FISH

                            FISH
 The first vertebrates, or animals with back bone to evolve were fish. They now make up about half of all vertebrates species , and include many of the most spectacular animals on the planet.

     Fish are vertebrates that have always lived under water, since the evolution of their earliest ancestors around 530 million year ago. Other vertebrates , including whales and seals, also live in the water , but they must breathe air and they have many other feature that shows their ancestor were land animals. By contrast , a fish skeleton, internal organ gills, skin, muscles, and sense are specialized for aquatic life and they are amazingly efficient.

                                  TYPES OF FISH
   The term " fish"  cover three group of very different . unrelated animals that share the same aquatic habitat and have many of the same adaptation to it. These are the few surviving species of jawless fish , The bony fish such as tuna (which form by fat the biggest group ). and cartilaginous fish such as sharks. 

                JAW LESS FISH
  The earliest fish to evolve had muscular mouth with no jawbones. There were many types , but today the jawless fish are reduced to around 43 species of lampreys , which have sucker - like mouths lined with rasping teeth.

                 BONY FISH
  There are at least 32,300 species of fish with skeletons made of hard bone. They include a wide range of body forms- such as eels , flat fish and seahorses- but most have the streamlined shape of  this bluefin tuna.



                 CARTILAGINOUS FISH
  The 1200 species of sharks,rays, and chimaeras  have skeletons made of flexible cartilage- the same kind of tissue that supports your ears. Many sharks, including this shortfin mako, are highly efficient predators.