1. Class Chondrichthyes (Cartilaginous Fishes): These include sharks, rays, and chimaeras. They have skeletons made of cartilage instead of bone, and most possess gill slits.
2. Class Osteichthyes (Bony Fishes): This is the largest class of vertebrates, encompassing the majority of fish species. They have skeletons made of bone and are characterized by their bony scales and swim bladders.
3. Class Agnatha (Jawless Fishes): This group includes lampreys and hagfish. They lack jaws and have eel-like bodies, with no paired fins.
While these three classes are distinct, they are often grouped together under the informal term "fish" due to their shared aquatic lifestyle and general appearance. However, it's important to remember that this grouping is not a scientifically rigorous classification.