Cartilage is the flexible substance found that gives human noses and ears their shape! Only their teeth, and sometimes their vertebrae, have calcium in them! That why when you find fossils of sharks you find teeth and vertebrae but no other bones. Species in this class have paired fins, hard scales, a two-chambered heart, and a pair of nostrils. Most species have 5-7 gill slits on each side of their body. (gills are exposed outside) Some species produce egg cases, others give birth to live young. The ampullae of Lorenzini are sensing organs that respond to waterborne vibrations and to pressure changes. Fertilization is internal in the cartilaginous fishes.
Chondrichthyes adapted a cartilaginous skeleton that assists in deep water life. Chondrichthyans have gills to extract oxygen from the water. They have a specific jaw suspension called 'holocephaly' in which it is connected with ligaments and tissue to the underside of the skull.

Skin is made of flat boney scales.

lateral Line System

Gills covered by a protective flap

Operculum

Reproduction is mainly oviparous.
These fishes have a skeleton much stiffer than that of the cartilaginous fish (sharks). Bony fish have an excellent sense of smell, like sharks and rays, but their sense of vision is also excellent. The bony fishes have special adaptations to control their buoyancy. A special organ called the swim bladder acts as a gas-filled chamber to control their depth in the water. Another prominent feature of the bony fishes is the operculum. This constitutes the lateral fleshy grooves on the fish that cover the chambers housing the gills. The operculum allows the fish to breathe (even if it is not swimming) by moving water currents over the gills. Other features include paired fins, dermal scales in the skin (in most species) and numerous vertebrae.

Dry Skin: The dry skin allows reptiles to move around for periods of time without water. The dry skin stops evaporation of the organism's moisture.
Eggs: Reptile eggs are tough enough to survive on land. Amphibians needed moist areas for their eggs to develop and for fertilization to occur. Reptiles have made the jump to laying eggs in the open and on land.
Skeleton: It is true that fish and amphibians have skeletons. They developed larger skeletons that could hold larger organs and help them increase in size.
Leg shift:  Basically the legs of reptiles shifted to the bottom of the body.Reptiles' legs are directed down towards the ground. That change in direction allows reptiles to pick their bellies off the ground and helps them move more efficiently. It's obvious that this trait doesn't apply to snakes that have no legs. They have their belly on the ground or on a tree limb all day long. 

The key characteristics of squamates include:

The key characteristics of tuataras include:

The key characteristics of turtles include:

Anatomical characteristics:
Feathers are the defining characteristic of birds, found on every living species of bird and no other class of animal. Feathers are made of keratin, the same substance that forms hair and nails in other animals and are highly modified scales. Feathers are critical not only for flight but also for warmth and protection against the elements -- and in many species, for males to attract mates. Differences in wing shape provide different advantages to the various bird species. The narrow, sharp-tipped wings of the falcon provide speed; albatrosses soar high on wings much longer than they are wide. All birds have beaks, or bills, made of a bony core surrounded by a thin layer of keratin. Birds do not have true teeth, but many species have tomia -- sharp ridges along the edges of their beaks. Birds do not chew food but grind or rip it into pieces small enough to swallow. The shape of a bird's beak indicates the bird's general diet. Meat-eaters like hawks and owls have sharp, hooked beaks for ripping and tearing. Strong, cone-shaped beaks help seed eaters break through shells. Ducks and geese have broad, flat beaks for straining food out of the water.All birds lay eggs, some very colorful or covered with spots. Most birds have lightweight skeletons with hollow bones. This keeps them light enough for flight. Many fused bones, including the collarbones or wishbones, make birds’ skeletons rigid in comparison to mammals'. This helps brace the birds' wings during flight. Their sternums, or breastbones, are large, providing sturdy attachment points for powerful wing muscles. Flightless penguins have heavy bones filled with marrow, helping them to survive in their freezing home range. Ostriches have heavy, solid bones in their legs, helping the birds run and defend themselves with powerful kicks.

Some mammals have a short life cycle, while others have a longer one. Large mammals often live longer than small mammals. For example, tiny cotton mice live only four to five months in the wild. However, giant elephants live for 55 to 70 years.

People are mammals, too. Today, many people live into their 90s. Eating good food and getting plenty of exercise can help you live a long and healthy life.