

MAMMALIA 289 



out from the developing embryo and becomes fastened to 

 the uterus of the mother in such a way that there is an 

 exchange of nourishment between mother and offspring 

 through the thin walls of the blood-vessels. All mamma- 

 lian embryos, like those of reptiles and birds, are covered by 

 a protective envelope, the amnion, before birth. This is 

 filled with a watery fluid and its walls enter into the forma- 

 tion of the placenta. 



Mammals are divided into large groups primarily on the 

 basis of the degree of specialization in the reproductive and 

 developmental processes. The groups are as follows : 



Subclass I. Prototheria; egg-laying (oviparous) mammals. 



Subclass II. Eutheria; viviparous mammals. 



Division 1. Didelphia; mammals which carry the young in a mar- 

 supial pouch and nourish them before birth through a placenta which is 

 usually very primitive. 



Division 2. Monodelphia; mammals which nourish the young 

 through a typical placenta, and never carry them in a pouch after birth. 



Fossil records show that the Prototheria and Didelphia 

 appeared on earth before the Monodelphia. There are 

 also rudimentary structures on various representatives of 

 the latter group which indicate that they came from 

 marsupial ancestors. The fourteen orders of mammals 

 which have living representatives will now be considered. 



Order 1. Monotremata. This order includes two primi- 

 tive egg-laying mammals which live in Australia, New Gui- 

 nea, and Tasmania. The skeleton and some other mor- 

 phological features show affinities with birds and reptiles. 

 The eggs hatch in a few hours and the young are at once 

 placed in a hollow in the abdomen which is lined with mam- 

 mary glands. There is no teat and the young lick the 

 milk from the hairy surface of the skin. 



The spiny ant eater, Echidna aculeata, has a long head 

 and a mouth without teeth. Its tongue is very sticky and 

 extensile, serving to capture ants. The duckbill, Orni- 

 thorhynchus anatinus (Fig. 107) is adapted to an aquatic 

 life. It possesses webbed feet, thick fur, and a duck-like 



