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That's a great question. Most bony fish have something called a swim bladder inside them. This is basically an inflatable bag. Fish use the swim bladder to control their buoyancy by inflating it with oxygen from their gills using a gland called a gas gland. This increases their volume and pushes them towards the surface of the water. Some fish, however, have a tube that runs from their swim bladder to their gut, and they gulp air from the surface and push this into the bladder. Most of these fish live in murky, muddy waters that are low in oxygen content. It's no coincidence that lungs are also basically bags that are covered in blood vessels that allow gas exchange. The first lungs evolved from swim bladders in fish that gulped air and so were already using the gas bladder as a primitive lung. Lungs eventually developed many tiny pockets called alveoli and were covered in blood vessels to prmote gas exchange, but fundamentally they are still inflatable bags. I hope this helps.
Barracuda image released to the public domain by author, Laban712
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