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Placentalia 有胎盘类 有胎盘类 Placentalia \ Plac` en* ta" li* a\, n. pl. [ NL.] ( Zool.) A division of Mammalia including those that have a placenta, or all the orders above the marsupials. [ 1913 Webster]
Mammalia \ Mam* ma" li* a\, n. pl. [ NL., from L. mammalis. See { Mammal}.] ( Zool.) The highest class of Vertebrata. The young are nourished for a time by milk, or an analogous fluid, secreted by the mammary glands of the mother. [ 1913 Webster] Note: Mammalia are divided into three subclasses; [ 1913 Webster] I. { Placentalia}. This subclass embraces all the higher orders, including man. In these the fetus is attached to the uterus by a placenta. [ 1913 Webster] II. { Marsupialia}. In these no placenta is formed, and the young, which are born at an early state of development, are carried for a time attached to the teats, and usually protected by a marsupial pouch. The opossum, kangaroo, wombat, and koala are examples. [ 1913 Webster] III. { Monotremata}. In this group, which includes the genera { Echidna} and { Ornithorhynchus}, the female lays large eggs resembling those of a bird or lizard, and the young, which are hatched like those of birds, are nourished by a watery secretion from the imperfectly developed mammae. [ 1913 Webster]
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