Explain what is Class Osteichthyes ?
Class Osteichthyes takes its name from the Greek root "osteo," referring to bone, and "ichthys," referring to fish. The bony fish have a skeleton composed of hard bone, made up of hydroxyapatite, or calcium phosphate mineral, instead of cartilage. In addition, the scales that cover bony fishes are different from the teeth embedded in the sharks' skin. Fish scales are flattened, and the entire body is coated with a mucus secretion that reduces drag in the water.
Bony fishes have a hard plate, called an operculum, that covers the gills located in a gill chamber. They are able to take in water through their mouths, which bathes the gills and then passes out through the operculum. Bony fishes also differ from the cartilaginous sharks and rays in the way they maintain their depth in the water column. They have a swim bladder, which controls buoyancy by varying the volume of gas inside according to the water depth. Like the Chondrichthyes, bony fish have a lateral line system that senses pressure and vibration.
Reproduction takes various forms among the bony fishes. Many fish cover eggs laid in the water with sperm in order for fertilization to occur. In other species internal fertilization can take place.
There are two major lines of bony fishes: the ray-finned fishes and the lobe-finned fishes. We are more familiar with the ray-finned fishes. Ray-finned fishes include the families we know as bass, salmon, perch, tuna, and herring.
Lobe-finned fishes and the lungfishes are rare and can be traced back about 400 million years to the Devonian Period. Their names pretty much describe their unique features. Lobe-finned fishes have sturdy pectoral and pelvic fins with bones arising from the skeleton and fleshy muscles to help support their method of "walking" under water. Two populations of a living lobe-finned coelocanth named Latimeria have been discovered so far. One lives off the East Coast of Africa and Madagascar in the Comoro Islands, and the other population, discovered in 1997, inhabits Indonesia.
Lungfishes, on the other hand, have an air sac in addition to their gills that enables them to breathe under drought conditions. There are only three genera of lungfishes that survive today.