Write the characteristics of class $Osteichthyes$.

(N/A) The characteristics of class $Osteichthyes$ are as follows:
$1$. Habitat: They include both marine and freshwater fishes.
$2$. Endoskeleton: Their endoskeleton is primarily bony.
$3$. Body Shape: The body is generally streamlined.
$4$. Mouth: The mouth is mostly terminal.
$5$. Gills: They have four pairs of gills which are covered by an operculum on each side.
$6$. Skin: The skin is covered with cycloid or ctenoid scales.
$7$. Air Bladder: An air bladder is present which regulates buoyancy.
$8$. Heart: The heart is two-chambered (one auricle and one ventricle).
$9$. Temperature: They are cold-blooded $(poikilothermous)$ animals.
$10$. Sexes: Sexes are separate. Fertilization is usually external.
$11$. Development: They are mostly oviparous and development is direct.
Examples: $Hippocampus$ (Sea horse),$Labeo$ (Rohu),$Catla$ (Catla),$Clarias$ (Magur),$Betta$ (Fighting fish),$Pterophyllum$ (Angel fish).