classesfishes

Fishes are a diverse lineage of aquatic vertebrates. In traditional and many modern classifications they are divided into jawless fishes (cyclostomes) and jawed fishes, the latter further split into cartilaginous fishes and bony fishes. The two largest living lineages are Osteichthyes (bony fishes) and Chondrichthyes (cartilaginous fishes), while jawless forms occupy the cyclostomes.

Jawless fishes include hagfishes (Myxini) and lampreys (Petromyzontida). They lack jaws and have a skeleton made of cartilage or partly cartilaginous structures. They typically retain a notochord, gill slits, and a round or elongated body. Lampreys often have a suction feeding mechanism; hagfishes are mostly scavengers and produce slime.

Chondrichthyes are characterized by a skeleton of cartilage rather than bone. This class encompasses sharks, rays, skates, and chimaeras. They usually possess placoid scales, a lateral line system for sensing water movements, and internal fertilization in many species. Reproduction modes range from eggs laid (oviparous) to live-bearing (viviparous).

Osteichthyes comprises the bony fishes, the largest and most diverse group. They are divided into Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes). Actinopterygians include most freshwater and marine species, with fins supported by bony rays. Sarcopterygii includes lungfishes and coelacanths, close relatives of the earliest tetrapods.

Extinct groups such as Acanthodii (spiny sharks) are frequently discussed in the context of early jawed fishes. Because taxonomic schemes vary, some classifications treat Osteichthyes as a clade rather than a formal class. Overall, the term "fishes" encompasses several major lineages with a wide range of life histories and morphologies.