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The species in this class have a flexible skeleton made of cartilage instead of bone.

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.

skull

jaws with teeth

a vertebral column (backbone)

fins (unpaired fins: dorsal, anal and caudal and paired fins: pectoral and pelvic)

layer of tough skin covered with small tooth-like scales (placoid scales)

unlike the bony fishes, they lack ribs

Teeth vary in size and shape as well.

Some have teeth that are not connected to the jaw and are continuously replaced.

Throughout all of the chondrichthyan species there are at least eight well-developed sensory systems

Some are formed from highly sensory cells that can release an electrical impulse.

A) Endoskeleton is entirely cartilaginous (bone is entirely absent)
B) Digestive system with J-shaped stomach and a spiral valve in the intestine
C) Skin with placoid scales or naked in elasmobranches ( shared in all cartilaginous fishes)
E) Heterocercal rather than homocercal tail

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scoliodon(dog fish), rhincocodon(whale shark), and carcharodon(white shark). Rays- Torpedo (electric ray), Trygon (sting ray and Latimeria.
Distinctions within groups: Though some sharks have long, flatter bodies, the bodies of rays are really flattened. They have disc-like, circular shaped bodies with very large connected pectoral fins. First of all rays have gill slits on the underneath side of their bodies, while sharks tend to have gill slits on the sides of their heads. Rays breathe by sucking in water through their spiracles and exhale it over their gills. Sharks on the other hand breath in two ways depending on the species. Rays use their tails to help them steer and maintain balance. Sharks on the other hand use their tails to swim. Rays tend to eat shellfish, crustaceans, and mollusks. Sharks on the other hand eat a whole variety of ocean animals depending on the species. They have sharp teeth in multiple rows are more aggressive hunters. Rays and sharks have different defensive mechanisms:  Some rays have poisoned barbs on their tails, while others can emit electric shocks when attacked. They also bury themselves in the sand and mud to hide from predators. Sharks on the other hand don’t have as many evolutionary adaptations to protect them from predators.

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The life cycle of the shark starts off with the egg of the female shark being fertilized by a male. After this happens, the sharks carry their young inside of them. The pups that are hatched inside are in competition in a cannibalistic way. The first pup that is born may eat the other eggs or pups, this process of the first offspring eating the others is called oophagy. After the pup is born they are developed enough to live their own life and become strong adults themselves after a long period of time. Then the same reproductive process hapens again and this becomes the life cycle of a shark. Generally the average sharks may live up to anywhere from 20 to 40 years although there are a couple species that are able to live up to about a whole century.

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Bony fish are characterized by a relatively stable pattern of cranial bones, rooted, medial insertion of mandibular muscle in the lower jaw.  In many bony fish these have evolved into swim bladders, which help the body create a neutral balance between sinking and floating. (The lungs of amphibians, reptiles, birds, and mammals were inherited from their bony fish ancestors). They do not have fin spines, but instead support the fin with  lepidotrichia. They also have an operculum, which helps them breathe without having to swim.Bony fish have no placoid scales.  Ossified endoskeletons (calcium phosphate)

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.

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Major Group Within Class:  A superclass containing the Bony fish, formerly in the now obsolete clade Teleostomi. It is divided into two classes: Actinopterygii − Ray-finned fish — contains most of the superclass's species. Sarcopterygii − Lobe-finned fish.
Distinctions within groups: The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines ("rays"), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also possess lepidotrichia. The living sarcopterygians are the coelacanths, lungfish, and the tetrapods. Early lobe-finned fishes had fleshy, lobed, paired fins, joined to the body by a single bone.Their fins differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. •In contrast to the tiny, pointed placoid scales of cartilaginous fishes, bony fish usually have cycloid or ctenoid scales, which are thin, flexible, and overlapping

–Cycloid scales have a smooth outer edge

–Ctenoid scales have a toothed outer edge

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5 Major Development Periods:

Embryonic- Baby

Larval- Fish Hatching from the egg

Juvenile- Not Sexually mature yet

Adult- Sexually Mature and has the ability to fertilize an egg

Senescent- Fish aging slowly  and become early

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The first and foremost characteristic of the reptiles is that are cold blooded. They are able to regulate their inner body temperature to the temperature of the environment. They have scaly skins but absence of hair or fur. The scales of the reptiles develop as a surface cells filled with Keratin.

Anatomical characteristics:

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.

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Major Group Within Class:
Crocodilia(Alligator),  Squamata and Testudines(turtles).
Distinctions within groups:
The key characteristics of crocodilians include:

elongated, structurally reinforced skull

wide gape

powerful jaw muscles

teeth set in sockets

complete secondary palate

oviparous

adults provide extensive parental care to young

The key characteristics of squamates include:

most diverse group of reptiles

exceptional skull mobility

The key characteristics of tuataras include:

slow growth and low reproductive rates

reach sexual maturity at 10 to 20 years of age

diapsid skull with two temporal openings

prominent parietal eye on top of head

The key characteristics of turtles include:

keratinised plates in place of teeth

body enclosed in a shell that consists of carapace and plastron

keen sense of smell, good colour vision, poor hearing

bury eggs in ground

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In most species of lizards, the life cycle begins with the lizard hatching from an egg, although a few species give birth to live young. Juvenile lizards then grow into adults without undergoing a metamorphosis that is seen in amphibian species. Adults mate and lay eggs and the cycle begins again.

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Generally accepted to have evolved from reptilian dinosaurs, birds share several characteristics with other classes of animals, including a skeletal backbone housing a spinal cord, a four-chambered heart and warm blooded.

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.

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Major Group Within Class:

Strisores (nightjars, frogmouths)

Columbaves ( cuckoos, pigeons)

Gruiformes ( cranes, rails, crakes)

Aequorlitornithes (penguins)

Inopinaves (hawks, owls, toucans)

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A bird life begins within an egg.  A special hard structure called the egg tooth forms on the baby bird’s beak to help it break the egg shell. This special tooth drops off a few days later.  A baby bird that has just hatched is called a hatchling. While the hatchling is growing in the nest and being fed by its parents we call it a nestling. When a nestling grows its flight feathers and is ready to leave the nest it takes its first flight or fledges. We call a bird that has just fledged a fledgling. A fledgling has fluffy down feathers and is often fed by its parents for several weeks more. Its flight feathers continue to grow and its wing muscles get stronger. When a fledgling or juvenile has finished growing it becomes a mature or adult bird. An adult bird attracts a mate, builds a nest and raises young to start the cycle all over again. Some birds migrate or travel long distances as part of their life cycle. Other birds stay in one area throughout the seasons.

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All mammals:

Are vertebrates (which means they have a backbone or spine).

Are endothermic. Also known as “warm-blooded,” endothermic animals regulate their own body temperate which allows them to live in almost every climate on Earth.

Have hair on their bodies.

Produce milk to feed their babies.

Jaw joint - The dentary (the lower jaw bone, which carries the teeth) and the squamosal (a small cranial bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and quadrate (a small bone at the back of the upper jaw).

Middle ear - In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.

Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.

Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.

Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob.

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Mammals can be divided into three more groups based on how their babies develop. These three groups are monotremes, marsupials, and the largest group, placental mammals. Monotremes are mammals that lay eggs. The only monotremes that are alive today are the spiny anteater, or echidna, and the platypus.

Distinctions between groups:
Monotremes are egg-laying Mammals. There are only five living monotreme species: the duck-billed platypus and four species of echidna (also known as spiny anteaters). All of them are found only in Australia and New Guinea.
A distinctive characteristic common to marsupials is that most of the young are carried in a pouch. Well-known marsupials include kangaroos, wallabies,koalas, possums, opossums, wombats, and Tasmanian devils.The placental mammals include such diverse forms as whales, elephants, shrews, and armadillos. They are also some of the most familiar organisms to us, including pets such as dogs and cats, as well as many farm and work animals, such as sheep, cattle, and horses. And humans, of course, are also placental mammals.

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Nearly all mammals form inside their mother and are born live. Whales, dolphins, sea otters, and other water mammals all give birth to live babies. Mammals care for their helpless babies and keep them safe. For example, bats keep their babies in warm, dry caves where they are safe from enemies. New mammal babies drink the milk their mother produces. Later, young mammals eat solid food. At first, most parents bring this food to their young. Over time, young mammals grow bigger and stronger. They learn to find their own food and become adults.

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.