The swim bladder is connected to the back of the digestive tube, which is a slender gourd-shaped airbag, and the air is communicated with the throat. The wall of the swim bladder has certain elasticity, and it can expand and contract freely, becoming bigger and smaller. When the swim bladder contracts, the air inside decreases, the proportion of fish increases and the fish sinks; On the contrary, the swim bladder expands, the air inside increases, the proportion of fish decreases and the fish floats.

There is a very old lungfish, which can only be seen in Australia, South America and Africa. The swim bladder of lungfish is very developed, and its wall is covered with capillaries, which can be used to breathe like lungs, so people give it the reputation of "lungfish". When the pond where lungfish lives dries up in the dry season, it will get into the soil to spend the hot summer. When the rainy season comes, the pond is full of water, and the lungfish will return to life in the water.