THE VERTEBRATES 205 



plates, while among the turtles and crocodiles there may be, 

 in addition to the epidermal plates, a real deposit of bone 

 in the skin whereby the effectiveness of the armor is increased. 

 The epidermal covering of snakes and lizards is periodi- 

 cally moulted, or, as we say, the skin is shed. The bright 

 colors and patterns of snakes and of many lizards are due 

 to the presence and arrangement of pigment cells in the 



* * O 



FIG. 102. The giant land-tortoise of the Galapagos Islands, Testude 

 sp. These tortoises attain a length of four feet. (Photograph from 

 life by Geo. H. Cole man, of a specimen brought to Stanford University 

 by Snodgrass and Heller.) 



skin. With some reptiles, notably the chameleons, the 

 colors and markings can be quickly and radically changed 

 by an automatic change in the tension of the skin. 



Specimens of some pond or land turtle common in the 

 vicinity of the school should be obtained. The red-bellied 

 and yellow-bellied terrapins, or the painted or mud-turtles 

 are common over most of the United States. They may be 

 raked up from creek bottoms or fished for with strong hook 

 and line, using meat as a bait. They will live through the 



