Monotreme
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Monotremes are a small relict[1] group of Australasian mammals, consisting of the duck-billed platypus and the several echidnas.
In addition to their unusual physical conformations, monotremes are remarkable for being the only mammals to lay eggs, and to have poison glands (spurs on the back legs of the male platypus); these were an unpleasant surprise for several early zoologists. A feature is the absence of well-developed mammary glands, the young platypi and echidnae being nourished by nutritious emissions from their mothers unmodified sweat glands.