Teeth. Variations in the jaw spine may indicate the evolutionary advantage of a jaw mouth. It is not clear if the advantage of obscure jaws is more biting strength, improved breathing, or a combination of any ingredients.
Fish can evolve from an animal like a coral-like ocean current, whose larvae are importantly similar to primitive fish. The first ancestors of fish left the larval form in their youth (as some beaches do today), but this is probably the opposite picture.
Fish is a paraphyletic group: it means that any fish contains all the fish that contain tetrapods, which are not fish. For this reason, groups such as the fish species seen in the old reference works are no longer used in formal classification.
The Ladycythis is the largest known fish in the subclass Actinoptergaei.
Dition The endemic classification divides fish into three existing classes, and with extinct forms sometimes classified into trees, sometimes as their own class:  
Class Agnith (netless fish)
Subclass Cyclostomata (Hagfish and Lampress)
Subclass ostracodermy (armored netless fish).
Clond Chandrichthais (Cartilaginous fish)
Subclass Elasmobranchi (Shark and Ray)
Subclass Holocephaly (Chimeras and extinct relatives)
Class placoderma (armored fish).
Class akanthodii (“spiny shark”, sometimes classified under bone fish).
Subclass sarcoparty (fleshy fine fish, ancestor of tetrapods)
The above scheme is most commonly seen among non-specialist and general works. Many of the above groups are paraphyletic, as they have given birth to successive groups: the Agnathanas are the ancestors of the Chandrichia, who in turn gave birth to the ancestor of the Ostethis, Acanthodia. With the advent of phylogenetic nomenclature, fish have been divided into more detailed schemes, with the following major groups:
Class Maxini (Hugfish)
Class Peterspidomorphy early (fish without fish at the beginning)
Class Petromizantida or Hyperortia
Class Cephalaspidomorphi early (early netless fish)
Infraphilium ganathostomata (jaw spine)
Class Placodermae 6 (armored fish)
Clond Chandrichthais (Cartilaginous fish)
Class Acondodiai sp (cunning shark)
Clint actinoptregai (beaver fish)
Order asymptomatic forms (starzones and paddlefish)
Order polypitiformes (redfish and beech).
Infraclass Holocaust (Gars and Boffin)
Infraclass teleostei (many orders of conventional fish)
Class sarcoparty (lob-fine fish)
Subclass Actinistia (Koelenkethes)
Subclass Dipnoi (sister group on lungs, tetrapods)
† – Indicates extinct tokens
Some paleontologists claim that these are primitive fish because they are condonta cordates. For a complete treatment of this labor style, see the article on the spine.
Hugfish’s position in the film Chardata is not settled. Phylogenetic studies in 1999 and 1999 support the idea that Hugfish and Lampre form a natural group, the cyclostomata, a sister group of the ganothostomata.  
Different fish groups make up more than half of the vertebrate species. There are about 26,000 endemic species, including about 2,000,000 bone fish, 97070 sharks, ray and chimera, and about 100 hogfish and lampreys.  One-third of this species belongs to the nine largest families; These families, from the largest to the smallest, are Cyprinidi, Gobidi, Cichlidi, Charactidi, Loricaride, Balitaridae, Seranidae, Labridi and Scorpenidae. About 4 families are monotypic, where there is only one species. The final total number of species could exceed 32,500.  Gas is exchanged using gills on both sides of most fish bones. The gills have a threadless structure called a filament. Each filament has a capillary network that provides a large surface area for the exchange of oxygen and carbon dioxide. Fish exchange gases by drawing oxygen-rich water through their mouths and pumping them into their gills. In some fish the capillary blood flows against the water, causing a reciprocal exchange. The gills expel oxygen-weak water to the side of the alley. Some fish, such as sharks and lampreys, keep shaking multiple lakes. However, bone fish have a single gill open on each side. This opening is hidden behind a protective bone called an aperculum.
Adolescent beeches have external gills, a very primitive feature that they share with larval amphibians.
The term “fish” most precisely describes a non-tetrapod cranite (meaning a skull and, in most cases, a vertebrate) that lives throughout its life and whose limbs, if any, are wing-like [] in groups such as birds or mammals. In contrast, fish are not a clade, but taxa paraphyletic collections, including hogfish, lampreys, sharks and ray, radiant fish, quellencanth and lungs.   In fact, lungfish and quailcats are more common than re-finned fish or sharks. Mammals, birds, amphibians, etc.) are close relatives, so the last common ancestor of all fish is also the ancestor of tetropods. Since paraphyletic groups are no longer recognized in modern systemic biology, the use of the term “fish” as a biological group should be avoided.
Many types of aquatic animals are commonly called “fish” not fish in the above sense; Examples include shellfish, cattlefish, starfish, crayfish and jellyfish. By definition, however, not all mammals, including whales and dolphins, including cetaceans, are fish. In some contexts, especially in aquatic farming, real fish are called finfish (or fin fish) to distinguish them from other animals.
A typical fish is ecothermic, has a well-formed body for quick swimming, uses gills to extract oxygen from the water, or uses an accessory respiratory organ to breathe atmospheric oxygen, has two sets of paired birds, usually one or two (rare three). It has a dorsal fins, a rectal fins, and a tail fin, has a jaw, has skin that is usually covered with fibers and lays eggs.
Each criterion has exceptions. cannot swim a length of 0.5 feet per second.  Many freshwater fish extract oxygen from the air as well as from water using a variety of different structures.as the corridorus expel oxygen through the intestines or stomach.  Body size and wing system are highly variable, covering seemingly un-fish species such as seahorses, pufferfish, angelfish, and gallopers. Similarly, the surface of the skin may be bare (like twisted inlays), or covered with a variety of different sizes, usually plaqueid (characterized by sharks and ray), cosmoids (lungfish and quelinkanth), ganoid (various fossils), and various fossils. Cycloids and stenoids (these last two are found in most bone fish)  There are even fish that live mostly on land or lay eggs in land near water.  Mudskippers feed and interact with each other on mudflats and go underwater to hide their aging.  A single, indescribable species of fretobias, known as a true “landfish” because these worm-like catfish are waterlogged. Many species live underground in lakes, underground rivers or wetlands, and are known as kafish fish.  
Small to 8-millimeter (0.3-inch) scout infanfish from 16-meter (52-foot) whale sharks in size.