Class Mammalia (Latin, mammae = breasts).  As the name implies, all mammals share a common feature, which is a mammary gland that produces milk.   Another common feature among mammals is hairs that insulate and maintain the body temperature.

Mammals are divided into three groups o sub-classes.  The Monotremes are the egg-laying mammals.  The duckbill platypus, and spiny anteaters live in Australia and New Guinea.  The females lay two eggs, after hatching young nurse by licking milk secreted onto mother's hair.

The Marsupials are also known as pouched mammals.  The members of this group are opossums, kangaroos, and koalas.  The females give birth to tiny embryonic offspring and the offspring complete its development in the mother's pouch.  The nipples are in the mother's pouch, where the young nurses and sleeps.

The Placental mammals are the most successful group of mammals.    The female gives birth to a well developed offspring.  The embryonic development takes place within the uterus of the mother, in an organ called placenta.

Humans, whales, dolphins, and bats are the few  that represent the mammals.