fin

In aquatic animals, flattened extension from the body that aids balance and propulsion through the water.

In fish they may be paired, such as the pectoral and ventral fins, or singular, such as the caudal and dorsal fins, all being supported by a series of cartilaginous or bony rays.

The fins in cetaceans (whales and dolphins) are simple extensions of the soft tissue and have no bony rays. The flippers of seals are modified five-fingered limbs and contain the same bones as the limbs of other vertebrates.