The marsupials, or marsupialia, are an order of pouched mammals, which include the kangaroos, koalas, and opposum, all inhabitants of Australia. The marsupial embryo stays in the utterus (womb) for only a short time. When the young are born they are little more than embryos. They crawl from the vagina into a pouch called marsupium on the mother's abdomen. The mother may lick a pathway through her fur to smooth their way. Once they are inside the pouch, the babies seek out a nipple which they grasp in their mouth and hang onto for weeks as they complete their development. Thus, the pouch serves the same function as the incubator in which we rear premature babies.
Marsupials have several other features which set them apart from the placental mammals. Their brains have a number of reptilian characteristics and their skeletons have two bones called marsupial bones attached to the pelvis. Apparently, marsupials split off early from the main stem of mammalian evolution.