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Fish are members of the subphylum Vertebrata of the phylum Chordata.  There are three classes of fish, the class Agnatha consists of the Lampreys and the Hagfish.  These animals are eel like in shape, and have cartilage rather than bone for their skeleton.  Many, but not all, of the lampreys are parasitic as adults and neither the Lamprey nor the Hagfish are commonly kept in aquariums.

The second class of fish is the Chondrichthyes.  These are the Sharks, Rays, Skates and the Ratfish.  These fish have a cartilaginous skeleton like the Agnatha, but unlike them, the Chondrichthyes are descended from fish with bony skeletons.  These fish are also not commonly kept in home aquariums although they occasionally are, and are frequently on display in public aquaria.  There are only a few fact sheets on this site about fish from this class.

The Whale Shark and the Great White shark are Chondrichthyes, but the Black Shark, Silver Shark and the Redtail Black Shark are not.

Most species of fish alive today belong to the third class of fish, the Osteichthyes.  These are often called the bony fish because they have calcified bones.  These fish come in a wide range of shapes and sizes.  Most of the Osteichthyes have a swim bladder.

Comparison of the DNA of the different species of bony fish indicates that they are not all as closely related as their appearance suggests.  In particular, the Lungfish are more closely related to the Mammals that they are to other bony fish.

Most of the fish kept in aquariums are bony fish, and nearly all the fact sheets on this site are about fish from this class.