MICROANATOMY

Detailed explanation-1: -Mammals are divided into three groups-monotremes, marsupials and placentals, all of which have fur, produce milk and are warm-blooded. Monotremes are the platypus and echidnas and the females lay soft-shelled eggs.

Detailed explanation-2: -The placental mammals include such diverse forms as whales, elephants, shrews, and armadillos. They are also some of the most familiar organisms to us, including pets such as dogs and cats, as well as many farm and work animals, such as sheep, cattle, and horses. And humans, of course, are also placental mammals.

Detailed explanation-3: -Marsupials (e.g. kangaroo, opossum) and monotremes (e.g. platypus) differ from placental mammals in many characteristics, particularly reproduction. With their early divergence from placentals, they fill the phylogenetic gap between the mammal-reptile divergence 310 MYA and the placental radiation 100 MYA.