Subphylum Vertebrata contains those chordates that have replaced the simple notochord with a segmented skeletal rod that wraps around and protects the brain and nerve cord. The skeletal segments, called vertebrae, are made of bone or cartilage, and the entire series of segments is called the vertebral column. The portion encasing the brain is called the skull. There are seven main classes of vertebrates.

 

    Jawless fish:

These fish are bottom-dwelling filter feeders without jaws. They breathe through gills and lay eggs. Examples are lampreys and hagfish.

 

    Cartilaginous fish:

With a flexible endoskeleton made of cartilage, these fish have well-developed jaws and fins, and they breathe through gills. Their young hatch from eggs. Examples are sharks, eels, and rays.

 

    Bony fish:

Bony fish mark an advance since they have much stronger skeletons made of bone rather than cartilage. Bony fish are found in both salt water and fresh water. They breathe through gills and lay soft eggs. Almost every fish you can think of is a bony fish, from goldfish to trout.

 

    Amphibians:

Amphibians such as frogs and salamanders embody the transition from aquatic to terrestrial living. Born initially as fishlike tadpoles living in the water, they undergo a metamorphosis and develop legs and move onto land as adults. Most adult amphibians breathe through lungs that develop during their metamorphosis, though some can breathe through their skin. Their eggs lack shells, must be laid in water, and receive little parental care.

 

    Reptiles:

With the development of the fluid-filled amniotic sac, reptiles, including dinosaurs, were the first animals to be able to hatch their eggs on land and make the full transition to terrestrial life. Reptiles lay few eggs and provide some parental care. Reptiles also have thick, scaly skin that resists water loss and efficient lungs.

    All classes of vertebrates that evolved before birds are cold-blooded (ectothermic). The metabolism of these earlier classes is dependent on the environment. When the temperature drops, their metabolism slows and speeds up as the temperature rises. Birds and mammals, in contrast, are warm-blooded (endothermic). They have developed structures such as feathers, hair, and fur to help them maintain body temperature. The metabolism of birds and mammals stays constant through far larger extremes of temperature, making these two classes much more versatile.

 

    Birds:

Birds have specially evolved structures such as wings, feathers, and light bones that allow for flight. In addition, birds have four-chambered hearts and powerful lungs that can withstand the extreme metabolic demands of flight. Birds lay hard eggs but provide a great deal of care for their eggs and developing young.

 

    Mammals: