MAMMALIA. 317 



the young, but in the Marsupials (Kangaroos, Opossums, &c.) the 

 milk is forced into the mouth of the young animal by the action of 

 a special muscle. 



It will be sufficient for our present purpose to divide the Mam- 

 malia, in accordance with the way in which the young ai'e developed, 

 into three primary sections — viz., the Placental Mammals, the Non- 

 placental Mammals, and the Oviparous Mammals. In the Placental 

 Mammals, not only are the young brought forth alive, but they are 

 nourished within the body of the mother by means of a structure 

 called the placenta, through which the nutrient materials of the 

 mother's blood reach the young. In consequence of this, the young 

 of the Placental Mammals can be retained within the body for a 

 considerable period, and when born, they are ble to obtain their 

 natural food — the milk — by their own exertions. This section in- 

 cludes all the higher gi-oups of Mammals. In the Non-placenial 

 Mammals, on the other hand, the young are born alive, but at an 

 extremely early period of their development, before there is any 

 necessity that a placenta should be formed for the nourishment of 

 the foetus. In these cases, therefore, the young, when born, are 

 much more immature and helpless than in the case of the Placental 

 Mammals. So helpless are they, that they are even unable to suck, 

 andl have in most cases to be fixed by the mother herself upon the 

 teats, whilst the milk is forced into their mouths by a muscle which 

 is spread over the mammary gland. This section includes only the 

 Marsupials or Pouched Mammals. Lastly, in the Oviparous Mam- 

 mals, the young are not born alive but are produced as eggs, as in 

 the Birds and Eeptiles. The young, however, are liberated from the 

 egg at a very early period of their development, and are then 

 nourished by the milk of the mother. This section includes only the 

 Monotrematous Mammals — viz., the Duck-mole {Ornithorhynchus) 

 and the Spiny Ant-eater {Echidna). 



Adopting the above primary sections as practically sufficient in 

 an elementary work, the whole class of the Mammalia may be 

 divided into the following fourteen orders : — 



Division A. — Oviparous Mammals. 

 Order 1. — Monotremata. 



Division B. — Non-placbntal Mammals. 

 Order 2. — Marsupialia. 



Division C. — Placental Mammals. 

 Order 3. — Edentata. 

 Order 4. — Sirenia. 



