What is the difference between cartilaginous fish and bone fish?

The main difference between cartilaginous fish and bone fish is the skeleton, which consists of cartilage. In addition, they do not have a swim bladder, which is why they must constantly be in motion so as not to sink to the bottom. They also differ in the structure of the gills, they do not have gill covers, and the number of gill slits varies from 5 to 7. The gills themselves are more like plates, in contrast to the petals of the gills of bony fish. The outer cover of bony fish is scales, and in cartilaginous fish it is bony scales or bony plates or their absence at all. Another difference between cartilaginous and bony fish is the way of reproduction. In bony fish, external fertilization, fry develops in eggs, and in cartilaginous fish, fertilization is internal, fry develops inside the body – this is the so-called viviparity.