284 



Descriptive Zoology. 



CLASSIFICATION OF MAMMALS. 



SUBCLASSES. 



ORDERS. 



2. Theria^j (Placentals) 



i. Prototheria i. Monotremata Duckbill and' Spiny Ant-eater 



f (Nonplacentals] . 2. Marsupialia Opossum, Kangaroo 



3. Edentata Sloth, Armadillo 



4. Rodentia Rabbit, Squirrel 



5. Insectivora Mole, Shrew 



6. Cheiroptera Bat 



7. Cetacea Whale, Porpoise 



8. Sirenia ; Sea Cow 



9. Ungulata Horse, Cow, Deer 



10. Proboscidea Elephant 



11. Carnivora Cat, Dog, Bear, Seal 



12. Primates Lemur, Ape, Man 



CHARACTERISTICS OF CHORDATA. 



1. There is a notochord, a long skeletal rod, a sort of 

 forerunner of the backbone. 



2. There are gill slits opening from the throat to the 

 outside. 



3. The central nervous system is a hollow tube and is 

 wholly dorsal to the digestive tube. 



CHARACTERISTICS OF VERTEBRATA. 



In the Vertebrata the characters given for the Chordata 

 are distinctly displayed, and a permanent backbone is de- 

 veloped. A cross section of a vertebrate shows two cavities 

 (see Fig. 150), the dorsal containing the cerebro-spinal 

 nervous system, and the ventral containing the digestive, 

 circulatory, and respiratory organs. In the vertebrates a 

 liver is always present. The nervous system is usually 

 well developed. 



Classification of Animals. An early classification divided 

 animals into two groups, the vertebrates, or the animals 

 having backbones ; and the invertebrates, those lacking a 



