^w^ Sharks, skates, rays, and chimeras are grouped together as Chondrichthyes because their skeletons are made of cartilage.
^w^ Cartilage is a tough, rubbery material that is lightweight and extremely flexible.
^w^ This skeletal structure is useless when the shark is out of the water. Without the support of the water, gravity would cause the body structure to collapse and the internal organs of the shark to crush and break.
^w^ Rings on the sharks' vertebrae offer some of the few clues we have as to how sharks age.
^w^ There is no bone marrow in cartilage. Since bone marrow is the site of the production of the immune system's cells (`leukocytes` or white cells), this function has been taken over by other organs in sharks.
