Q.Select the animal having venous heart.

Bony fishes (Class Osteichthyes) are marked out by a bony (ossified) endoskeleton, operculum-covered gills, and an air bladder used for buoyancy, distinguishing them from cartilaginous fishes.

Class Osteichthyes includes the majority of living fishes (e.g., Labeo, Catla, Exocoetus/flying fish, Hippocampus/seahorse) and is characterised by several distinctive features, of which three important ones are:

1. Bony endoskeleton: unlike cartilaginous fishes (Class Chondrichthyes, e.g. sharks and rays), bony fishes have a skeleton made of true, ossified bone, giving strength and rigidity to the body.

2. Operculum-covered gills: each of the paired gills is covered and protected by a bony flap called the operculum, and water is drawn in through the mouth and expelled out through a single opening behind the operculum on each side — unlike cartilaginous fishes, which have separate, exposed gill slits without an operculum.

3. Air bladder (swim bladder): most bony fishes possess an internal, gas-filled sac called the air bladder or swim bladder, which they can inflate or deflate to control their buoyancy, allowing them to remain at a chosen depth in the water column without constant swimming effort — a structure absent in cartilaginous fishes.

Other features include body typically covered with cycloid or ctenoid (bony) scales, and mostly external fertilisation with a streamlined body shape, but the three above are the most commonly cited distinguishing characteristics.