ALL KINDS OF FISH
Most fish are classified as bony fish, which refers to the fact that their skeleton is made of bones. Bony fish also have some paired—right and left—fins. Sharks and their relatives (skates, rays, and ratfish) have paired fins too, but their skeleton is made of firm, flexible connective tissue called cartilage. (You've got cartilage too, at the tip of your nose and in your ears.) Lampreys and hagfish belong to yet another group of cartilaginous fish. These—more primitive—fish lack paired fins, along with jaws; their mouth is a sucking disc.
WHAT'S FOR DINNER?
Depending on the species, a fish may be an herbivore (plant-eater), a predator, or a detritivore (an animals that eats decomposing plants and animals). Some fish are omnivores that eat just about anything.