There are three main types of modern fish: bony fishes, cartilaginous fishes and the bizarre and ancient jawless fishes. Bony fishes are far and away the most common fishes in rivers, lakes and oceans. Jawless fishes, while once very common, are now extremely rare, and only a few species survive.
Bony fishes make up most of the fishes in the Earth's aquatic habitats. These creatures exist in a wide array of sizes, from tiny guppies to enormous sunfish. The most characteristic feature of bony fishes is their hard, bony skeleton. However, bony fishes also possess an operculum, a membrane that covers their gills and that both other fish families lack. Another organ unique to bony fish is a depth-regulating structure called the swim bladder.
Cartilaginous fishes include sharks, rays, skates and sturgeon. These fish are characterized by their lack of hard, bony skeletons everywhere except their jaws. Cartilaginous fish have placoid scales, which increase their speed while swimming. Unlike bony fish, cartilaginous fish have internal egg fertilization, and many species are viviparous, giving birth to live young.
Jawless fishes are strange creatures unlike virtually all other living things. They have a bony skull but no jaws or extensive skeleton. They are largely parasites that tear through the host's skin and suck out nutrients. Jawless fish are considered "living fossils," as the vast majority of their relatives went extinct millions of years ago.