CHAPTER VIII 

 MAjNIMALIA 



Mammals can be defined as animals with hair and milk glands. 

 Either part of the definition will serve, for only mammals se- 

 crete milk for their young; and although some mammals are 

 almost lacking in hair, even the whale has a few whiskers. One 

 extinct reptile has been described as having hair, but this is 

 probably a hair-like structure, rather than true hair. However, 

 it is not beyond the realm of possibility that some therapsid rep- 

 tiles had hair between the scales. In any event, the above defini- 

 tion will serve to identify the living mammals. 



INIammals include a heterogeneous assemblage of animals. 

 Some lay eggs like a reptile, others give birth to very immature 

 young, and others, which make the majority group, give birth 

 to young in a well developed condition. The last group includes 

 the mice, porcupines, cats, dogs, seals, whales, elephants, pigs, 

 monkeys and men — in fact, practically all the mammals which 

 any student outside of Australia would know. The mammals 

 are divided into three sub-classes and a number of orders. 



Sub-class Prototheria. This group includes one order and a 

 few genera. They are the most primitive of living mammals and 

 are restricted in their distribution to Australia, New Zealand, 

 and the adjacent islands. The animals have the body covered 

 with hair, and there are milk glands. The latter have no teats, 

 the secretory ducts opening separately into depressions on the 

 mother's abdomen, and the young lick the milk off. 



The order is called the Monotreiuata (one opening) in refer- 

 ence to the cloaca into which the digestive and urinogenital 

 openings empty. This structure is typical of the amphibia, rep- 

 tiles and birds, and is not found elsewhere among the mammals. 



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