What is a non-placental mammal?

A non-placental mammal is a mammal that does not have a placenta, which is a specialized organ that provides nourishment to a developing fetus. Non-placental mammals give birth to live young, but the embryos develop within the mother's uterus without a placenta. Instead, they receive nutrients from the mother's uterine lining through a yolk sac or other specialized structures. Non-placental mammals include marsupials, such as kangaroos and koalas, and monotremes, such as the platypus and echidna.