Fish in general
The term “fish” is applied to a class of animals that includes some 21,000
extremely diverse species. Fish can be roughly defined (and there are a few
exceptions) as cold-blooded creatures that have backbones, live in water,
and have gills. The gills enable fish to “breathe” underwater, without drawing
oxygen from the atmosphere. This is the primary difference between fish
and all other vertebrates. Although such vertebrates as whales and turtles live
in water, they cannot breathe underwater. No other vertebrate but the fish is able to live without breathing air. One family of fish, the lungfish, is able to breathe air when mature and actually loses its functional gills. Another family of fish, the tuna, is considered warm-blooded by many people, but the tuna is an exception. Fish are divided into four groups: the hagfish, the lampreys, cartilaginous fish, and bony fish. The hagfish and the lampreys lack jaws, and as such, they form the group called jawless fish; the cartilaginous fish and the bony fish have jaws. The bony fish are by far the most common, making up more than 95 percent of the world’s fish species. Cartilaginous fish, including sharks, rays, and skates, are the second largest group, numbering some 700 species. There are 32 species of hagfish and 40 species of lampreys.
in water, they cannot breathe underwater. No other vertebrate but the fish is able to live without breathing air. One family of fish, the lungfish, is able to breathe air when mature and actually loses its functional gills. Another family of fish, the tuna, is considered warm-blooded by many people, but the tuna is an exception. Fish are divided into four groups: the hagfish, the lampreys, cartilaginous fish, and bony fish. The hagfish and the lampreys lack jaws, and as such, they form the group called jawless fish; the cartilaginous fish and the bony fish have jaws. The bony fish are by far the most common, making up more than 95 percent of the world’s fish species. Cartilaginous fish, including sharks, rays, and skates, are the second largest group, numbering some 700 species. There are 32 species of hagfish and 40 species of lampreys.