Class – Osteichthyes

Class Osteichthyes contains the bony fishes — fishes whose skeleton is made of true bone. Like Chondrichthyes, they belong to the super class Pisces, but they differ from the cartilaginous fishes in several important ways.

Habitat and body

This class includes both marine and freshwater fishes. The body is streamlined and supported by a bony endoskeleton. The mouth is usually placed at the front end of the body (terminal position).

Gills, scales and buoyancy

• They have four pairs of gills, and on each side these are protected by a bony cover called the operculum.
• The skin is covered with cycloid or ctenoid scales.
• An air bladder is present, and it allows the fish to regulate its buoyancy — a key advantage over the cartilaginous fishes, which lack this organ and must keep swimming.

Heart, temperature and reproduction

• The heart is two-chambered, with one auricle and one ventricle.
• They are cold-blooded animals.
• Sexes are separate, fertilisation is usually external, they are mostly oviparous (egg-laying), and development is direct (the young resemble the adult without a distinct larval stage). …