ia 
94 GENERAL STRUCTURE OF REPTILES, 
* the same group ; 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, Z’wrtles, 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 Reptiles 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food is withdrawn; and when not excited to activity by 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 Zurtle 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 flat 
plate (termed the plastron), which is often incomplete, is 
