A fish is a creature that is found in the waters all over the world. They come under the classification of vertebrates and they are cold blooded species. There have been classified more than 30 thousands of species of fish in the world.

Most of the fishes have a bony skeleton while some have cartilages. Almost all of them lay eggs for reproduction. They breathe through gills, which is used to take oxygen from the surrounding water. They use fins for their movement. There are many types of fins in a fish. Either the fins are paired or unpaired. Fins like dorsal fin, fat fin, caudal fin, anal fin, pelvic fins and pectoral fins are found in a fish. The pelvic and the pectoral fins are paired fins.

Like other vertebrates fish too have a digestive system, respiratory system, circulatory system, excretory system and the central nervous system. Using their jaws the fish eat any food and the food enters the esophagus from which it enters the stomach. Here the food is broken and sent to pouch like organ where it further broken down with the help of the digestive enzymes. After this process the nutrients of the food are absorbed and the waste is sent to the excretory system.

The gills are the important organs in a fish in the respiratory system. Gills are used to exchange oxygen from the surrounding water and carbon dioxide. Most of the fishes gulp water and pass it to the gills for this exchange to happen. If you think that all the fishes take oxygen from water, you are wrong. There are fishes that get oxygen from air. They come to the surface of the water to gulp air and then go into water. Many catfishes come in this category.

In the circulatory system the heart plays an important role in pumping the blood to the other parts of the body. From the heart the blood is pumped to the gills where the exchange of oxygen is done and then the blood is pumped to the other parts of the body of the fish and then back to the heart. Fishes release ammonia as wastes and the level of ammonia in the body is controlled by the kidneys of the fish. Some of the wastes are released by the gills too. Fishes have very small brains when compared to the other vertebrates. Fishes have well developed eyes with color vision. Although they have ears in their head most of them can’t hear very well. The lateral line of the fish is too sensitive that it can sense the vibrations created by the movement of other fishes. This sense is used to identify other living organisms and prey by its side.

Fish plays an important role in the human life in many ways right from the earlier days till now. Fish is an important food for the human beings. It is also used for entertainment and relaxation. Sports fishing are in practice now in many parts of the globe. Aquariums with different species which are considered as lucky and relaxing are available in many parts of the world. Fishing industry is an important thriving industry in the world.

Acipenseriformes - Acipenseridae—sturgeons Polyodontidae—paddlefishes Chondrosteidae—extinct Errolichthyidae—extinct

Albuliformes - The bonefishes are a family of ray-finned fish that are popular as game fish in Florida, select locations in the South Pacific, and the Bahamas where two bonefish are on their 10 cent coin, and elsewhere.

Amiiformes - Bowfins are found throughout eastern North America, typically in slow-moving backwaters and ox-bow lakes.

Anguilliformes - True eels are an order of fish, which consists of four suborders, 19 families, 110 genera and approximately 800 species.

Ateleopodiformes - The scientific name means "Ateleopus-shaped", from Ateleopus + the standard fish order suffix "-formes".

Atheriniformes - Atheriniformes, also known as the silversides, is an order of ray-finned fish that includes the Old World silversides and several less-familiar families, including the unusual Phallostethidae.

Aulopiformes - Macristiidae

Batrachoidiformes - The toadfish comprise the family Batrachoididae, the only family in the ray-finned fish order Batrachoidiformes.

Beloniformes - The order is sometimes divided up into two suborders, the Adrianichthyoidei and the Belonoidei.

Beryciformes - Beryciformes is an order of ray-finned fishes.

Carcharhiniformes - The ground sharks, order Carcharhiniformes, are the largest order of sharks; they are also called whaler sharks.

Ceratodontiformes - Lungfish are freshwater fish belonging to the Subclass Dipnoi.

Cetomimiformes - The Cetomimiformes or whalefishes are an order of small, deep-sea ray-finned fish.

Characiformes - The Characiformes are an order of ray-finned fish, comprising the characins and their allies.

Chimaeriformes - Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , or rabbitfishes .

Clupeiformes - Clupeiformes is the order of ray-finned fish that includes the herring family, Clupeidae, and the anchovy family, Engraulidae.

Coelacanthiformes - Coelacanth , which is a reference to the hollow spines of the fins, is the common name for an order of fish that includes the oldest living lineage of gnathostomata known to date.

Cypriniformes - The Cypriniformes are an order of ray-finned fish, including the carps, minnows, loaches and relatives.

Cyprinodontiformes - Microcyprini Regan, 1909

Elopiformes - Elopiformes is the order of ray-finned fish that includes the tarpons, tenpounders, and ladyfish, as well as a number of extinct types.

Esociformes - Esociformes is a small order of ray-finned fish, with two families, the Umbridae and the Esocidae .

Gadiformes - Gadiformes is an order of ray-finned fish, also called the Anacanthini, that includes the cod and its allies.

Gasterosteiformes - Gasterosteiformes is an order of ray-finned fishes that includes the sticklebacks and relatives.

Gobiesociformes - Clingfishes are fishes of the family Gobiesocidae.

Gonorynchiformes - Gonorynchiformes is an order of ray-finned fish that includes the important food source, the milkfish , and a number of lesser-known types, both marine and freshwater.

Gymnotiformes - Aside from the electric eel, gymnotiformes are slender fish with narrow bodies and a tapering tail, hence the common name of "knifefishes".

Heterodontiformes - The bullhead sharks are a small order of basal modern sharks .

Hexanchiformes - Hexanchiformes is the order consisting of the most primitive types of sharks, and numbering just five extant species.

Lamniformes - Lamniformes is an order of sharks, also known as mackerel sharks .

Lampriformes - Lampriformes is an order of ray-finned fish.

Lepidosireniformes - Lungfish are freshwater fish belonging to the Subclass Dipnoi.

Lepisosteiformes - In American English the name gar is strictly applied to members of the Lepisosteidae, a family including seven living species of fish in two genera that inhabit fresh, brackish, and occasionally marine, waters of eastern North America, Central America, and the Caribbean islands.

Lobata - Lobata is an order of Ctenophora in the class Tentaculata with smaller tentacles than other ctenophores.

Lophiiformes - Some anglerfish are pelagic , while others are benthic .

Mugiliformes - The taxonomic order Mugiliformes of vertebrate fish contains the single family Mugilidae, the mullets.

Myctophiformes - Myctophiformes is an order of ray-finned fish consisting of two families of deep-sea marine fish, most notably the highly abundant lanternfishes .

Myxiniformes - Hagfish are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti.

Notacanthiformes - Notacanthiformes is an order of deep-sea ray-finned fishes, consisting of the families Halosauridae and Notacanthidae

Ophidiiformes - Ophidiiformes is an order of ray-finned fish that includes the cusk eels , pearlfishes , brotulas , and others.

Orectolobiformes - The carpet sharks are an order, Orectolobiformes, of sharks, so called because many members have ornate patterns reminiscent of carpets.

Orthoptera - Suborder Caelifera

Osmeriformes - Osmeriformes is an order of ray-finned fish that includes the true or freshwater smelts and allies, such as the galaxiids and noodlefishes; they are also collectively called osmeriforms.

Osteoglossiformes - Suborder Osteoglossoidei

Perciformes - The Perciformes, also called the Percomorphi or Acanthopteri, is the largest order of vertebrates containing about 40% of all bony fish.

Percopsiformes - Percopsiformes is a small order of ray-finned fish, comprising the trout-perch and its allies.

Petromyzontiformes - A lamprey is a parasitic marine/aquatic animal with a toothed, funnel-like sucking mouth.

Pleuronectiformes - The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes.

Polymixiiformes - The beardfishes are a small family of deep-sea marine ray-finned fish named for their pair of long hyoid barbels.

Polypteriformes - The bichirs are a family, Polypteridae, of archaic-looking ray-finned fishes, the sole family in the order Polypteriformes.

Pristiformes - Sawfishes are a family of marine animals related to rays.

Pristiophoriformes - Sawsharks have a pair of long barbels about halfway along the snout.

Rajiformes - Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body.

Saccopharyngiformes - Saccopharyngiformes is an order of unusual ray-finned fish superficially similar to eels, but with many internal differences.

Salmoniformes - Salmonids have a relatively primitive appearance among the teleost fish, with the pelvic fins being placed far back, and an adipose fin towards the rear of the back.

Scorpaeniformes - Scorpaeniformes is an order of ray-finned fish, but it has also been called the Scleroparei.

Siluriformes - Catfish are a diverse group of ray-finned fish.

Squaliformes - Squaliformes is an order of sharks that includes about 80 species in seven families.

Squatiniformes - While the forward part of the angel shark's body is broad and flattened, the rear part retains a muscular appearance more typical of other sharks.

Stephanoberyciformes - The Stephanoberyciformes are an order of marine ray-finned fishes, consisting of about 45 species, the majority of which belong to the ridgehead family .

Synbranchiformes - Synbranchiformes, often called swamp eels, is an order of ray-finned fishes that are eel-like but have spiny rays, indicating that they belong to the superorder Acanthopterygii.

Syngnathiformes - Syngnathiformes is an order of ray-finned fishes that includes the pipefishes and seahorses.

Tetraodontiformes - The Tetraodontiformes are an order of highly derived ray-finned fish, also called the Plectognathi.

Torpediniformes - The electric rays are a group of rays, flattened cartilaginous fish with enlarged pectoral fins, that comprise the order Torpediniformes.

Zeiformes - Suborder Caproidei