-well developed brain
-lungs, two way, two circuits: pulmonary and systemic
-four chamber heart
-well developed hormonal system
Order Placental: most mammals, uterus until developement is complete, half maternal
and half embryonic.
Order Marsupial: uterus holdsembryo for first stage, have skin and hair, complete
developement in pouch.
Order Monotremes: egg laying, platypus and spiny anteater