MAMMALIA. 429 



437. General Survey. There are three subclasses of 

 mammals which differ in mode of reproduction and in degree 

 of development. 



1. The Monotremata are the lowest and are characterized 

 by the fact that they lay eggs, like reptiles and birds ; there is a 

 cloaca into which the alimentary, urinary, and genital canals 

 open ; the milk glands are poorly developed. The class is rep- 

 resented by the duck-mole, an aquatic form, and the spiny 

 ant-eater, both natives of Australia and neighboring islands 

 (Fig- 235). 



2. The Marsupialia possess a marsnpium or pouch, a fold of 

 the skin into which the prematurely born young are placed and 

 nourished until able to take care of themselves. The period 

 of gestation is short and the connection between the embryo 

 and the wall of the uterus is slight. In the group are embraced 

 the kangaroo and other Australian forms, and the opossums of 

 America. It is an interesting fact that the native Australasian 

 mammalia all belong to these two lower classes (see Figs. 

 50, 60). 



3. In the Placentalia there is a placenta or mass of closely 

 interwoven maternal and embryonic tissue which unites the 

 fcetus with the wall of the uterus, by which arrangement the 

 young gets its food and oxygen from the blood of the mother. 

 The young are retained much longer in the uterus, and are 

 consequently much more mature when born. All the common 

 mammals belong to this group, which is distributed over the 

 habitable part of the earth. 



438. Form. The axis of the body is usually separable into 

 head, neck, trunk, and tail, though the last may be reduced 

 to a very small number of segments. The proportions of these 

 parts of course differ much and are to be connected with the 

 habits of life. Most of the Mammalia are quadrupeds (except 

 the allies of the whales, the porpoises and the- sea-cows) ; and 

 all except man, and some of apes most like him, have the axis 

 of the body in a horizontal position supported by all four 

 appendages, or by the medium. The aquatic forms whales, 



