44° ZOOLOGY 



determined by that of the surrounding medium; red blood 

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



6. Two occipital condyles. 



7. Chiefly viviparous (Prototheria 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 two subclasses of mam- 

 mals, Prototheria and Eutheria, which differ in mode of repro- 

 duction and in degree of development. 



1. The Prototheria 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 Eutheria, Division Didelphia (Marsupials) possess a 

 marsupium or pouch, a fold of the skin into which the pre- 

 maturely 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 the Prototheria and the Marsupials (see Figs. 52, 62). 



3. In the Division Monodelphia (Placentalia) 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 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. 



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 



