542 REPTILIA. 



who merely watches the changes perceptible from day to day in 

 their external form, they acquire a tenfold interest to the physio- 

 Fig. 241 . 



logist who traces the progressive evolution of their internal viscera ; 

 more especially when he finds that in these creatures he has an 

 opportunity afforded him of contemplating, displayed before his 

 eyes, as it were, upon an enlarged scale, those phases of develope- 

 ment through which the embryo of every air-breathing vertebrate 

 animal must pass while concealed within the egg. The division, 

 therefore, of Reptiles into such as undergo a metamorphosis, and 

 such as do not, is by no means philosophical, although convenient 

 to the zoologist : all Reptiles undergo a metamorphosis, though 

 not to the same extent. In the PERENNIBRANCHIATA the change 

 from the aquatic to the air-breathing animal is never fully com- 

 pleted ; in the CADUCIBRANCHIATA the change is accomplished 

 after the embryo has escaped from the ovum ; and in the REP- 

 TILIA proper, as well as in BIRDS and MAMMALS, which are gene- 

 rally said to undergo no metamorphosis, the changes referred to 

 are accomplished in ovo during the earliest periods of the formation 

 of the fetus. 



(590.) The second order of Reptiles (OPHIDIA) includes the 

 Serpent tribes, animals entirely deprived of external locomotive ex- 

 tremities, and nevertheless endowed with attributes at once formid- 

 able and surprising. Absolutely without limbs or any apparent 

 means of progression, the scale-clad Serpent makes its way in either 

 element with equal facility ; and walks or leaps, or climbs or swims, 

 at will. Destitute of any prehensile members, it seizes and devours 

 the strongest and most active prey : it binds its victim in a living 

 rope ; or, with a single scratch inflicted by its venomed fangs, 

 speedily destroys the stoutest assailant. 



(591.)The transition from the OPHIDIA to the Lizards (SAURIA), 

 composing the third order of Reptiles, is very gradually accom- 

 plished by several intermediate forms, in which the first buddings 



