356 
VERTEBRATA. 
there are some not fecundated until after tliey have been excluded ; but it is otherwise with the 
animals of this class ; in some of them, as the snakes, for instance, the young are already consid- 
erably advanced within the egg at the time the mother deposits it, and there are some species 
which may be rendered viviparous by retarding the laying of the eggs. 
Reptiles not only present extremely varied forms, but great diversity of character and modes 
of gait ; Cuvier remarks that in their formation nature seems to have tried to imagine grotesque 
forms, and to have modified in every possible way the general plan adopted for vertebrated 
animals. 
ORDER 1. CHELONIA. 
The distinguishing characteristic of the Chelonian reptiles, and the on e whic in fact enables 
us to recognize them at the first glance, consists in their possession of a complete bony case, 
within which the head and limbs can 
generally be more or less completely 
retracted. This case consists of two 
large bony plates, the upper one, which 
is more or less convex, being called the 
Carapace^ while the loAver one, which 
is flat, is denominated the Plastron. 
These two plates are united at their 
lateral margins, leaving an anterior and 
posterior aperture for the protrusion 
of the head, tail, and limbs. At first 
view it would seem impossible to refer- 
these singular animals to the ordinary 
vertebrate type, but a little examination 
of their anatomy, as represented by the 
annexed engraving, shows that their 
different parts are only modifications 
of the same general structure that we 
have seen to prevail throughout the 
preceding groups. 
In their general internal structure 
the Chelonia agree pretty closely with 
the other reptiles. The heart is com- 
posed of three cavities, the partition 
between the ventricles being very in- 
N ' complete, so that the venous and arte- 
rial blood can mix freely in that cavity. 
The lungs are of very large size, and 
extend far into the cavity of the body ; 
but as the ribs are immovable, respira- 
tion is effected by a process very anal- 
ogous to swallowing. The tongue is short, fleshy, and completely movable ; the ears are dis- 
tinctly visible, and the eyes well formed, and furnished with movable lids. 
The Chelonian Reptiles are usually sluggish and inactive animals, the slowness of the terres- 
trial species being even proverbial. They are, however, exceedingly tenacious of life ; they will 
live for a long period without any nourishment, and will even continue to give signs of life for 
some time after they have been deprived of their heads. They are found principally in the 
warmer regions of the earth, where they generally subsist upon vegetable substances, although 
many of the aquatic species also devour small animals. They are oviparous, and the eggs are 
covered with a hard shell. 
This order, called Testudinata by many naturalists, is divided into five families : the Chelg- 
SKELETON OF A TUETLK. 
