Fish is a kind of aquatic vertebrate with variable temperature. It breathes through gills, swims through the swinging of tail and trunk, and feeds on the upper and lower jaws. It belongs to the vertebrate subfamily of Chordata. Vertebrates are generally divided into fish (53%), birds (18%), reptiles (12%), mammals (9%) and amphibians. According to the statistics of the late Canadian scholar Nelson in 1994, there are 246 18 species of fish in the world, accounting for more than half of the named vertebrates, and new fish species are constantly being discovered, with an average of about 150 species per year, which should increase by more than 1500 species in more than ten years. At present, there are about 32,438 species of named fish in the world.

There are about 32,000 kinds of fish in the world. Fish live in water, distributed in the ocean and fresh water, of which two thirds live in the ocean and the rest live in fresh water. There are 2,500 species in China, of which 100 is a kind of medicine. Common medicinal animals are the main raw materials of cod liver oil (vitamin A and vitamin D) of hippocampus, syngnathus, Monopterus albus, carp, crucian carp, sturgeon (swim bladder glue) and Pseudosciaena crocea (otolith is brain stone).

Hydrolyzed protein, cytochrome C, lecithin and cephalin can be extracted from various fishes. The liver and ovary of puffer fish contain a lot of tetrodotoxin, which can be extracted to treat diseases such as neuropathy, spasm and tumor. The bile of big fish can be extracted as "bile pigment calcium salt", which is the raw material for artificial production of bezoar. The difference between fish species is as great as that between terrestrial vertebrates. It is generally believed that fish are slippery, spindle-shaped, streamlined, finned and gilled aquatic animals, but more species do not meet this definition.

Some fish are particularly long and some are particularly short; Some sides are flat, some are flat; Some fins are large or complex in shape, and some degenerate or even digest; The shape and position of the mouth, eyes, nostrils and gill openings vary greatly. Fish is an important food for human beings. Overfishing, pollution and environmental changes will destroy fish resources, which will help control mosquito-borne diseases such as malaria. Fish is an important experimental animal in behavior, physiology, ecology and medicine.