The pouch of a female Eastern Grey kangaroo; the most obvious distinguishing characteristic between marsupials and placentals. The joey, when newly born and in a very early stage of development, crawls up the female's fur from the birth canal in the cloaca (see below) to the pouch, clamps onto a nipple and continues its development in there.
A look inside the pouch of a red kangaroo.
For more photos of the pouch look under anatomy.
The cloaca of the same kangaroo, in its normal state. Marsupials, instead of having separate openings for anus and vagina, have only a shallow cloaca which serves both functions.
The cloaca when relaxed and open.
A close-up of the cloaca on the Common Wallaroo, and next to it the Eastern Grey Kangaroo.
A Whiptail wallaby exposing her cloaca.
A female Red Kangaroo's cloaca.
For more, see Doe Urinating.