Fish belong to the subphylum Vertebrata of the phylum Chordata. This kind of animal is characterized by breathing with gills, swimming by swinging its trunk and tail, and feeding by its upper and lower jaws, which belongs to a kind of poikilotherm. Common fish are generally Chondrichthyes or Osteichthyes, which can also be divided into freshwater fish and saltwater fish according to the water area, or into tropical fish, temperate fish and cold fish.
1. Gills: No matter what kind of fish, there are gills, because the characteristic of this kind of animal is to breathe with gills. When water flows through the gill filaments, the fish can complete gas exchange, thus obtaining oxygen and releasing carbon dioxide. If the fish leaves the water, the gill filaments will stick together and lose the ability to breathe.
2. Fish scales: Most fish have fish scales, which are skin derivatives, usually calcium, covering the whole body or part of the region, can play a role in protecting the body, but also to prevent some irritation. In addition, the structure of fish scales is very special, which can make the fish swim with less resistance.
3. Fin: When a fish is swimming, it relies on the tail fin to provide power. At the same time, it can also change the direction. It relies on the pectoral fin to slow down and brake. The other fins are mainly responsible for maintaining balance. No matter which fin is lost, the fish will sway from side to side when swimming.
4, swim bladder: Not all fish have swim bladder, it is mainly responsible for regulating the gas, so as to change their density, to facilitate the fish to float or dive. If the fish suffers from swim bladder loss, it will cause it to roll over and can no longer maintain its balance.
