Final answer:
The swim bladder is not a mechanism used by some animals to stay afloat. It is a gas-filled organ derived from the gut that helps control the buoyancy of bony fish.
Step-by-step explanation:
The swim bladder is not a mechanism used by some animals to stay afloat. It is actually a gas-filled organ derived from the gut that helps control the buoyancy of bony fish. It is believed to be homologous to the lungs of lungfish and land vertebrates.