Class Pisces

Class Pisces

Class Pisces (fishes) comprises exclusively aquatic, gill-breathing vertebrates, and is broadly divided into cartilaginous fishes (sharks and rays) and bony fishes (the great majority of living fish, such as Catla), which differ in several clear, comparable ways.

Fins and scales — a shark's paired and unpaired fins are typically fleshy-based and less flexible than those of a bony fish, and a shark's scales are of the placoid type — small, tooth-like structures with an enamel-covered spine, embedded in the skin and pointing backward, giving shark skin its characteristic rough texture. A bony fish such as Catla instead bears thin, flexible, overlapping cycloid (or ctenoid) scales, made of bone-like material, without the tooth-like structure of placoid scales, and generally more flexible, thinner fins supported by bony fin rays.

Air bladder (swim bladder) — a gas-filled sac found in most bony fishes (including Catla) but absent in cartilaginous fishes such as the shark. The air bladder's principal importance is hydrostatic: by adjusting the volume of gas within the bladder, a bony fish can control its buoyancy and maintain a stable depth in the water column without continuous muscular effort, unlike cartilaginous fishes, which lack this organ and must rely on continuous swimming and an oil-rich liver for buoyancy control instead. In some fish the air bladder additionally assists in sound production or, in a few groups, in accessory respiration.

"Branchial heart" — in fishes, the heart is often descriptively referred to as a "branchial heart" because it pumps deoxygenated blood forward, in a single circuit, directly to the gills (branchiae) for oxygenation, before that oxygenated blood is distributed to the rest of the body — the fish heart's entire pumping action is thus oriented specifically toward driving blood to the respiratory (branchial) surface, unlike the hearts of air-breathing vertebrates, which pump blood to a separate lung circuit distinct from the general body circuit. …