Cartilaginous Fish vs. Bony Fish
What are Cartilaginous Fish?
Fish are divided into different classes around 400 million years ago. One class is called Chondrichthyes in which cartilaginous fish are included. Skeleton of the fish of this class is made up of cartilage. Sharks, rays, skates are some examples of this class. Almost 1100 species of this class still exist. There is no connection of upper jaw of cartilaginous fish with its skull, so they move independently. The skull consists of 10 cartilage elements and eyelids protect their eyes. They also have a third membrane for the protection of eyes which is called nictating membrane. All cartilaginous fish have four chambers of the heart and the fourth chamber is called conus arteriosus which is a contractile cardiac muscle. Intestine of this class is short but has a spiral internal structure for nutrient absorption. Kidneys and genital of these fish empty into one opening which is called cloaca. Cartilaginous fish lack ribs and bone marrow. So, red blood cells are produced in the spleen. Dermal denticles cover the whole skin and similar to the structure of human teeth. These enamel coated denticles are called placoid scales. The mouth is sub dermal because it is located ventrally. There are 5 to 7-gill slits remain open and visible at all the time, and no operculum covers the gills. The caudal fin is not symmetric, and the two lobes of the fin are unequal in size. Another interesting feature of this class is that their fin is parallel to the longitudinal axis of the body which keep them to balance rather than providing power to swim through the water column. Their light weight skeleton along with the oil filled liver provide buoyancy against the massive body. Their heavy weight could crush the internal organs outside water. Cartilaginous fish excrete urea as the nitrogenous waste product. They are living fossils because cartilaginous fish started to evolve before 420 million years and there are almost 970 species are living in the sea.
What are bony Fish?
The skeleton of this class is made up of bones which are calcified and ossified. Bony scales are called cycloid or ctenoid according to their edges either smooth or spiny. The upper jaw of bony fish is connected with the skull and skull has 63 small bony elements. Eyes of bony fish do not have eyelids, so eyes are always open. Heart of bony fish have four chambers, and the fourth chamber is known as bulbous arteries which are made of noncontractile muscles and elastic fibers. Their caudal fin is symmetric, and the whole body is covered with scales. Their pectoral fin lies perpendicular to the longitudinal axis. Bony fish have a gas filled swim bladder which is helpful in buoyancy and performs hydrostatic functions. They also have a flap to cover the gills called operculum. Bony fish excrete ammonia as nitrogenous waste. Bony fish show rapid growth and reach maturity younger, so they reproduce more. They inhabit both freshwater and saltwater and almost 27000 species exist. Moreover, bony fish account more than half of all vertebrate species on the earth.