| 
MARSUPIALS IN GENERAL. 9 
forced injection of milk from the mammary glands. This is 
brought about by the upper part of the larynx, or extremity of 
the wind-pipe, being at this period of existence so much 
elongated as to project through the fauces or hinder aperture 
of the mouth and reach the internal nostrils. By this means 
a closed passage is formed from the nostrils to the lungs, and 
the young creature can thus breathe securely while a stream 
of milk is flowing down its mouth and throat into the 
stomach. It is noteworthy that this condition, which is 
merely transitory among the Marsupials, persists throughout 
life in the Cetaceans. 
The whole of the animals presenting the foregoing charac- 
teristics constitute one Order, to which zoologists apply the 
name of Marsupialia. The differences by which the Marsu- 
pialia are distinguished from the higher orders of Mammalia 
are, however, so much greater than those by which the latter 
are severally differentiated from one another, that it has been 
further considered advisable to regard the whole of the latter 
as constituting one great Sub-class, while the Marsupials form 
a second Sub-class to themselves. For the Sub-class compris- 
ing all the higher Mammals naturalists have adopted the names 
of Monodelphia, Placentalia, or Eutheria ; while for the one re- 
presented solely by the Marsupialia, the names of Didelphia, 
Implacentalia, and Metatheria have been proposed. 
So different in general form and appearance from one 
another are many of the Marsupials—some, like the Thylacine, 
being carnivores, adapted for walking, while others, like the 
Kangaroos, are herbivorous, with limbs suited for leaping— 
that it might at first sight appear that the Sub-class Didelphia 
might well be subdivided into distinct orders, in the same 
manner as the Monodelphia or Eutheria are split up into the 
Primates, Carnivora, Rodentia, Ungulata, &c. It has been 
found, however, that structurally all the Marsupials are so 
| ia 
