FISHES, BATRACHIANS, AND REPTILES 221 



some reptiles, notably the chameleons, the colors and 

 markings can be quickly and radically changed by an au- 

 tomatic 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 

 ma}' be raked up from creek bottoms or fished for with 



Fig. 179. — The giant land-tortoise of tlie Galapagos Islands, Tcsliido sp. 

 These tortoises reach a length of four feet. (Phot(jgraph from life by 

 Geo. Coleman from specimen brought to Stanford University by Snod- 

 grass and Heller.) 



strong hook and line, using meat as a bait. They will 

 live through the winter, if kept in a cool place, without 

 food or special care of any kind. Observe their swimming 

 and diving, the retraction of head and limbs into the shell, 

 the use of the third eyelid (nictitating membrane), and 

 the swallowing of the air. Note the "shell," consist- 

 ing of a dorsal plate, the carapace and ventral plate, plas- 

 tron, and the lateral uniting parts, the bridge. Almost 

 all the fresh-water and land turtles are carnivorous, but 

 few catch any very active prey. While some are strictly 



