38 
GUIDE TO TIIE 
in addition to these, there is on the north side an enclosed 
plot of grass into which any of the animals on that side 
can be let out. This side of the house is entirely devoted 
to Marsupials. 
The great class of Mammalia is divisible into three sec¬ 
tions ; the Ornithodelphia , Didelphia , and Monodelphia ; 
and typical of the first group is the Ornithorynchus or 
Duck-bill Platypus of Australia ; typical of the second 
is the Kangaroo ; and included in the third are all the 
ordinary Mammals from man downwards. 
The Ornithodelphia present certain characters by which 
they are connected both with birds and reptiles. It 
includes only two genera, viz., Ornithorynchus and Echidna , 
both being restricted to Australia, Tasmania and New 
Guinea. 
The Didelphia , or Marsupials are so called because 
the majority of them possess a pouch on the belly of 
the animal, in which the imperfectly developed young arc 
received, being apparently transferred thither by the mother, 
although the act has never been witnessed, and there is no 
passage between the womb and the pouch.- Each becomes 
attached to a long nipple within the pouch, and which 
exactly fills the mouth of the young creature, down 
whose throat the milk is squirted by the cremaster 
muscle of the mother. Suffocation is prevented by a 
special modification of the larynx of the young, which, 
as in the case of dolphins, porpoises and whales is pro¬ 
longed upwards into the posterior opening of the nose 
at the back of the mouth, so that respiration takes place 
entirely through the nostrils, and thus also the milk is 
prevented finding its way into the wind-pipe. The 
young remains attached to the nipple until it attains 
