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marsupium A pouch (from the Greek marsupion, ‘pouch’) on the surface of the body of an adult animal in which its young are held securely. Marsupia occur in some invertebrates (e.g. some Isopoda), some Anura, but most notably in the Marsupialia, which derive their name from the possession of marsupia, although not all species have them. In those female marsupials that do, and also in echidnas (see TACHYGLOSSIDAE), the marsupium is a fold of skin, supported by epipubic bones, containing the mammary glands into which the eggs of echidnas and young of marsupials are placed.