MAMMALIA 439 



not determined by that of the surrounding medium; red blood 

 corpuscles not nucleated; one (the left) aortic arch persists. 



6. Two occipital condyles. 



7. Chiefly viviparous (Monotremes are oviparous); foetus 

 nourished during early development in the uterus of the mother, 

 often being closely connected therewith by a complex structure 

 known as the placenta. 



448. 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. 238). 



2. The Marsupialia (pouched) possess a marsupium 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. 52, 62). 



3. In the Placentalia (placenta) there is a placenta or mass 

 of closely interwoven maternal and embryonic tissue which 

 unites the foetus with the wall of the uterus, by which arrange- 

 ment 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. 



449. 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 



