A marsupial is a mammal belonging to the infraclass Marsupialia, which is characterized by premature birth and continued development of the newborn while attached to the nipples on the mothers lower belly. The young are born in a very incomplete state, usually around the size of a lentil or a jelly bean, and they crawl to the pouch where they continue to grow. The pouch, or marsupium, is a flap of skin covering the nipples, which is a distinctive characteristic common to most of these species. However, not all marsupials have pouches, and in some species, the nipples are fully exposed or are bounded by mere remnants of a pouch. Marsupials have a short-lived placenta that nourishes their young for just a few days before theyre born, and the rest of their nutrition comes from the mothers teats inside the pouch.

Marsupials have typical mammalian characteristics such as mammary glands, three middle ear bones, and true hair. However, there are striking differences as well as a number of anatomical features that separate them from placental mammals. There are over 330 species of marsupials, and they are found in Australasia, Wallacea, and the Americas. The largest and most varied assortment of marsupials, some 200 species, is found in Australia, New Guinea, and neighboring islands, where they make up most of the native mammals found there. In addition to larger species such as kangaroos, wallabies, wombats, and the koala, there are numerous smaller forms, many of which are carnivorous, with the Tasmanian devil being the largest of this group. About 70 species live in the Americas, mainly in South and Central America, but one, the Virginia opossum, ranges through the United States into Canada.