MAMMALS OF THE WORLD
Although only three subclasses of mammals exist in the class Mammalia, they each exhibit vastly different features and adaptations.

Unlike the common conception of mammals, the monotreme subclass (Holotheria) does not include animals with nipples to nurse their young. Instead, females lay eggs that may be carried in a pouch or kept warm in a nest. The hatching young then lick milk off the fur on the surface of their mother's mammary glands. Monotremes are found only in Australia, Tasmania, and New Guinea.

Baby animals in the marsupial subclass (Metatheria) depend on the shelter of their mother's pouch where they reside during development.

The most common subclass of mammals, the placental mammals, or Eutheria (including polar bears), develop a placenta, an organ of exchange between developing embryo and mother. Through it the embryo receives its nourishment and oxygen and rids its blood of wastes. This adaptation enables young placental mammals to complete embryonic development before they are exposed to perilous conditions outside the womb.