian order, Heterodontiformes, still has surviving members.

Modern sharks and rays arose during the Jurassic Period, about 135,000,000 to 190,000,000 years ago, probably from an older group, the hybodont sharks. Presumably marine cladoselachians gave rise to the hybodont Heterodontiformes during the close of the Devonian. These had the placoderm amphystylic jaws but had paired fins of a more efficient type. In turn the hybodonts are thought to have given rise to the living but archaic mollusk-eating Port Jackson sharks (heterodonts). The relationships of the surviving (but archaic) hexanchiform sharks are unknown. The two main orders of modern Selachii, the Lamniformes (sharks) and Rajiformes (skates and rays), appeared between 140,000,000 and 180,000,000 years ago during the Jurassic Period. They are characterized by a hyostylic jaw (in which articulation involves only the hyoid bone), an improvement allowing greater mobility of the jaws and an important feature in the methods of predation used by modern selachians.

Skates (skate) and rays (ray) evolved from some bottom-living sharklike ancestor during the Jurassic. The primary evolution and diversification of modern sharks, skates, and rays took place in the Cretaceous Period and Cenozoic Era. Thus, along with the teleost fishes (discussed below) most surviving sharks, skates, and rays are essentially of relatively recent origin, their main evolutionary radiation having taken place within the last 140,000,000 years.

The class Holocephali, the chimaeras (chimaera) or ratfishes as their modern survivors are called, first appeared in the Upper Devonian but were most common and diversified during the Mesozoic Era. Only one of the seven known orders survived beyond the close of the Cretaceous Period about 65,000,000 years ago. Although not many modern species of chimaeras are known, they are sometimes relatively abundant in their deep-sea habitat. The relationships of these fishes are in question. It has been proposed that they are related to the Devonian ptyctodont arthrodires, which had a chimaera-like shape and pelvic claspers. It has also been suggested that they are closely related to the Selachii because both selachians and holocephalians have many characters in common, such as placoid scales, pelvic claspers, and absence of true bone. It has been suggested recently that both holocephalians and selachians are related to the acanthodians on the basis of the gill arch structures. Further evidence is needed to solve the problem of their classification and relationships.

The class Sarcopterygii are extremely ancient in origin, their first remains appearing in Lower Devonian strata of Germany, about 390,000,000 years old. The most important group, the rhipidistians, which gave rise to the amphibians by the end of the Devonian, became extinct about 120,000,000 years later, near the beginning of the Permian. Two lesser groups, the coelacanths and the dipnoans (lungfishes), have barely survived. Recognition of the class Sarcopterygii is controversial in that some ichthyologists believe that the two major groups, the Crossopterygii (including the rhipidistians and coelacanths) and the Dipnoi, have arisen from independent origins, the present structures of the two groups being widely divergent. The primitive members, however, show several similarities, supporting the view that they had a common ancestor. The nature of the ancestor or ancestors remains a mystery. The Sarcopterygii probably evolved from unknown Silurian jawed freshwater fishes that may also have been ancestral to the actinopterygians.

The rhipidistian (Rhipidistia) crossopterygians apparently flourished in the fresh waters of the Middle Devonian where, in adapting to a habitat subject to seasonal droughts, some evolved pectoral and pelvic appendages strong enough and flexible enough to enable them to leave drying pools to seek out those ponds that retained water. Paradoxically, terrestrial amphibians first rose through the need to survive in water.

The early coelacanths of the Upper Devonian were small freshwater and inshore fishes, and it was not until the Late Permian and Triassic that they became marine and grew larger and more diverse. They are not known as fossils later than the Cretaceous, and it was therefore a great surprise when in 1938 a live, 160-centimetre (63-inch) specimen was taken at 120 metres (approximately 390 feet) off the coast of eastern South Africa.

The dipnoans first appeared in the Lower Devonian and were fully differentiated at that time. They flourished until the close of the Triassic, when their numbers became greatly reduced. The modern Australian lungfish differs little from one of the Triassic forms. The living South American and especially African lungfishes are elongated, specialized fishes adapted to live and survive in more or less annual ponds.

The Actinopterygii, or “ray-finned” fishes, is the largest class of fishes. In existence for about 390,000,000 years, since the Lower Devonian, it consists of some 52 orders containing more than 480 families, at least 80 of which are known only from fossils. The class contains the great majority of known living and fossil fishes, with about 20,000 living species. The history of actinopterygians can be divided into three basic stages or evolutionary radiations, each representing a different level of structural organization and efficiency.

The Chondrostei (chondrostean) have a 300,000,000-year history. They arose first in the Lower Devonian, increased in numbers and complexity until about the Permian, and thereafter declined, becoming almost extinct by the middle of the Cretaceous, 100,000,000 years ago. The chondrostean order Palaeonisciformes is the basal actinopterygian stock from which all other chondrosteans and the holosteans evolved. They were the most common fishes of their time, relatively small and typically like later fishes in appearance. In comparison with today's fishes they had peculiar looking jaws and tails. Their tails were heterocercal. On their bodies were thick ganoid scales that abutted each other, rather than overlapping as in most modern fishes. Palaeonisciformes often had large eyes placed far forward, long mouths with the upper jaw firmly bound to the fully armoured cheek, and a relatively weak lower jaw muscle. They gave rise to a great variety of types, with elongate bodies and jaws, bottom-living types that fed on microorganisms, deep-bodied marine reef fishes and coral-eating reef fishes. Almost all of these were replaced by modern teleosts. Surviving Chondrostei are the bottom-feeding marine and freshwater sturgeons, the strange plankton-feeding paddlefishes of the Mississippi of North America and the Yangtze River of China, and the freshwater bichirs and reed fishes (family Polypteridae) of Africa. The relationship of the polypterids is in some doubt, and the group is sometimes placed in the Sarcopterygii.

Several of the chondrostean orders developed characteristics that approached the holostean level of anatomic organization and are sometimes called subholosteans. One of these orders, the Parasemionotiformes, evolved from the Palaeonisciformes in the Lower Triassic and may have given rise to at least some of the holosteans (holostean). This evolutionary line leads to the Pholidophoriformes, which gave rise to modern bony fishes, or teleosts.

The holosteans are thought to be of mixed origin and to represent a stage in the evolution of a group of chondrostean orders. If so, the infraclass Holostei does not represent a single lineage. Important holostean characteristics are the approach of the tail toward the homocercal condition and the equal number of fin rays and basal elements of the fin rays. Both of these conditions make the holostean a more efficient swimmer than the chondrostean, as does thinning of the holostean body scales. Another important advance of holosteans was the freeing of the upper jaw from the preopercular bone of the cheek, allowing greater movement of the gill chamber and jaws, with more powerful development of the lower jaw muscle.

Five orders of holosteans are known, with their greatest evolutionary radiation occurring during the Triassic, Jurassic, and Cretaceous periods, when the chondrosteans were declining and the teleosts just beginning to expand. Two holostean groups survive today: the bowfin, Amia calva, and the several species of gars, Lepisosteus, all found in North America.

The modern bony fishes, infraclass Teleostei, include the great majority of living fishes. They first appear in the fossil record about 190,000,000 years ago (as the family Leptolepididae) with their homocercal caudal fin and caudal skeleton already fully developed. They arose from an order of holosteans now extinct, the Pholidophoriformes. This group was intermediate in character between the chondrosteans and the teleosts (teleost). Teleosts have reached their fullest extent within the last 50,000,000 years and represent a distinct functional advance over their holostean ancestors. They have greater swimming ability, owing to the improvement in the tail structure, and have a still more efficient feeding and gill ventilating apparatus.

The bony fishes represent the culmination of a long evolution toward a body plan with maximum swimming efficiency. Particularly important in this evolution have been changes in fins and in the tail. Some authorities believe that the paired fins arose from a single continuous tail and anal fin that was divided at the vent and extended forward along each side to the head. Later the sections between the pectoral, pelvic, anal, and caudal fins were lost. The fin rays of sharks and rays are of a horny material, but those of many primitive fossil fishes are of bone. The bony fin rays of sarcopterygians and actinopterygians probably arose from scales lying in the fin folds. Modern teleost fishes have flexible fin rays (called soft rays) of jointed segments of bone, or spiny rays, each of solid continuous bone. The first dorsal fin of acanthopterygian fishes is of the spiny type. The original tail fin of primitive fishes was not an effective swimming organ, because of its asymmetry. The steady improvement in tail shape over 400,000,000 years is one of the prominent features of fish evolution. In primitive fishes the tail (vertebral) axis turned upward (heterocercal) or downward (hypocercal) and a lobe of flesh projected from it. This form of tail cannot provide a powerful driving mechanism, because the driving force is unevenly distributed relative to the body axis. With an asymmetrical tail, the fish swims by an undulating motion of the body and tail. In some fishes with a diphycercal tail (with the axis of the vertebrae extending down the middle of the fin lobe), developed in both modern and ancient fishes, the tail remains relatively ineffective because it has remained too rigid for proper propulsive action. The development of a true homocercal tail fin, in which powerful muscles move strong fin rays with a very flexible basal joint and in which the upper and lower lobes are about equal, is a development exclusive to teleost fishes.

As suggested by the existence of more than 400 families, teleosts are extremely varied in anatomical form and in the habitat occupied. They can be divided into about nine superorders, each with distinct evolutionary significance. The Leptolepidimorpha, an extinct, relatively primitive group, has uncertain relationships with other teleosts and is as yet poorly understood. The second group, the Elopomorpha, retains some relatively primitive living members, such as the tarpons, but is mostly represented by the large variety of specialized true eels. The Clupeomorpha includes the herrings and anchovies, relatively primitive fishes, mostly specialized for existence near the surface of the open ocean. A few species are anadromous, breeding in fresh water but spending most of their lives in the sea. The fourth superorder, the Osteoglossomorpha (osteoglossomorph), consists of a group of relatively primitive teleosts, most of which are now extinct. The few surviving members are tropical and worldwide in distribution but adapted for restricted habitats. The Protacanthopterygii is a varied collection of relatively primitive orders, marine, deep-sea, and freshwater in distribution; trouts, smelts, and argentines are examples. The sixth superorder, the Ostariophysi, is an important group of primarily freshwater fishes, including the characins, carps, minnows, loaches, suckers, and catfishes.

The remaining three superorders have a complex fossil history and are not yet fully understood, but all seem to possess similar evolutionary trends. Each group shows a tendency to develop spiny fin rays in the dorsal and anal fins (reduced in some) and a shelf of bone under the eye. There is a tendency for the pelvic fins to move forward on the body, with a reorganization of swimming methods and a slight gain in manoeuvrability. All three groups probably are related and presumably arose from some early protacanthopterygian ancestor. The Scopelomorpha includes a wide variety of deep-sea open-ocean plankton feeders and predators, some of which bear light organs. The Paracanthopterygii is a rather miscellaneous collection of fishes, the most important to man being the cods. The final superorder, the Acanthopterygii, is the result of the great radiation of modern spiny-rayed fishes and contains the dominant fishes in marine shore habitats, tropical, temperate, and Arctic. They also have penetrated the freshwater environment, especially lakes, slow-moving streams, and ponds. The superorder has some important open-ocean members, such as tunas. The key to the successful acanthopterygian radiation probably has been their mobile, protractile mouth.

In forming hypotheses about the evolution of fishes and in establishing classifications based on these hypotheses, ichthyologists place special emphasis on the comparative study of the skeleton. There are two primary advantages of this approach. First, direct comparison between extant and fossil groups is possible, the latter usually represented only by bony remains. The second advantage is that the bones of living fishes are relatively easy to observe and to study, compared with other body structures. Proper preservation and special preparation of the nervous system, for example, is difficult and expensive when the fishes compared are from the far ends of the earth. In the study of the relationships of species within a group major use has been made of similarities and differences in the dimensions of external features such as head and body length, and of counts of external characters, such as teeth, fin rays, and scales. Colour pattern is also important. In recent years valuable data on classification of fishes has been obtained from studies of comparative behaviour, physiology, genetics and functional anatomy.

The following classification has been derived primarily from the works of C. Patterson, Miles, P.H. Greenwood and co-workers, D.E. Rosen and C. Patterson, and K.S. Thomson. Fish classification has undergone major revisions in recent years and further modifications can be expected in the future. Ichthyologists frequently disagree on major as well as minor concepts of phyletic relationships. There remains much to learn about both living and fossil fishes. The geographical distribution given for a poorly known fossil group usually represents only the location of fossil finds not necessarily the true distribution of the group. In the classification presented here groups indicated by a dagger (†) are known only from fossils.