[78] Dasypus Peba (Desmarest).
[79] Dasypus (Tolypeutes) apar (Geoffroy).
[80] Chlamydophorus truncatus (Harlan).
[81] The Macropodidæ.
[82] Macropus giganteus (Shaw).
[83] The presence of the pouch, or marsupium, containing the teats, involves many structural and physiological peculiarities which separate the Marsupialia, in a classificatory sense, from the rest of the Mammalia. The Great Kangaroo, which may be considered a fair example of the Marsupials, has in the female a set of skin muscles, around the pouch, beneath the skin, which close it. The milk, or mammary gland, has four long, slender teats in the pouch, and beneath the skin of it is a muscle called the cremaster, which is largely developed. It spreads over the surface of the gland, and its action is to squeeze it and to force out the milk through the teat. There is thus protection for the young, and milk is given forth, without the effort of the young in sucking. The reason for this is obvious. The Great Kangaroo, which is often as tall as a man, is pregnant for about thirty-nine days only, and then a little one, not bigger than a thumb, is born; it is not completely formed, and is blind and cannot move itself. The mother places it in her pouch, and it fixes on to a teat, where it hangs for about eight months, and then it begins to look out of the pouch. The duration of the life of the young in the womb is thus very small, and it has no placenta there, which in the other and non-marsupial Mammalia forms the life-union between the mother and the offspring before its birth. Thus, the Marsupials form one great group of Mammalia which are “implacentalia,” without placentas or “after-births,” and all the other Mammalia are “placentalia,” and have this link between mother and young. In all the Mammalia hitherto described the young come into the world by a single passage. In those now under consideration (the Marsupialia) there is a double passage, and the womb is separated into two portions, being double; so they are termed Didelphia. The marsupium has two remarkable bones more or less in relation to it, and all animals thus furnished are termed Marsupialia, and they form two sections or sub-orders—(1) The Marsupiata proper, with marsupial bones, mostly with pouches, and with inflected lower jaws. (2) The Monotremata, which have marsupial bones, depressions in the skin, when suckling, like ill-developed pouches, and beak-like jaws in front, which are not inflected.
[84] See Footnote 83 on previous page.
[85] Waterhouse’s “Natural History of the Mammalia,” order Marsupiata, from which much of this description of the order has been taken.
[86] R. Owen, “Marsupialia;” “Todd’s Cyclopædia of Anatomy and Physiology.”
[87] See also Vol. I., page 58, Note.