35G 



VERTEBRATA. 



tluTO are some not fcciiinlatfil until aftor tlu-y have l)c'eii excliidctl ; but it is otherwise with the 

 animals of this cl.iss; in some oftlii-m, as the sn:ikes, for instance, the young are already consid- 

 « rahlv a<lvanccd within tin- cgiX :'t \hr. time the mother deposits it, and there are some speeies 

 which may he reiulered vivipan.us l.y refardini;: the hiying of the eggs. 



Kcptiles not onlv jtresent extrenuly saiicd forms, but great diversity of character and modes 

 of gait ; (.'uvier remarks that in their formation nature seems to have tried to imagine grotes(jue 

 forms, anil to have modilicd in every possible way the general plan adopted for vertebrated 



ammais. 



OllDER 1. CHELONIA. 



'riie distinguishing characteristic of the Chelonian reptiles, and the on e ■wliic in fact enables 

 us to recognize them at the lirst 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, wliicb 

 is more or less convex, being called the 

 Carapace, while the lower one, which 

 is flat, is denominated the Plastron. 

 These two plates arc united at their 

 lateral margins, leaving an anterior and 

 posterior aperture for the protrusion 

 of the head, tail, and limbs. At tirst 

 view it would seem impossible to refer 

 these singular auimals to the ordinary 

 vertebrate type, but a little examination 

 of their anatomy, as represented by the 

 annexed engraving, shows that their 

 difterent 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- 

 ^ ' 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 Ciielo- 



^:>^ 



SKELETON OP A TDRTLK. 



