REPTILES. 191 



They generally deposit their eggs in warm sandy places, 

 leaving them to be hatched by the warmth of the atmos- 

 phere. 



313. There are five orders of Reptiles: 1. The Turtles 

 or Tortoises. 2. The Crocodiles. 3. The Lizards. 4. The 

 Serpents. 5. The Amphibia. 



314. The Tortoises are unlike all other animals in their 

 covering. They are in a fortified house of bone and horn, 

 which they carry around with them. Into this they can 

 wholly retire when attacked. In some of the Land Tur- 

 tles this covering is so jointed that they can close the 

 openings before and behind after drawing in the head, 

 legs, and tail, thus shutting the doors of their portable 

 house against their enemies. The construction of this 

 covering is worthy of examination. It is composed of 

 two shields, an upper and a lower one. The upper one, 

 called the carapace, has a coating of plates of horn. As 

 the turtle grows, each plate grows by enlargement around 

 its edge. The tortoise-shell, so much used in making 

 combs, comes from this coating in one species. On re- 

 moving this, we see that the carapace is composed of a 

 large number of plates of bone, very nicely and firmly 

 joined together. There is a row of eight plates through 

 the middle, and these are appendages of the vertebrae of 

 the back of the animal. These vertebrae you see in Fig. 

 6, where the lower shield is removed, so that you have a 

 view of the under surface of the carapace. As the ribs 

 extend from the vertebras they expand, thus making some 

 of its side plates. The lower shield, called the plastron, 

 is the same thing as the breast-bone of other animals, 

 only it is enormously large. 



315. Life in these animals goes on at a low rate, and 

 lasts a long time in some cases even over two hundred 

 years. Their sensibilities are dull, and it is very difficult 

 to kill them, as they survive the severest injuries. They 

 vary considerably in the form of their feet and of their 

 shell, especially the former, according to their mode of 



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