Final answer:
Monotremes are a unique order of mammals that include the platypus and echidnas. They lay eggs and the babies hatch in a fetal state, completing their development in nests before being nourished by milk.
Step-by-step explanation:
The monotremes are one of the oldest orders of mammals, and include only the echidna and the platypus today. Monotremes are unique among mammals because they lay eggs, instead of giving birth to live young. The platypus and echidnas are found in Australia and New Guinea. Monotremes retain their eggs through about two-thirds of the developmental period, and then lay them in nests. The babies hatch in a fetal state and complete their development in the nest, nourished by milk secreted by mammary glands.