Living mammals are placed into one of three major groups, depending on the way the animal completes its embryonic developement. One small group of mammals are the monotremes, which are egg-laying mammals. The monotremes include only three species, the duckbill platypus and two kinds of echidnas or spiny ant-eaters. All three are found only in Australia and New Zealand. These bizarre creatures differ from all other mammals because the female lays eggs, but when the young hatch she feeds them with her milk; the mother has no breasts or nipples, instead the milk seeps from small openings in her abdomen and is lapped up by the young.
Other mammalian features of the monotremes are the presence of hair and warm-bloodedness. However, they also have a number of reptilian features; for intance, their skeletons have reptilian shoulder girdles, and like reptiles, they have only one external opening for eliminating all body wastes as well as for reproduction. Most zoologists believe that monotremes separated early in evolution from the main mammalian line and do not belong to a group which was ancestral to the other mammals.