What are the similarities and differences between marsupials monotremes and placentals?

Marsupials, monotremes and placental mammals are all mammals. They share the following characteristics: * vertebrates * warm-blooded * have fur, skin or hair * breathe through lungs (not gills) * the young feed on mother's milk The major differences, however, include: * monotremes are the only mammals which lay eggs * marsupial young are born undeveloped and continue most of their growth and development whilst they are in the mother's pouch, attached firmly to the teat for several months, where they receive all their nutrients.