230 TEXT-BOOK OF ZOOLOGY 



activity of reptiles increases, while it is lessened with the reduction of 

 the outside temperature. As the temperature of the air rises, the 

 respiration becomes more active, the blood courses more rapidly through 

 the body, the demand for food becomes keener, digestion proceeds more 

 rapidly, and all the movements of the body are accelerated. This 

 explains the great love of sunshine displayed by reptiles, especially 

 the more active species (e.g., lizards), and their greater abundance in 

 the warmer regions of the earth (as well as in earlier warmer periods of 

 the earth's history). Eeptiles thrive particularly in a dry atmosphere, 

 though they are also able to live in a moist air. (Notice the advance 

 they display in this respect, in common with birds and mammals, in com- 

 parison with amphibians and fishes.) With a decrease in the temperature 

 of the air, however, these animals become more and more sluggish, until 

 finally, in colder districts, they become quite torpid and sink into a 

 hibernating sleep, from which they are only aroused by the reviving 

 warmth of returning spring. Thus the vital activities of reptiles vary 

 with the annual changes of the external temperature. 



5. Covering of the Body. Since the heat of the body is of no 

 importance to reptiles, they are not, like warm-blooded birds and 

 mammals, in need of special heat-preserving envelopes (hair, feathers, 

 or fat layers). Their body covering serves principally as a protection 

 against external injuries and against their drying up in a hot and dry 

 air, though in some (lizards, and especially snakes) it also renders aid in 

 locomotion. 



The lower skin, or derma, is thickened locally, and hardened (e.g., in 

 most lizards) or ossified (e.g., in chelonians). The epidermis covering 

 these portions, which is shed and renewed periodically (sloughing), is like- 

 wise thickened and cornified. 'When these thickened portions are apposed 

 by their edges they are termed scutes, but if they overlap each other, like 

 tiles, they are described as scales. In many cases (especially in the croco- 

 dile) smaller or larger bony plates are developed in the deeper layers of 

 the skin by the deposit of calcareous material. 



6. Dentition. All reptiles, with the exception of the chelonia, 

 possess teeth, which are inserted in the mandibles, the maxilla, and in 

 some also in the palate. They are all of similar conical shape (compare, 

 on the other hand, with mammals) and directed posteriorly, being con- 

 sequently adapted rather for holding the prey (see lizards and snakes) 

 than for tearing it up or masticating it. In the crocodiles the teeth 

 are fixed in sockets, while in lizards and snakes they are united firmly 

 with the mandibles, maxillae, and palatine bones. (For the poison fangs 

 see viper.) 



7. Reproduction. Most reptiles propagate themselves by eggs, which 



