Fishes, or more properly "fishlike animals", include several classes of marine and aquatic creatures. Among these are the jawless agnathans ("jawless ones," Greek), including lampreys and hagfishes; chondrichthyes ("cartilaginous fishes," Greek), including ratfishes, sharks and rays; and osteichthyes ("bony fishes," Greek), the bony fishes that, with some 23,000 described species, are the largest and most diverse group of vertebrates today. 

The agnathans are the earliest vertebrates known from the fossil record, and were once far more numerous and diverse than they are today. Another prehistoric group, the placoderms ("plated skin," Greek), are completely extinct now, but are notable as the first vertebrates with true jaws. Some species grew to gigantic proportions and were the dominant life forms in the early oceans.

All fishlike animals are characterized by respiring via gills, which exchange carbon dioxide for oxygen via a countercurrent exchange. Bony fishes have but a single pair of gill slits, while other fishes may have five or more. Fishes have closed circulatory systems, two-chambered hearts, and lack moveable eyelids.

 
Left, a reef-dwelling puffer; right, a deep-sea fish with light organs.