• Bony fishes are characterized by three key features: scales on the body, lungs or a swim bladder, and bone in the skeleton.

• Lobe-finned fishes have fleshy fins with a central bony axis, while ray finned fishes have non-fleshy fins supported by long flexible bones called rays.

• The external anatomy of a bony fish has several distinct characteristics—an operculum, fins, and scales.

• A fish's heart has two chambers that work together to move blood through the body.

• Water flows over fishes gills in a direction opposite to blood flow. Oxygen diffuses from the water into the blood very efficiently as a result of this process, which is called countercurrent flow.

• Fish adjust their overall density by regulating the amount of gas in the swim bladder, enabling them to move up or down in the water.

• Unlike cartilaginous fishes, most bony fishes reproduce by external fertilization in a process called spawning.

Vocabulary swim bladder (p. 787) ray-finned fish (p. 787) lobe-finned fish (p. 787) operculum (p. 788)

countercurrent flow (p. 791) spawning (p. 792) optic tectum (p. 792)

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