CHORD ATA — VERTEBRATA — MAMM.\LS 



377 



1. Monotremata 



2. Marsupialia 



3. Insectivora 



4. Chiroptera 



5. Carnivora 



6. Rodentia 



7. Edentata 



8. Ungulata 



9. Sirenia 



10. Cetacea . 



11. Primates . 



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The species of mammals represent an ascending series as 

 follows, (i) The very primitive, reptile-like sub-class Proto- 

 theria (represented by Order i). These are egg-layers ; the egg 

 is (in Echidna) placed by the mother in a ventral pouch con- 

 taining milk glands without nipples ; here it is hatched and then 

 nourished by the milk poured out around it. Ornithorhynchus 

 lays its eggs in a nest in its burrow, brooding over them bird- 

 like. The body temperature is variable (changing as much as 

 15° according to the temperature of the environment). A cloaca 

 is present, in which terminate the ducts of the urinary, repro- 

 ductive and digestive systems. (2) The more advanced sub- 

 class Metatheria (including Order 2). They produce the young 

 alive, but in so rudimentary a condition that directly after birth 

 they are placed by the mother in a ventral pouch which con- 

 tains the false nipples and are there sheltered until able to take 

 care of themselves ; a placenta is absent or functional only for 

 a short period. Cloaca absent. (3) The sub-class Eutheria 

 (including Orders 3-1 1) are the most highly evolved. The young 

 are nourished through a placenta until well advanced (whence 

 the name placental mammals for this sub-class) ; after birth 

 they are nourished by milk through true nipples. The young are 

 never carried in a pouch. Cloaca absent. 



