Fish classes

Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or bony fish, which have skeletons primarily composed of bone tissue. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.

Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that constitutes nearly 99% of the over 30,000 living species of fish. The vast majority of extant actinopterygian species are teleosts, and by species count they dominate the subphylum Vertebrata, comprising over 50% of all living vertebrates. They are the most abundant nektonic aquatic animals and are ubiquitous throughout freshwater, brackish and marine environments from the deep sea to subterranean waters to the highest mountain streams. Extant species can range in size from Paed

Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fishes commonly referred to as lobe-finned fishes. These vertebrates are characterised by prominent muscular limb buds (lobes) within their fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fishes, the Actinopterygii, which have only skin-covered bony spines supporting the fins.

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