Osteichthyes are defined by

• A. Cartilaginous skeletons
• B. A bony skeleton and a swim bladder
• C. Jawless animals lacking paired fins
• D. A cartilage-based endoskeleton throughout life

Answer: (B)

Osteichthyes are defined by having a skeleton made of bone and a swim bladder. The bony skeleton distinguishes them from cartilaginous fishes, whose skeletons are cartilage. The swim bladder is a gas-filled organ that allows these fish to regulate their buoyancy, helping them stay at different depths with less energy. The other descriptions point to different groups: a cartilage-based endoskeleton is characteristic of cartilaginous fishes, and jawless animals lacking paired fins refer to agnathans.