Classification of Living Things: Classes of Vertebrates
Mammalia
Three of the   vertebrate classes are fish.   The most primitive of these is Agnatha
.  It consists of jawless fish    that do not have scales.  These are the lampreys and   hagfish.  Fish that have skeletons consisting of hard    rubber-like cartilage rather than bone are   members of the class Chondrichthyes
.  These are the sharks and rays.  All of the   bony fish are members of the class Osteichthyes
.  Tuna, bass, salmon, and trout are examples of Osteichthyes.
Ray (class Chondrichthyes) and       bony fish (class Osteichthyes)
Animals   in the   class Amphibia
spend part of their lives   under water and part on land.  Frogs, toads, and salamanders are amphibians.  Many of these species must keep their skin moist by periodically returning to    wet areas.  All of   them must return to water in    order to reproduce because their eggs would dry out   otherwise.  They start life with gills, like fish, and later develop lungs to breathe   air.
Salamander and frog (class Amphibia)
The   class Reptilia
includes turtles, snakes,   lizards, alligators, and other large reptiles.  All of    them have lungs to breathe on land and skin that does not need to be kept wet.  They produce an amniote
egg which usually has a calcium carbonate rich, leather hard shell that protects the embryo from drying out.  This   is an advantage over fish and amphibians because the amniote egg can be laid on land where it is   usually safer from predators than it would be in lakes,    rivers, and oceans.
 
Amniote egg
The   class Aves
includes all   the   birds.  They also produce amniote eggs but usually give them greater   protection from predators by laying them high off of the ground or in other relatively   inaccessible locations.  In the case of both reptiles    and birds, the eggs are fertilized within the reproductive tract of females.  There are other striking similarities between reptiles and birds in their   anatomies and reproductive systems.  This is not surprising because   birds are descendents of theropod dinosaurs (two-legged mostly carnivorous   dinosaurs).
Birds (class Aves)
Dogs,   cats, bears, humans and most other large animals today are members of the vertebrate class Mammalia
.  All mammals conceive their young within the reproductive   tract of the mother and, after birth, nourish them with milk produced by their mammary glands
.  Mammals are heterodonts
with strong jaws.  That is to say, they have a   variety of specialized teeth (incisors, canines, premolars,    and molars).  This allows them to chew their food into small pieces before swallowing it.  Subsequently, they can   eat any size plant or animal.  Many reptiles must swallow their prey whole, which limits them to hunting    smaller game.
Mammalian heterodontism
Like   birds, mammals are endothermic
,    or warm blooded.  They are able to maintain a    relatively constant body temperature regardless   of external environmental conditions mainly by using internal physiological   mechanisms.  In other words, they are homeothermic,    or stable in core body temperature, as a result of endothermy.  All of the living species of insects, fish, reptiles, and   amphibians are ectothermic
, or cold blooded.  They    keep their body temperature in a    normal range mainly by avoiding exposure to environmental    temperature extremes.  For instance, reptiles    usually remain in shaded areas on hot days to prevent fatal overheating.  On cold nights, their lowered body temperature can cause them to become    sluggish and inactive.  In contrast, endothermic animals are   able to remain active at night and often in the winter when the air   temperatures are   especially cold.  They can    also move about in the heat of very warm days.  This ability   most likely provided an advantage for the early small mammals in   surviving alongside dinosaurs and other large reptiles,   which apparently were mostly ectothermic.  The downside of endothermy is the need to consume far   more calories relative to body size in order to maintain a constant    core body temperature.  Small mammals, such as moles   with their rapid metabolism rates, must eat insects or other high calorie   foods every half hour or so in order to stay alive.  By comparison,   cold blooded rattlesnakes usually eat only once every 3-6 weeks and have been known to go without food for as long as   two years.
Aiding in mammal body   temperature control is their insulating hair and sweat glands.  Sweating helps to dissipate heat by evaporative cooling.  Compared to    most other land mammals, humans are relatively hairless, but they have far more    sweat glands.  Mammals have four chambered hearts (like   birds), complex nervous systems, and large brains relative to the size of their bodies.  This broad range of   useful features has made mammals highly adaptive and successful.  They first appeared   about 200,000,000 years ago, early in the age of dinosaurs, and replaced reptiles as the dominant class of land animals after 65,000,000 years ago.  As   the rapidly changing environment at that time led to the mass extinction of   most large reptiles, it left vast evolutionary possibilities which mammals   took advantage of by rapidly diversifying through adaptive radiation.  
Important to   mammalian success is their reproductive system.  Their bodies took the amniote egg revolution of reptiles and birds one step   further.  In effect, the uterus
functions as the   protective eggshell.  Young mammals spend a long period of their early development   within their mother's uterus.  After birth, they are provided with protein and fat   rich milk to eat and are usually protected until maturity.  Pregnancy and milk   production require mothers to significantly increase their own calorie consumption   in order to provide nutrients for their infants.  A nursing human female   normally uses about 30% of her body's energy just to produce milk.
Mammalian mother and her baby
Despite their   success, mammals still only make up about .4% of known animal species.  It is humbling to realize that all chordates together are only just over 3.7%   of known animal species.  By comparison, well over 1/2 of all animal   species are insects.