Jawless and jawed fishes

All living cyclostomes are ectoparasites on some fishes. The elongated body has 6 to 15 pairs of gill slits for breathing and a circular sucking mouth with no jaws. There are no scales and no paired fins; cranium and vertebral column are cartilaginous, and circulation is closed.

Cyclostomes are marine but migrate to fresh water to spawn, and die within a few days after spawning. Their larvae metamorphose and return to the sea. Examples: Petromyzon (lamprey), Myxine (hagfish).

Chondrichthyes are marine with a streamlined body and a cartilaginous endoskeleton. The mouth is ventral, the notochord lasts throughout life, and the gill slits are separate with no operculum.

Their tough skin carries minute placoid scales, and their teeth are modified placoid scales pointing backwards. The jaws are very powerful and they are predators. With no air bladder they must keep swimming to avoid sinking.

The cartilaginous fish heart has two chambers, one auricle and one ventricle. Some have electric organs (Torpedo) and some a poison sting (Trygon). They are cold-blooded (poikilothermous). Sexes are separate, males have claspers on the pelvic fins, fertilisation is internal and many give birth to live young. Examples: the dog fish Scoliodon, the great white shark Carcharodon, the saw fish Pristis and the sting ray Trygon.

Osteichthyes live in the sea and in fresh water and have a bony endoskeleton, a streamlined body and a mostly terminal mouth. Four pairs of gills are covered by an operculum on each side, and the skin has cycloid or ctenoid scales.

Bony fishes have an air bladder that regulates buoyancy, a two-chambered heart, and are cold-blooded. Sexes are separate, fertilisation is usually external, most are oviparous and development is direct.

Bony fish examples. Marine: the sea horse Hippocampus and the flying fish Exocoetus. Freshwater: rohu (Labeo), katla (Catla) and magur (Clarias). Aquarium: the fighting fish Betta and the angel fish Pterophyllum.