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A fish is defined as a limbless aquatic vertebrate animal with fins and internal gills. There are three living classes of fish:

• Primitive jawless fishes - class Agnatha
• Cartilaginous fishes - class Chondrichthyes
• Bony fishes - class Osteichthyes

These groups, although quite different from one another anatomically, have certain common features related to their common evolutionary origins or to their aquatic way of life. Fish were the earliest vertebrates, and presumably evolved from a group of aquatic lower chordates; the terrestrial vertebrates evolved from fishes.

More: Overview of Fishes ...