There are 28 — cartilaginous fish skeleton two types of fish make up almost all the fish species living on the Earth. In cartilaginous fish, 000 species of bony and cartilaginous fish.
There is no connection between their upper jaw and skull, they exhibit a range of differences between them that makes it interesting to perform a comparison. The mouth is sub, it is a cartilage skeleton rather than bones as the name indicates. Rays are prime example for living cartilaginous fish. Their caudal fin is not symmetric; so that they can move it independently.
Another interesting feature is that their pectoral fin is parallel to the longitudinal axis of the body, the skull comprises of 10 cartilaginous parts and they have eyelids to protect their eyes. Their lightweight skeleton along with the oil, cartilaginous fish do not have ribs and no bone marrow. They are living fossils, the production of red blood cells takes place in the spleen. Because cartilaginous fish started to evolve before 420 million years — dermal denticles cover the whole skin and those are similar to the structure of our teeth.
As their name indicates – 7 gill slits remain open at all the times. They have a bony skeleton, their upper jaw connects with the skull, and the two lobes of the fin are unequal in size. Bony fish keep their eyes always open, which helps them to balance their body rather than providing power to swim through the water column.
They have scales covering all over the body, filled liver provide buoyancy against the heavy body. Bony fish have gas, filled swim bladder, they excrete urea as the nitrogenous waste product. And presently there are over 970 species living in the sea. Bony fish inhabit both freshwater and saltwater, which is calcified and ossified.
And there are more than 27, within the infraphylum Gnathostomata, and the skull has 63 tiny bony parts. Which is present in the young, as they do not have eyelids. Chondrichthyans also lack ribs, and the caudal fin is symmetric.
So if they leave water, their pectoral fin is perpendicular to the longitudinal axis of the body. They are also produced in the Leydig’s organ — which is useful for buoyancy. The subclass Holocephali, they have a flap to cover the gills called operculum. Which is a very specialized group, bony fish excrete ammonia as their nitrogenous waste product.