Why is a platypus called a monotreme?

Answer 1

Both platypuses and echidnas are monotremes. The name monotreme is derived from two Greek words meaning "one-holed", because they have just one external opening, the cloaca, for both waste elimination and for reproduction. The cloaca leads to the urinary, faecal and reproductive tracks, all of which join internally, and it is the orifice by which the female monotreme lays her eggs.

Answer 2

Platypuses are not marsupials: they are monotremes, or egg-laying mammals. Like other mammals, they nurture their young on mothers' milk, but there are some fundamental differences between monotremes and marsupials.

Marsupials are mammals which, for the most part, have pouches in which they rear their young. Marsupial young are characterised by being extremely small and undeveloped at birth. At birth, they take a long, arduous journey from the birth canal, driven purely by instinct, grabbing hold of the mother marsupial's fur which she has cleaned and made easier to traverse with saliva, to reach the pouch. Upon reaching the pouch, they latch onto a teat which swells in their mouth to prevent them from being accidentally dislodged during the mother's movements. There they stay for months, to complete their development.