Fish belongs to the subphylum Vertebrata in the phylum Chordata, and vertebrates are generally categorized into five major groups: fish (53%), birds (18%), reptiles (12%), mammals (9%), and amphibians (8%). According to Nelson (1994) statistics, the global living species of fish **** 24,618 species, accounting for more than half of the named vertebrates, and new species of fish are constantly being discovered, an average of about 150 species per year, more than 10 years should have increased by more than 1,500 species, the world's current named species of fish should be more than 26,000.

The character for fish, pinyin yu, means "fish", and fish are the oldest vertebrates. They inhabit almost every aquatic environment on Earth - freshwater lakes and rivers to saltwater seas and oceans.

What exactly are the animals that belong to "fish"? Modern taxonomists define a fish as a vertebrate that spends its entire life in water, breathes through its gills, and swims with its fins. Fish include the three major groups of the Orthostomatidae, Chondrichthyes and Sclerostomatidae. There are more than 26,000 known species of fish in the world, which is the largest group of vertebrates, accounting for 48.1% of all vertebrates. The vast majority of them live in the ocean, there are about 8600 species of freshwater fish, China's existing fish nearly 3,000 species, of which about 1,000 species of freshwater fish.

The general body shape of fish, suitable for swimming in the water, the whole body is fusiform and slightly flat. In the three body axis, the head and tail axis is the longest, the dorsal and abdominal axis is the second, the left and right axis is the shortest, so that the whole body is streamlined or slightly flattened, in order to facilitate the movement in the water to move forward to reduce the resistance, so this kind of fish is good at swimming. They often inhabit the middle and upper layers of water. They can migrate long distances. Examples: carp, crucian carp, shark.

The appendages of fish are fins, which are the motor organs for swimming and maintaining body balance. The fins consist of branched fin stretcher bones and fins, fins are divided into two types, one angular fins are not segmented or branched, occurring from the epidermis, seen in cartilaginous fishes; the other is the squamous fins, or bony fins, derived from the scales and segmented, branched or unbranched, seen in hard-boned fishes, the fins are connected by thin fin rays. Bony fins are divided into two types, fin spines and soft strips. Fin spines are formed by deformation of one type of fin and are hard spines that are neither branched nor segmented, and are found in higher fishes. Soft strips are soft and articulated, with either a distal branch (called a branching fin) or no branch (called an unbranched fin), and are formed by merging the left and right halves. There are two types of fins: odd and even. Even fins are paired fins, including one pair of pectoral and one pair of ventral fins, which are equivalent to the anterior and posterior limbs of terrestrial vertebrates; odd fins are unpaired fins, including the dorsal, caudal, and anal (anal) fins. The basic function of the dorsal and anal fins is to maintain the balance of the body, prevent tilting and swaying, and help swimming, while the caudal fin, like a rudder, controls the direction and pushes the fish forward. Generally, all common fish have the above five types of fins: pectoral, ventral, dorsal, anal and caudal. However, there are a few exceptions, such as eels without even fins, odd fins are also degraded; eels without ventral fins; electric eels without dorsal fins and so on.

The movement of the fish has a very close relationship with the changes in body shape and fins, and its swimming power mainly relies on the following three ways: the use of the trunk and tail muscle contraction wave-like movement.

Relying on the fins swing paddling movement.

Using the gill holes to spray water backward caused by the reaction force to make the fish forward. Fish movement in addition to swimming, a few fish also have a special form of movement, that is, jumping or flying, such as chub can jump diagonally out of the water very high, and then fall into the water vertically. Flying fish can jump diagonally out of the water with great force, but also can open wide pectoral fins, and soar up to about 300 meters in the air. Salmon can repeatedly jump over many obstacles in rivers and migrate from the sea to the middle and upper reaches of rivers to spawn. In addition, there are very few fish that can crawl, such as the monkfish and the bouncing coat.