fish, in zoology: The Bony Fishes

The Bony Fishes

The bony fishes are distinguished from other living fishes by their bone skeletons and by the presence of either a swim bladder (which functions as a float) or, in a few fishes, lungs. The bony fishes are divided into two subclasses, the lobe-finned (or fleshy-finned) fish and the ray-finned fish. The latter group includes over 95% of all living fish species.

The earliest bony fishes were fleshy-finned; they gave rise to the amphibians (the first terrestrial vertebrates, or tetrapods). The only surviving fleshy-finned fishes are the lungfishes and coelacanths (see lobefin). These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs.

Ray-finned fishes, now predominant in both fresh and marine waters, represent an advanced adaptation of the bony fishes to strictly aquatic conditions; they are the most highly successful and diverse of the fishes. In nearly all of these fishes the lung has evolved into a hydrostatic organ, the swim bladder. The fins in this group consist of a web of skin supported by horny rays. Each ray is moved by a set of muscles, giving the fin great flexibility. Most ray-finned fish have overlapping scales made of very thin layers of bone. Their skeletal structure is light but strong and most have excellent vision.

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