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SECOND BOOK. 
7. Breathing is effected by lungs (two) in the thorax. 
8. The thorax is bounded by ribs. 
9. The regular number of limbs is four: legs, or legs and arms. 
10. Most Mammals have teeth in sockets in the jaw. 
Orders. — There are about 3000 kinds of Mammals, and these are 
arranged in orders. (See p. 234). 
Vertebrata, Class II. — BIRDS (pp. 25-31). 
Character istics : 
1. The second class of the Vertebrata in order of development. 
2. The covering always consists of feathers. 
3. The young are hatched from eggs. 
4. The head is singly -jointed with the backbone. 
5. The heart is four-chambered ; the blood warmer than in Mammals. 
6. The chest (thorax) and belly (abdomen) are not separated by a dia- 
phragm. 
7. Breathing is effected by lungs and air-sacs. 
8. The thorax is bounded by ribs. 
9. The regular number of limbs is four: viz. a pair of legs and a pair 
of wings. 
10. There are no teeth, but some birds have gizzards. 
Orders. — Birds differ greatly in the structure of their bills and toes. 
These differences enable us to group them into orders. (See p. 234.) 
Vertebrata, Class III. — REPTILES (pp. 31-36). 
Characteristics : 
1. The third class of the Vertebrata in order of development. 
2. The covering consists of horny scales or plates. 
3. The young are hatched from eggs. 
4. The head is singly -jointed to the backbone. 
5. The heart is three-chambered ; the blood is cold. 
6. The chest (thorax) and belly (abdomen) are not separated by a dia- 
phragm. 
7. Breathing is effected by lungs. 
8. The thorax is bounded by ribs (generally). 
9. The limbs are absent or four in number (generally). 
10. The Tortoises and Turtles have a horny beak, and no teeth. The 
other reptiles have teeth, but these are not sunk in separate sockets, except 
in the Crocodiles. 
Orders. — (See p. 234.) 
