CHORD ATA — VERTEBRATA — M AM]\L\LS 377 



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1. Monotremata 378 



2. Marsupialia 378 



3. Insectivora 379 



4. Chiroptera 379 



5. Carnivora 379 



6. Rodentia 381 



7. Edentata 382 



8. Ungulata 382 



9. Sirenia 398 



10. Cetacea 398 



11. Primates 399 



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 

 aHve, 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 3-11) 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. 



