Fish are generally referred to as cartilaginous fish and bony fish, which can be divided into freshwater fish and salt water fish, or tropical fish, temperate fish and cold water fish. Most fish are cold-blooded animals, and very few fish are warm-blooded animals, breathing through gills.
Fishbone is divided into cartilage and hard bone according to its nature. Cartilaginous fish keep cartilage for life, because there is calcium precipitation in softness, it is also called calcified cartilage. The skin of fish consists of epidermis and dermis. The epidermis is very thin and consists of several layers of epithelial cells and mucus layers. The epidermis can continuously secrete viscous liquid, lubricate the body surface and protect the fish, such as reducing skin friction resistance.