REPTILES. S 



In one respect in which Reptiles agree among 

 themselves, they agree also with the Classes of 

 Birds and Fishes ; the reproduction of their spe- 

 cies is effected by means of eggs. For the most 

 part these are encased in a calcareous covering, 

 either hard and shelly as in the Tortoises, Croco- 

 diles and some Saurians, or tough and leathery as 

 in most of the Serpents : in the Amphibia, however, 

 they are destitute of any covering. In a few in- 

 stances, the young are brought forth alive ; but 

 these are rather apparent than real exceptions to 

 the general rule, the eggs in such cases being- 

 hatched in the body of the parent, or ruptured in 

 the act of deposition. Our native smaller Lizard, 

 Viper, and Slow-worm, afford familiar examples of 

 this peculiarity. Reptiles do not in general incu- 

 bate their eggs ; but there is reason to believe that 

 the Boas and Pythons are exceptions to this rule. 

 The heart in this Class is so constructed that 

 at every pulsation only a part of the blood from 

 the system is thrown into the lungs> the remain- 

 ing portion returning into the circulation without 

 being aerated. As animal heat is derived from 

 the communication of oxygen to the blood, in 

 its exposure to the air when passing through the 

 lungs, it follows that the imperfect aeration of the 

 blood in these animals is attended with a diminu- 

 tion of vital heat, and that they are what is called 

 cold-blooded ; in other words, the heat produced 

 by respiration is so small, that the difference 



