94 GENERAL STRUCTURE OP REPTILES. 



the same group j yet the structure of the internal organs, on 

 which classification is founded, is essentially alike in all; 

 and their physiological condition presents no important dis- 

 similarity. Four obviously different tribes, Turtles, Lizards, 

 Serpents, and Frogs, are brought together by the following 

 characters. They are all oviparous, in this respect agreeing 

 with Birds and Fishes ; but they are cold-blooded, and have 

 not a complete apparatus for the double circulation of the 

 blood, in which respect they differ from Birds; and they 

 breathe air by means of lungs, instead of breathing water by 

 gills, in which respect they differ from Fishes. But by the 

 lowest group, that of Frogs and their allies, this class is 

 united to that of Fishes in a most remarkable manner ; for 

 these animals in their young state breathe by gills, and 

 lead the life of a fish ; and some of them retain their gills 

 during the whole of life, even after the lungs are developed 

 ( 87). The first three of the tribes just mentioned un- 

 dergo no such change : and they further agree in this, that 

 they breathe air during the whole of their lives, coming forth 

 from the egg in the same condition as that in which they are 

 subsequently to live, and also in having their bodies covered 

 with horny scales or plates, whilst the skin of the Frog tribe 

 is soft and unprotected. 



82. The class of Eeptiles presents a marked contrast to 

 that of Birds, in the comparative slowness and feebleness of 

 its movements, the dulness of its sensibility, and the in- 

 activity of its organic functions. As there is no fixed tempe- 

 rature to be maintained, one important source of demand for 

 food is withdrawn; and when not excited to activity by 

 external warmth, these animals may pass long periods without 

 fresh supplies of food. Their blood is very poor in red 

 corpuscles, and its circulation is comparatively languid. A 

 reduction of the temperature of their bodies to within a few 

 degrees of freezing point, induces complete torpidity, which 

 continues until they are roused by a renewal of warmth. 



83. The Turtle tribe is peculiarly distinguished by the 

 inclosure of the body in a bony covering ; of which the 

 upper arched portion (termed the carapace] is formed by 

 the coalescence of the ribs with a set of bony plates deve- 

 loped in the substance of the skin; whilst the lower fiat 

 plate (termed the plastron), which is often incomplete, is 



