Cartilaginous versus Bony Fishes
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Cartilaginous fishes, such as sharks, build their skeleton from cartilage; bony fishes, the more diverse group, build theirs from bone.
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A ray's flexible cartilage skeleton contrasts with a tuna's rigid bony skeleton, including the buoyancy-control swim bladder many bony fishes use.
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Cartilage is not an unfinished form of bone — cartilaginous fish retain it as a stable adult material.