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Agnatha
The class agnatha includes fish such as lampreys and hagfish. These fish are classified due to their jawless trait, meaning they do not have a hinged mouth. There are more than 50 known species within the agnatha class. These fish have other similar traits, including a lack scales and smooth, cylindrical bodies without fins. These fish eat other fish, including dead or hurt specimens as well as worms. They are most often bottom-dwellers and scavengers.
Chrondrichthyes
The class chrondrichthyes includes what are know as "true fish." These are fish whose skeletons are not made of bone, but are instead entirely composed of cartilage, which is hardened by lime. This makes chrondrichthyes much more flexible than their bone fish counterparts. These fish also qualify as jawed fish, since they exhibit a hinged mouth. Examples of fish in this class include sharks, rays and skates. There are over 600 species of fish classified as chrondrichthyes.
Osteichthyes
Osteichthyes are the third main class of fish. They are also known as the second class of "true fish." This class consists of fish with bone skeletons. These fish also exhibit most of the traits we attribute to fish as a whole, including scales and covered gills. Most of the fish we eat and keep as pets are of this class. Examples include goldfish, koi, clownfish, bass, salmon, tuna and flounder. There are more than 30,000 identified species of osteichthyes.
Actinopterygii
Ray-finned fish of the osteichthyes class are called actinopterygii. These fish have fins with a thin layer of skin supported by spiky bones, which fan out in a ray-like pattern. This is a subclass that includes 26,900 types of known fish species, including perch and catfish -- more fish species than any other class. Actinopterygii have been around for 400 million years. These species are found both living and from fossil evidence.
Sarcopterygii
Sarcopterygii, or lobe-finned fish, are the second subclass of osteichthyes. These fish do not have thin spiky fins, but instead have thick, lobe-like fins that attach to the body with a single bone. Fish of this subclass are speculated to have been the first to evolve feet and begin walking on land, constituting the first amphibians. Today sarcopterygii are not as numerous as actinopterygii, but notable examples include lungfish and coelacanths, which were once thought extinct.