Did you know sharks and stingrays are fish? They differ in a few ways from tuna, clownfish, and other typical fish you might find in the ocean, though: while the latter are bony fish, sharks, rays, chimaeras, lampreys, and hagfish are cartilaginous fish. Bony fish are so named because they have internal skeletons which are partly or wholly made of bone. Cartilaginous fish, however, have skeletons made of cartilage, a tough, flexible tissue.
Fish have vital organs--such as a heart--like other vertebrates, but have fins instead of legs and gills instead of lungs. Bony fish have a gill cover, called an operculum; sharks and other cartilaginous fish (except chimaeras) do not have gill covers. Oxygen from the water comes in through the gills and is brought into contact with the fish's blood and circulated throughout the body.
Usually bony fish have two dorsal fins along their backs, a caudal fin on their tails, and an anal fin on the their bellies near their tail. They also have a pectoral fin on each side (roughly where a shoulder would be) and a pair of pelvic fins on their bellies below the pectoral fins, which are used for steering and balance. Sharks have similar fins, designed to make them powerful swimmers.
Bony fish reproduce by external fertilization: the female's spawn is fertilized by sperm and hatches into fry, or young fish. Sharks, on the other hand, have complex reproductive systems--they reproduce by internal fertilization and some shark species, as well as manta rays, give live birth.
Besides their reputation, sharks have some unusual features: in addition to having a flexible jaw, they have teeth that are constantly falling out and being replaced. They also have a stomach that expands to hold what they've eaten! Sharks are nearsighted, but have a very sharp sense of smell, which is why the smell of blood can so excite them.
Rays, skates, and sawfish--generally grouped together as rays--breathe through openings called spiracles that are located behind their eyes. Stingrays, as their name implies, can be very harmful to people; but giant manta rays (or devilfish) are a minimum threat to anything bigger than a shrimp or small fish, even though they can weigh up to 3,000 pounds!
Rays look downright nice compared to the other group of cartilaginous fish. Lampreys and hagfish, who belong to a separate division of cartilaginous fish, do not have jaws. Lampreys are parasites--they live off of larger animals--and hagfish are scavengers, eating dead organisms.
If you want to get some hands-on experience while learning more about fish, we recommend dissecting a perch and dogfish. The perch is typical of bony fish and the dogfish, a small shark, is typical of cartilaginous fish.