Class Chondrichthyes; (Greek, chondros = cartilage, ichthys = fish).  Sharks, skates, and rays are the members of this class. These fish have a flexible skeleton made of cartilage.

Chondrichthyes lack scales, swim bladder (a balloon like organ that helps fish float present in bony fish).  These animals are fast swimmers, and when they are not moving they sink to the bottom of the ocean.  They also lack the operculum; therefore they need to swim all the time to breathe.

Sharks have huge livers that store a large amount of oil, which is lighter than water and therefore improves buoyancy.