Fish are cold-blooded aquatic animals with backbones, gills, and fins. Most fishes are torpedo-shaped (fusiform) for efficient travel through water, but much variation occurs, from flattened and rounded, as in flounders, to vertical and angular, as in sea horses. Fishes range in size from the pygmy goby, Pandaka pygmaea, of the Philippines, which reaches only 12 mm (0.5 in) long and about 1.5 g (0.05 oz) in weight and is sexually mature at 6 mm (0.25 in), to the whale shark, Rhincodon typus, which grows to 18 m (60 ft) long and over 20 tons in weight. Fish were among the first animals systematically hunted by primitive humans. Even today, relatively primitive societies in the South Pacific and South America depend largely on fish for food; while in many industrialized nations, fish still constitute a major part of the diet. It is said that the search for codfish led French fishermen to the discovery of Canada and that villages sprang up on the coasts of Norway, Scotland, Japan, and other countries wherever shoals of herrings regularly came close to shore. Today fishes are harvested for unprocessed human food, fish meal, animal feed, and oil. They also are pursued avidly by sport anglers, who contribute to the economy of fishing areas and to specific industries. Currently, however, the increasing human population, overfishing to supply this population, and pollution of the world's waters are all cutting heavily into the world supply of fish, and threatening the existence of a number of species. At the same time, regulations to curtail the taking of certain species or sizes are virtually unenforceable on an international level. DISTRIBUTION Fish are found throughout the world, from altitudes of more than 5,000 m (3 mi), as in Lake Titicaca, located 3,800 m (2.3 mi) above sea level in the Andes, to depths of about 10 km (6 mi) in the Pacific Ocean. Some, like certain killifishes, Cyprinodon, inhabit hot springs, where the water temperature may reach 45 deg C (113 deg F); others, like the icefishes, Chaenocephalus, are found in Antarctic seas, where water temperature may fall below 0 deg C (32 deg F). About 107 species, including the swordfish, Xiphias, are distributed worldwide in tropical and subtropical waters, but many species have very limited ranges, among the smallest being that of the killifish Cyprinodon diabolis, which is confined to a single spring in Nevada. About 70% of the Earth's surface is covered by oceans and seas, and about 3.5% of the land surface (1% of the Earth's total surface) is covered by fresh water. Inhabiting these waters are an estimated 20,000 or more fish species, equal to or exceeding the number of all other vertebrate species combined. Bird species number approximately 8,600; reptiles, 6,000; mammals, 4,500; and amphibians, 2,500. About 60% of the fish species live in marine waters; the remaining 40% are found in fresh water. Most of the world's fishes are continental in orientation, living either as part of the freshwater systems on land or as sea-dwellers staying near and influenced by the coastal environment. High densities of marine fish populations occur near coasts, because the waters there are extremely rich in nutrients. Coastal benefits include chemical and organic enrichment discharged by rivers, upwellings from the ocean depths that recycle previously deposited nitrates and phosphates, aeration caused by surf and tide, and the penetration of sunlight. ANATOMY The living species of fish are usually divided into three classes: the Agnatha, the jawless fishes, comprising the hagfishes and lampreys; the Chondrichthyes, the cartilaginous-skeleton fishes, such as sharks and rays; and the Osteichthyes, the bony-skeleton fishes, comprising all other living fishes. The skeletons of these three groups vary in fundamental ways. In the hagfishes and lampreys the backbone is basically a notochord, a rodlike structure composed of unique notochordal tissue. In sharks and rays the notochord is surrounded and constricted by spaced rings of cartilage, the vertebrae, to form a backbone. The remainder of the skeleton is also cartilaginous, not bony, but in many forms the cartilage is partly calcified, and thereby hardened, by the addition of calcareous salts. In primitive bony fishes, such as the sturgeon, the vertebrae spaced along the notochord are still largely cartilaginous, but in most advanced bony fishes the vertebrae are bony and are united to form the backbone, and the notochord is no longer present. Some fishes, such as lampreys, lack ribs; others have either a single or a double pair of ribs attached to each trunk vertebra. Among the higher bony fishes there also may be small, riblike intermuscular bones, which often render such fish difficult to eat. The body appendages of fish are of two kinds, cirrhi and fins. Cirrhi are flaps of flesh that may appear on any part of the body; they often serve as camouflage. Fins are either median or paired. Median fins are situated along the centerline of the body, at the top, the bottom, and the end. The top, or dorsal, fin may consist of one to several fins, one behind the other, and may include a fleshy fin, called the adipose fin, near the tail. The bottom, or anal, fin is located on the belly behind the vent, or anus. The end fin is called the tail, or caudal, fin. The dorsal and anal fins may be supported by cartilaginous rods, as in the lampreys, by cartilaginous rods and horny rays, as in sharks, by horny rays, as in the spiny-finned fishes, or by bony rays (derived from scales) in the soft-rayed fishes. The tail fin may be protocercal, the body continuing straight back as a middle support between the upper and lower lobes of the tail; heterocercal, with the end of the body turning up and continuing to the tip of the upper lobe; or homocercal, in which the last few vertebrae are fused and joined with other bony elements (hypurals) to support the tail-fin rays. A modification of the heterocercal tail so as to resemble the protocercal type is called diphycercal. The paired fins correspond to the arms and legs of land vertebrates. The pectoral fins are situated at the front of the body behind the gill openings and generally function to provide maneuverability, but may be highly modified to fulfill other functions. The simplest internal support for the pectoral fins occurs in the sharks, where a U-shaped cartilaginous skeletal structure, called the pectoral girdle, joins and helps support the two pectoral fins. In the higher bony fishes the pectoral girdle is composed of bone and is more complex in structure. The pelvic fins, also called the ventral fins, are located along the bottom of the body but vary considerably in their placement. They may be located in the middle of the belly, as in salmon; below the pectorals, as in the largemouth bass; or in front of the pectorals, as in cods. Pelvic fins also serve as maneuvering structures and also may be modified to serve other uses. The supporting pelvic girdle is lacking in many bony fishes; in most fishes in which the pelvic girdle is present it is represented by a single skeletal element on each side of the body. The scales of fish are colorless; a fish's coloring arises from structures beneath or closely associated with the scales. Not all species of fishes have scales, or the scales may be so small as to make the fish appear scaleless. Scales also may be present only on small areas of the body. The arrangement of scales may be imbricate (overlapping like the shingles on a roof) or mosaic (fitting closely together or just minutely separated). Four basic scale types can be distinguished on the basis of structure. Placoid scales, also called dermal denticles, are found on sharks and rays and are toothlike in structure. Indeed, modified and enlarged placoid scales have become the teeth of sharks. The placoid scale consists of an upper layer of enamellike substance called vitrodentine, a lower layer of dentine, a pulp cavity, and a disklike basal plate embedded in the skin. Placoid scales do not increase in size as do the scales of bony fishes, and new scales must be added as a shark grows. Cosmoid scales are found on the primitive coelacanth. They also occur on lungfishes, but in a highly modified, single-layered form. The cosmoid scale of the coelacanth is a four-layered bony scale. The upper layer is enamellike vitrodentine; the second layer is a hard, dentinelike substance called cosmine; the third layer is spongy bone, and the lowest layer is dense bone. Ganoid scales, as found on gars, are typically squarish (rhombic) in shape and consist of a single bony layer, a layer of cosmine, and a covering of a very hard enamellike substance called ganoin. Leptoid scales are believed to have been derived from ganoid scales by the loss of the ganoin layer; they consist of a single layer of bone. Leptoid scales are found on the higher bony fishes and occur in two forms: cycloid (circular) and ctenoid (toothed), the latter bearing tiny comblike projections. The single-layered cosmoid scale of lungfishes also may be classified as leptoid, although of a different derivation. CIRCULATION The blood of the fish serves, as does the blood of other vertebrates, to transport oxygen, nutrients, and wastes. The typical fish's circulation is a single circuit: heart-gills-body-heart. In contrast, mammals have two circuits: heart-lungs-heart and heart-body-heart. The fish heart proper is two-chambered, consisting of an upper atrium and a lower ventricle. Amphibians, basically, have a three-chambered heart, two atria and one ventricle; reptiles have a three- or four-chambered heart; and mammals and birds have a four-chambered heart consisting of two atria and two ventricles. The fish heart, however, has two accessory chambers, and all four chambers are contained within a single pericardial sac. One accessory chamber is the thin-walled sinus venosus, which collects blood and leads into the atrium; the other accessory chamber is the conus arteriosus, an enlargement of the main artery leading out of the ventricle. In some fishes, such as sharks, the conus arteriosus is muscular and pumps blood in the manner of the ventricle. RESPIRATION In order to live, fish must extract oxygen from the water and transfer it to their bloodstream. This is done by gills, lungs, specialized chambers, or skin, any of which must be richly supplied with blood vessels in order to act as a respiratory organ. Extracting oxygen from water is more difficult and requires a greater expenditure of energy than does extracting oxygen from air. Water is a thousand times more dense (heavier per unit volume) than air, and at 20 deg C (68 deg F) it has 50 times more viscosity (resistance to flow) than air and contains only 3% as much oxygen as an equal volume of air. Fishes, therefore, have necessarily evolved very efficient systems for extracting oxygen from water; some fishes are able to extract as much as 80% of the oxygen contained in the water passing over the gills, whereas humans can extract only about 25% of the oxygen from the air taken into the lungs. Gills are made efficient in a number of ways. (1) A large surface area for gaseous exchange means that more oxygen can enter the bloodstream over a given period of time. A single gill of a bony fish consists of a curved gill arch bearing a V-shaped double row of gill filaments. Each filament has many minute folds in its surface, giving it a sort of fuzzy appearance and increasing the amount of surface area along a given length of filament. Consequently, the surface area of the gills is commonly 10 to 60 times more than that of the whole body surface. (2) A short diffusion, or travel, distance for the oxygen increases the rate of oxygen entry into the blood. The blood traveling in the folds of the filaments is very close to the oxygen-containing water, being separated from it by a very thin membrane usually 1 to 3 microns (4/100,000 to 1/10,000 in) thick, and possibly less. (3) By using countercurrent circulation in the gill, the blood in the filament folds travels forward, in the opposite direction to the water flow, so that a constant imbalance is maintained between the lower amount of oxygen in the blood and the higher amount in the water, ensuring passage of oxygen to the blood. If the blood were to flow in the same direction as the water, oxygenated blood at the rear of the gills would be traveling with deoxygenated water and not only could not extract oxygen from the water but would even lose oxygen to it. (4) Gills have little physiological dead space. The folds of the filament are close enough together so that most of the water passing between them is involved in the gas-exchange process. (5) Water flows continuously in only one direction over the gills, as contrasted with the interrupted, two-way flow of air in and out of lungs of mammals. AIR BREATHING A fish out of water usually dies because its gills collapse, reducing the area of the respiratory surface, and become dry, effectively stopping the diffusion of oxygen into the blood. Many fishes, however, have evolved methods of extracting oxygen from air. Such adaptations permit these fishes to live in oxygen-poor waters, where they come to the surface to gulp air, or in waters subjected to drying; or they may enable a species to exploit environments, such as damp beaches, unavailable to other fishes. One method of air breathing is the development of gills that either secrete mucus or trap moisture that supports the gills and keeps them wet. Another method is to breathe through a damp skin, as do the freshwater eels. Very commonly, special chambers have been developed in the mouth, throat, or head in which inspired air is brought into contact with moist tissues richly supplied with blood vessels. Some fishes have thin-walled areas in the intestine where oxygen can be extracted from swallowed air. In still others, the swim bladder, often mistaken for a lung because of its inflated shape and shiny, silvery white walls, is modified into an air-breathing apparatus. Air breathing has become so important to some species that they will drown if not allowed access to air. BODY TEMPERATURE Fish are described as cold-blooded, meaning that their body temperature varies with the external temperature. Fish do, however, produce metabolic heat (that is, heat derived from the oxidation, or "burning," of food and from other processes), but much of this heat is lost to the outside at the gills. Blood passing through the gills loses heat to the water quite rapidly, so that a fish's body temperature is usually within a degree or so of the water temperature. Tunas and mackerel sharks, however, are warm-bodied fishes. They have evolved countercurrent circulatory networks that consist basically of paired ingoing and outgoing blood vessels. In this way the heat of the warm blood going to the gills is transferred to the cooled blood coming from the gills, and the heat is kept within the fish's body. By using these networks, yellowfin and skipjack tuna are able to keep their body temperature from about 5 deg to almost 12 deg C (9 deg to 21 deg F) above the water temperature. One skipjack taken in warm waters registered a body temperature of 37.8 deg C (100 deg F). The bluefin tuna does even better and might qualify as a warm-blooded (as opposed to warm-bodied) fish. It is able to maintain a fairly constant body temperature across different water temperatures, its body temperature varying only about 5 C degrees (9 F degrees) over a 20 C degree (36 F degree) range of water temperatures. One of the advantages of warm-bodiedness is an increase in muscle power. Muscles contract more rapidly when warm without loss of force. If with a 10 C degree (18 F degree) rise in body temperature a muscle can contract three times as fast, then three times the power is available from that muscle. More muscle power means more speed in pursuing prey, escaping enemies, and shortening the time required for long-distance migration. WATER BALANCE The blood of freshwater fishes is typically more salty than the water in which they live. Osmotic pressure, the force that tends to equalize differences in salt concentrations, causes water to diffuse, or enter, into the fish's body, primarily through the gills, mouth membranes, and intestine. To eliminate this excess water, freshwater fishes produce a large amount of very dilute urine. Lampreys, for example, may daily produce an amount of urine equal to as much as 36% of their total body weight; bony fishes commonly produce amounts of urine equaling from 5 to 12% of their body weight per day. As these fishes are gaining water, they are losing salts. Salts contained in their foods are insufficient to maintain the proper salt balance. Freshwater fishes have therefore developed the capacity to absorb salts from water by means of their gills. Marine bony fishes, in contrast, have blood that is less salty than sea water, and consequently they lose water and absorb salts. To offset this loss of fluid, marine fishes drink seawater and produce very little urine. The drinking of seawater, however, adds to the concentration of salts. These salts are eliminated in several ways. Calcium, magnesium, and sulfates are passed out through the anus along with wastes. Sodium, potassium, chloride, and nitrogenous compounds, such as urea, are excreted through the gills. The hagfishes and the sharks have approached the problem of fluid balance in other ways. Hagfish blood has a total salt concentration approximately equal to that of seawater. Sharks' gills do not excrete the nitrogenous waste product urea, retaining it instead in the blood. The presence of urea and another waste product, trimethylamine oxide, as well as various salts, keeps the shark's blood at a slightly higher solute concentration than that of seawater. SWIMMING Many fishes swim by contracting and relaxing a succession of muscle blocks, called myomeres, alternately on each side of the body, starting at the head and progressing down toward the tail. The alternate shortening and relaxing of successive muscle blocks, which bends part of the body first toward one side and then toward the other, results in a series of waves traveling down the fish's body. The rear part of each wave thrusts against the water and propels the fish forward. This type of movement is quite clearly seen in the freshwater eel. Because movement of the head back and forth exerts drag, which consumes additional energy and slows travel, a great many fishes have modified this snakelike motion by keeping the waves very small along most of the length of the body, in some cases showing no obvious movement at all, and then increasing them sharply in the tail region. It is the end of the traveling waves that moves the tail forcefully back and forth, providing the main propulsion for forward motion. A simpler form of tail propulsion is seen in such inflexible-bodied fishes as the trunkfish, which simply alternates contractions of all the muscle blocks on one side of the body with those on the other side, causing the tail to move from side to side like a sculling paddle. Some of the predatory bony fishes are the fastest swimmers; they can cruise at speeds that are between three and six times their body length per second and may be able to reach 9 to 13 body lengths per second in very short bursts. Some fishes, such as the blenny, which has been timed at 0.8 km/hr (0.5 mph), swim very slowly; others, such as the salmon, which may reach a sustained speed of 13 km/hr (8 mph), move much faster; and it has been estimated that tuna may reach speeds of 80 km/hr (50 mph), and swordfish, 97 km/hr (60 mph). GAS BLADDER Because a weightless, or buoyant, body requires a minimum of energy to keep it at a given depth, and because a weightless body requires less energy than a weighted body to move at a given speed, many fishes have evolved means of reducing their body weight, or density, relative to the density of water. A fish whose total body density equaled that of water would be effectively weightless, neither rising nor sinking. Because fat is less dense than water, one method of reducing body density would be to increase the proportion of fat within the body. Theoretically, about one-third of a fish's body weight would have to be made up of fat in order to make the fish weightless in seawater. This condition is approached in some species of deep-sea sharks having very large livers that contain a great amount of squalene, a fatty substance that is significantly less dense than seawater. Another method of reducing density is to include gases within the body. Many fishes have a gas-filled bladder that serves this function. The gases within the bladder are similar to those in air but are present in different and widely varying proportions. The degree of body volume that must be taken up by gas in order to achieve weightlessness depends mainly upon whether the fish is freshwater or marine. Fresh water is less dense than seawater and consequently provides less buoyancy. Freshwater fishes, therefore, require a larger gas bladder than do marine fishes to keep them from sinking. According to calculations, the capacity of a gas bladder should be about 7% of body volume for a freshwater fish and 5% for a marine fish. In actual measurements, freshwater fishes' gas bladders have been found to range from 7 to 11% of body volume, while those of marine fishes have ranged from 4 to 6% of body volume. If the gas bladder contained an unchanging quantity of gas, the fish possessing it would be weightless at only one depth. The reason for this is that as pressure increases with depth, the gas in the bladder is compressed, decreasing the bladder's volume and increasing the relative density of the fish. The fish would then have to use considerable energy to stop its increasingly denser body from sinking. Conversely, when a fish rises from great depths and pressure is decreased, the volume of the bladder expands and the fish becomes too light to remain at a given depth without considerable effort (in extreme cases, such as with certain deep-water fishes, excessive expansion of gases in the bladder can cause the bladder to burst). The quantity of gas within a fish's bladder must therefore be adjustable. If, as in the carp, the gas bladder is connected by a duct to the gullet, gas may be expelled through the mouth and gill cavities as the fish rises, and, in a similar manner, gas may be added to the bladder by swallowing air at the water surface. For most fishes, however, coming to the surface to gulp air prior to going deeper is impractical, and in many fishes the gas bladder has no connection to the outside. In these fishes there must be another means of adjusting the quantity of gas within the bladder. This is done by transferring gases from the gas bladder to adjoining blood vessels and back again. Deflating the gas bladder is a passive operation, working under the higher pressures building up within the gas bladder. This pressure forces the gas into surrounding blood capillaries, which then carry the gas away. These blood vessels may be scattered through the walls of the gas bladder, they may be confined to a compartment at the rear of the bladder, or they may be restricted to a region at the top of the bladder, which is separated from the bladder by a constrictive muscle and is known as the oval organ. Inflating the gas bladder is an active, or dynamic, operation because it is done against the high pressures within the bladder. The gases may be forced into the bladder by blood vessels covering large areas of the bladder walls, or, more commonly, by a combination of two units known as the gas gland and the rete mirabile. The gas gland is a modification of the inner lining of the bladder; the rete mirabile is a dense bundle of capillaries arranged side by side in countercurrent fashion. Blood leaving the gas bladder would be carrying a gas, such as oxygen, at a high pressure equal to that within the gas bladder. Blood arriving at the gas bladder would be carrying oxygen at quite a low pressure, equal to that within the water passing over the gills. The oxygen therefore diffuses from the outgoing blood into the incoming blood. This process is repeated continuously, with greater and greater concentrations and higher and higher pressures of oxygen collecting at the junction point of the outgoing and incoming capillaries, the gas gland. The gas gland may facilitate this buildup by secreting lactic acid, which acts to increase the pressure of oxygen within the blood. When the pressure of the oxygen in the gas gland exceeds that within the gas bladder, the oxygen diffuses into the bladder. LATERAL LINE SYSTEM The lateral line system, found in many fishes and in some aquatic amphibians, is sensitive to differences in water pressure. These differences may be due to changes in depth or to the currentlike waves caused by approaching objects. The basic sensory unit of the lateral line system is the neuromast, which is a bundle of sensory and supporting cells whose projecting hairs are encased in a gelatinous cap. The nueromasts continuously send out trains of nerve impulses. When pressure waves cause the gelatinous caps of the neuromasts to move, bending the enclosed hairs, the frequency of the nerve impulses is either increased or decreased, depending on the direction of bending. Neuromasts may occur singly, in small groups called pit organs, or in rows within grooves or canals, when they are referred to as the lateral line system. The lateral line system runs along the sides of the body onto the head, where it divides into three branches, two to the snout and one to the lower jaw. A swimming fish sets up a pressure wave in the water that is detectable by the lateral line systems of other fishes. It also sets up a bow wave in front of itself, the pressure of which is higher than that of the wave flow along its sides. These near-field differences are registered by its own lateral line system. As the fish approaches an object, such as a rock or the glass wall of an aquarium, the pressure waves around its body are distorted, and these changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action. Because sound waves are waves of pressure, the lateral line system is also able to detect very low-frequency sounds of 100 Hz or less. An interesting adaptation of the pressure-sensitive systems is seen in the modified groups of neuromasts called the ampullae of Lorenzini, which are found in sharks and certain bony fishes. The ampullae of Lorenzini act as electroreceptors and are able to detect electrical charges, or fields, in the water. Most animals, including humans, emit a DC field when in seawater. This is presumably caused by electrical potential differences between body fluids and seawater and between different parts of the body. An AC field is also set up by muscular activity (contractions). A wound, even a scratch, can markedly alter these electrical fields. The cat shark, Scyliorhinus, is known to catch prey by using its ampullae of Lorenzini to detect the electrical field generated by flatfish (plaice) buried beneath the sand. REPRODUCTION Most fishes are egg-layers, but many bear living young. Live-bearing fishes may be ovoviviparous, in which the eggs essentially simply hatch within the female, or viviparous, in which the unborn young are supplied nourishment through the mother's tissues. In some ovoviviparous fishes the embryo develops in the egg while the egg is still within its follicular covering within the ovary, and ovulation (or release of the egg) and birth occur at the same time. In other ovoviviparous forms the eggs are released from the protective follicles into the cavity of the hollow ovary, where development continues. In some viviparous fishes the walls of the egg follicle are in intimate contact with the embryo, supplying it with nourishment. In the viviparous sharks, a part of the oviduct, or egg channel, is developed into a uterus, where the modified yolk sacs of the young are closely joined to pockets within the uterus. In live-bearing fishes and in some egg-layers, fertilization occurs internally, and methods have been evolved for introducing the sperm into the female's body. In sharks the pelvic fins of the male are modified into intromittent organs called myxoptergia, and in the male topminnows the anal fin is modified into a similar-functioning intromittent organ called the gonopodium. At least three modes of reproduction--heterosexual, hermaphroditic, and parthenogenetic--are found in fishes. In the most common form, heterosexual reproduction, there are separate male and female parents, but even here there is considerable variation. In some live-bearing fishes, the female is able to store sperm for up to 8 or even 10 months, and this sperm is used to fertilize new batches of eggs as they develop. In some cases, a female may carry sperm from several males at once. In hermaphroditic reproduction, a single fish is both male and female, produces both eggs and sperm (either at the same time or at different times), and mates with other similar hermaphroditic fishes. External self-fertilization occurs in one hermaphroditic fish, which sheds egg and sperm simultaneously. In another, internal self-fertilization may occur. In certain fishes there is a time sequence of hermaphroditism, young fishes reversing their sex as they grow older. In parthenogenetic reproduction, unfertilized eggs develop into embryos. This is known to exist in at least one fish species, Poecilia formosa, of the Amazon River; however, even though development proceeds without fertilization in some of these females, mating with a male is still required to stimulate egg development. Parental care also shows great diversity. Some fishes, like the Atlantic herring, form huge schools of males and females and freely shed their eggs and sperm (milt), and then abandon the eggs. Other fishes build nests and care for both the eggs and newly hatched young. Others have evolved methods of carrying the eggs with them, commonly in their mouths, but also in gill cavities or in special pouches on the body. EVOLUTION The first fishes, and indeed the first vertebrates, were the ostracoderms, which appeared in the Cambrian Period, about 510 million years ago, and became extinct at the end of the Devonian, about 350 million years ago. Ostracoderms were jawless fishes found mainly in fresh water. They were covered with a bony armor or scales and were often less than 30 cm (1 ft) long. The ostracoderms are placed in the class Agnatha along with the living jawless fishes, the lampreys and hagfishes, which are believed to be descended from the ostracoderms. The first fishes with jaws, the acanthodians, or spiny sharks, appeared in the late Silurian, about 410 million years ago, and became extinct before the end of the Permian, about 250 million years ago. Acanthodians were generally small sharklike fishes varying from toothless filter-feeders to toothed predators. They are often classified as an order of the class Placodermi, another group of primitive fishes, but recent authorities tend to place the acanthodians in a class by themselves (class Acanthodii) or even within the class of modern bony fishes, the Osteichthyes. It is commonly believed that the acanthodians and the modern bony fishes are related and that either the acanthodians gave rise to the modern bony fishes or that both groups share a common ancestor. The placoderms, another group of jawed fishes, appeared at the beginning of the Devonian, about 395 million years ago, and became extinct at the end of the Devonian or the beginning of the Mississippian (Carboniferous), about 345 million years ago. Placoderms were typically small, flattened bottom-dwellers. The upper jaw was firmly fused to the skull, but there was a hinge joint between the skull and the bony plating of the trunk region. The cartilaginous-skeleton sharks and rays, class Chondrichthyes, which appeared about 370 million years ago in the middle Devonian, are generally believed to be descended from the bony-skeleton placoderms. The cartilaginous skeletons are considered to be a later development. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 395 million years ago. The early forms were freshwater fishes, for no fossil remains of modern bony fishes have been found in marine deposits older than Triassic time, about 230 million years ago. The Osteichthyes may have arisen from the acanthodians. A subclass of the Osteichthyes, the ray-finned fishes (subclass Actinopterygii), became and have remained the dominant group of fishes throughout the world. It was not the ray-finned fishes, however, that led to the evolution of the land vertebrates. The ancestors of the land vertebrates are found among another group of bony fishes called the Choanichthyes or Sarcopterygii. Choanate fishes are characterized by internal nostrils, fleshy fins called lobe fins, and cosmoid scales. The choanate fishes appeared in the late Silurian or early Devonian, more than 390 million years ago, and possibly arose from the acanthodians. The choanate fishes include a group known as the Crossopterygii, which has one living representative, the coelacanth Latimeria. During the Devonian Period some crossopterygian fishes of the order (or suborder) Rhipidistia crawled out of the water to become the first amphibians. Classification The classification of fishes is a subject of considerable debate. The living fishes are often divided into three different classes. Divisions within these classes, however--particularly within the bony fishes (Osteichthyes)--are much in dispute, with different names being applied to the same group and with a given group being regarded as either a subclass, order, or some other rank, depending upon the authority consulted. The following is a general classification; to help clarify terms that may be found in other sources, it also includes as many additional definitions as space allows. Class Agnatha or Cephalaspidomorphi, the jawless fishes Subclass (or order) Cyclostomata, the lampreys and hagfishes. (In certain classifications, the lampreys and hagfishes are each considered separate superclasses: Cephalaspidomorphi and Pteraspidomorphi, respectively.) Class Chondrichthyes, the cartilaginous-skeleton fishes Subclass Holocephali, the chimaeras, or ratfishes Subclass Elasmobranchii, the sharks, skates, and rays Class Osteichthyes, the bony fishes Subclass (or order) Crossopterygii, the coelacanth Subclass (or order) Dipnoi or Dipneusti, the lungfishes (In some classifications, the above two subclasses are treated as orders of a single subclass, the Choanichthyes or Sarcopterygii, the lobe-finned fishes.) Subclass Actinopterygii, the ray-finned fishes Infraclass (or superorder) Chondrostei, the primitive ray-finned bony fishes: sturgeons, paddlefish, and bichirs (In some classifications, the bichirs are placed in a subclass of their own, the Brachiopterygii.) Infraclass (or superorder) Holostei or Neopterygii, the intermediate ray-finned fishes: gars and the bowfin (In certain classifications, the gars are treated as a separate superorder, the Ginglymodi. The term Ginglymodi also has been used to designate the gars as an order, but this term has been replaced at the ordinal level by the term Lepisosteiformes; orders are now indicated by the ending -formes.) Infraclass (or superorder) Teleostei or Neopterygii, the advanced bony fishes: herring, salmon, perch. Edwin E. Rosenblum Bibliography: Alexander, R. M., Functional Design in Fishes 3d ed. (1974); Blake, R. W., Fish Locomotion (1983); Bone, Q., and Marshall, N. B., Biology of Fishes (1983); Dalrymple, Byron, Complete Guide to Game Fish (1981); Kennleyside, M. H., Diversity and Adaptation in Fish Behavior (1979); Madsen, Kjeld, Aquarium Fishes in Color (1982); McClane, A. J., McClane's Field Guide to Freshwater Fishes of North America (1978); Migdaklsi, E. C., and Fichter, G. S., The Fresh and Salt Water Fishes of the World (1976); Moyle, P. B., and Cech, J. J., Fishes: An Introduction to Ichthyology (1982); Nelson, J. S., Fishes of the World (1976); Norman, J. R., A History of Fishes, 3d ed., ed. by P. H. Greenwood (1975); Smith, Lynwood S., Introduction to Fish Physiology (1982).