The bony fish, or osteichthyes, are generally considered to be the more advanced group of the two. These fish have an ossified skeleton with a calcium bone matrix. They also have gills with bony supports, gill rakers, and a gill cover known as the operculum. The ray-finned fishes, which are the largest group of bony fishes, have fins that are supported by internal bony structures.
The cartilage fish, or chondrichthyes, have a skeleton made of cartilage, which never ossifies. Their fins lack bony internal support. They have 5 to 7 gills which are connected to the outside by a series of holes called spiracles. Many species of cartilage fish must swim continually in order to keep water flowing across their gills.