Final answer:
Chondrichthyes feature a cartilaginous skeleton and jaws with paired fins, unlike the jawless Agnatha. Chondrichthyes' keel-shaped scales offer reduced friction for more efficient swimming and provide additional defense and aggression mechanisms.
Step-by-step explanation:
Chondrichthyes, commonly known as cartilaginous fish, differ from jawless fish (Agnatha) in several distinct ways. One significant difference is that Chondrichthyes possess a flexible skeleton made entirely of cartilage, while Agnatha have a more primitive structure with no jaws or paired fins. Moreover, unlike jawless fish, Chondrichthyes have jaws and paired fins which greatly enhance their mobility and feeding capabilities.
Another notable difference lies in the keel-shaped scales of Chondrichthyes, known as placoid scales. These scales are beneficial as they reduce friction with the water when the fish moves, which increases their efficiency in swimming. The sharp, tooth-like structure of the scales also provides protection against predators and can inflict damage on prey.