The Actinopterygii is a particularly vast and diverse group, including forms as different as sturgeons and seahorses. Actinopterygian fishes have ray-supported fins, their defining character that distinguishes them from the sarcopterygian (or 'lobe-finned') fishes.
A typical actinopterygian has a streamlined body shape (seahorses being but one of numerous exceptions), relatively large eyes (although certain cave-dwelling forms are entirely eye-less) and is covered in flexible scales (although certain forms such as sturgeons have a reduced number of thick bony plates running down their bodies instead).
Actinopterygians also possess an air sac (if not two). The air sac functioned first as an air-breathing organ, as in the modern bichir. Most though use this organ as a flotation device, regulating the animal's overall buoyancy. The 'air sac' became entirely separated from the gut, losing its lung function and effectively becoming the "swimbladder" into which gas is secreted by specialised cells, or removed through the blood flow. Certain advanced actinopterygians have even extended the use of the swimbladder to act as a resonating chamber to amplify sounds.