Monotremes are members of the egg-laying mammalian order Monotremata, which includes the amphibious platypus and the terrestrial echidnas of continental Australia, the Australian island state of Tasmania, and the island of New Guinea. Monotremes are very highly modified for their particular ecological niches. The platypus is a semiaquatic carnivore-insectivore, and the echidnas have a specialized diet of small invertebrates such as ants, termites, and worms. The name Monotremata means “one-holed,” in reference to the fact that both sexes have only one opening at the rear of the body, which is used for both reproduction and excretion.

Source: Britannica