Marsupials
* Marsupials give birth to live young, but the young are born in a very immature state.
* The young are then carried in a pouch on the mother's body, where they continue to develop.
* Marsupials include kangaroos, koalas, and opossums.
Monotremes
* Monotremes are the only mammals that lay eggs.
* Monotremes include the platypus and echidna.
Placentals
* Placentals give birth to live young, and the young are born in a relatively mature state.
* The young are nourished through a placenta, which is an organ that connects the mother's bloodstream to the fetus's bloodstream.
* Placentals include humans, cats, dogs, and whales.
Similarities
* All mammals are warm-blooded.
* All mammals have hair.
* All mammals produce milk to feed their young.
* All mammals have a four-chambered heart.
Differences
* Marsupials give birth to live young, but the young are born in a very immature state. Monotremes lay eggs. Placentals give birth to live young, and the young are born in a relatively mature state.
* Marsupials carry their young in a pouch on the mother's body. Monotremes do not have a pouch. Placentals do not have a pouch.
* Marsupials have a relatively short gestation period. Monotremes have a long gestation period. Placentals have a long gestation period.