According to Nelson (1994) statistics, there are 24,618 living species of fish in the world, accounting for more than half of the named vertebrates, and new species of fish are constantly being discovered, with an average of about 150 species every year. According to species statistics, more than 1,500 species should have been added in more than ten years, so there should be more than 26,000 named fish species in the world.

Fish live their entire lives in seawater or freshwater, and a few can live out of water for short periods of time. Most of them have body shapes and fins suitable for swimming. It breathes through gills and catches food with its upper and lower jaws. The beating heart was divided into one atrium and one ventricle. Blood circulation is pressure, and the pressure in the deep sea can reach 1,000 atmospheres. The range of salt content in fresh water and sea water from fresh water to salt water is 0. 001～7%. In addition, the water temperature difference and oxygen content also vary greatly depending on the geographical environment. Due to the diversity of water environments such as these waters, water layers, water quality, and biotic and abiotic factors in the water, the body structure of fish has undergone different changes to adapt to different changes in the outside world. More advanced than Cylinostomata. The class Fish is the largest class among the extant vertebrate subphylums. From the perspective of animal evolution, this class is the beginning of the jawed class, so it is the most primitive and oldest class among the jawed classes. This is the largest taxonomic group in the vertebrate subphylum. It has derived many marginal branches as far back as the Devonian Period, and has developed and evolved into fish of various complex shapes to this day. Existing fish are divided into cartilaginous fish and bony fish. Scientific classification: Jawless species have a cylindrical spine, exist for life, and have no upper and lower jaws. The gills originating from the endoderm are sac-shaped, so they are also called cystobranchials. They have a well-developed brain, usually with 10 pairs of cranial nerves, and have paired visual and auditory organs. The inner ear has 1 or 2 semicircular canals. There is a heart and red blood; the epidermis is composed of multiple layers of cells. The even fins are underdeveloped, and some ancient bony turtles have pectoral fins. There are different classifications of jawless animals, which are generally divided into: Hagfish, Cephalothurans, Lampreys, and Fin Turtles.