For terrestrial animals, eyelids have two functions, one is to protect the eyes from damage, and the other is to keep the eyeball moist. For fish, because they live in water all their lives, their eyes will not be dry, and they will not encounter foreign bodies such as wind and sand like terrestrial animals, so they do not need eyelids.
1. Gills: Fish live in the water. Unlike terrestrial animals, their respiratory organs are not lungs, but gills. Gills grow in the head, after the water flows through, because the internal blood flow direction is just opposite to it, so it can efficiently complete gas exchange, absorb oxygen from the water, and discharge carbon dioxide.
2. Fins: Fishes want to swim in the water, and the role of fins is indispensable. Most fish have five kinds of fins, namely pectoral fin, pelvic fin, anal fin, dorsal fin and caudal fin, each fin can play a role in balancing the body, and the caudal fin can also provide power, but also let the fish swim in the direction of change, it is essential.
3. Swim bladder: Not all fish have swim bladder. It only exists in the bodies of most bony fish. There are many shapes of swim bladder. It is filled with oxygen, nitrogen and carbon dioxide. It can serve as an auxiliary respiratory organ for fish. At the same time, it can change the density of fish by contracting or expanding, so that it can float up or sink down.
4. Scale: Scale is a unique structure of fish. It is a derivative of skin, which can protect the fish body and prevent water loss. The special structure allows the fish to swim with less resistance.
