<P> The skeleton of a shark is mainly made of cartilage . </P> <P> Most sharks have eight fins: a pair of pectoral fins, a pair of pelvic fins, two dorsal fins, an anal fin, and a caudal fin . The members of the order Hexanchiformes have only a single dorsal fin . The anal fin is absent in the orders Squaliformes, Squatiniformes, and Pristiophoriformes . Shark fins are supported by internal rays called ceratotrichia . </P> <P> The tail of a shark consists of the caudal peduncle and the caudal fin, which provide the main source of thrust for the shark . Most sharks have heterocercal caudal fins, meaning that the backbone extends into the (usually longer) upper lobe . The shape of the caudal fin reflects the shark's lifestyle, and can be broadly divided into five categories: </P> <Ul> <Li> Fast - swimming sharks of open waters, such as the mackerel sharks, have crescent - shaped tails with upper and lower lobes of almost equal size . The high aspect ratio of the tail serves to enhance swimming power and efficiency . In these species, there are usually also lateral keels on the caudal peduncle . The whale shark and basking shark also have this type of tail, although they are generally more sedate animals than the other examples . </Li> <Li> "Typical sharks", such as requiem sharks, have tails with the upper lobe longer than the lower . The upper lobe is turned upwards at a moderate angle relative to the body, which balances cruising efficiency with turning ability . The thresher sharks have an extreme example of this tail in which the upper lobe has evolved into a weapon for stunning prey . </Li> <Li> Bottom - dwelling sharks such as catsharks and carpet sharks have tails with long upper lobes and virtually no lower lobe . The upper lobe is held at a very low angle, which sacrifices speed for maneuverability . These sharks generally swim with eel - like undulations . </Li> <Li> Dogfish sharks also have tails with longer upper than lower lobes . However, the backbone runs through the upper lobe at a lower angle than the lobe itself, reducing the amount of downward thrust produced . Their tails cannot sustain high speeds, but combine the capability for bursts of speed with maneuverability . </Li> <Li> Angel sharks have unique tails among sharks . Their caudal fins are reverse heterocercal, with the lower lobe larger than the upper . </Li> </Ul>

Fins of a shark and flipper of a whale are examples of