One particular feature found in bony fishes but is absent in cartilaginous fishes:

A Swim Bladder is not present in cartilaginous fishes. Also called a fish maw or gas bladder the swim bladder is a gas-filled bladder that allows many bony fish to control their buoyancy by filling or releasing gas form the bladder. This allows them to change bouyancy without wasting metabolic energy by swimming. 

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