• Chondrichthyes, jawed fishes that possess skeletons made of cartilage; and sharks, rays, skates and ratfishes.
• Osteichthyes, jawed fishes that have skeletons made of bone.
As divers we don’t have much, if any, contact with the primitive jawless fishes (also known as roundmouth fishes) that comprise the class Agnatha. The approximately 75 species of hagfishes and lampreys are the only living representatives of this once large class.
Worldwide, there are roughly 1,200 species of cartilaginous fishes, and all are described in the class Chondrichthyes. Many of these, especially a variety of sharks and rays, are among the oceans’ more spectacular creatures. I think any diver that has encountered a manta ray, squadron of spotted eagle rays cruising through the blue, whale shark, school of scalloped hammerhead sharks, or any of a number of other species ranging from mako sharks to southern stingrays would surely agree.
As the name suggests, the skeletons of cartilaginous fishes are made of cartilage, not bone. Some common distinguishing characteristics of sharks, rays and their kin include the lack of the swim bladder found in many bony fishes; that males fertilize females internally; the possession of five to seven gill arches; and that their skin is composed of toothlike denticles that give the skin an abrasive texture.