The ancestor of fish is amphioxus. The origin of this animal is very early, incomplete fossils were found 400 million years ago, and in the Devonian period, many modern fish have appeared. Ancient fish can be divided into four groups, namely, placoderm, jawless, Chondrichthyes and bony fish, of which placoderm has been extinct, only a few jawless fish remain, while Chondrichthyes and bony fish are the main branches of fish.
1. Breathing with gills: The common characteristic of fish is to breathe with gills, because fish live in water. When water enters through its mouth and is discharged from the gills, the gill filaments inside can exchange gas with the water. However, some fish have auxiliary respiratory organs, which can use intestinal tract, swim bladder and skin to breathe.
2. Scales on the body surface: Most fish have scales on the body surface. Fish scales are skin derivatives that can protect the body. In addition, the structure of fish scales is special, which can reduce the resistance of fish in swimming.
3, swimming by fins: Fish live in the water, so usually swimming, no limbs, replaced by a variety of fins. The tail fin is responsible for providing power and controlling the direction of progress, the pectoral fin can brake, and the other fins are mainly responsible for maintaining its balance.
