There are three classes, or main types, of mammals: monotremes, marsupials, and placental mammals. Monotremes are the most primitive, and there are only three species: the duck-billed platypus and two species of echidna. These mammals have hair and produce milk, but they also lay eggs. The eggs are leathery, similar to reptile eggs, and hatch into tiny young that are not well developed. The young cling to the fur on the mother’s belly and suck at her milk, which comes from pores in the skin instead of from a nipple.
Marsupials also have tiny, undeveloped young, but they grow inside the mother’s body instead of in an egg. When they are born, they climb up the mother’s fur to a pouch on her belly and settle inside. They latch onto a nipple and nurse almost continually until they have grown enough to emerge from the pouch. Some well-known marsupials are koalas and kangaroos.