168 COMPARISONS OF STRUCTUEE IN ANBIALS. 



to the articulating cavity of the latter the 

 head of the arm-bone is fitted, and farther 

 secured by a round ligament, {ligamentum 

 teres,) similar to that which binds the head 

 of the thigh-bone to its socket in most quad- 

 rupeds. This security of the shoulder-joint 

 in a reptile, which proceeds on land by a 

 series of vigorous leaps, is doubtless requi- 

 site, in order that it may bear unrestrained, 

 or -Nvithout dislocation, the shock to which 

 it is liable, when the animal comes down 

 on its fore-limbs, after making its spring. 

 The hind-limbs are very long and muscular, 

 and are the chief organs of progression, both 

 in the land and in the water. 



The changes which take place in the 

 osseous system of the frog — and Ave here say 

 nothing of the respiratory and circulating 

 system, from its tadpole to its mature state — 

 axe indeed very wonderful; they are, more- 

 over, within the observation of all who are curi- 

 ous to examine for themselves, and watch the 

 stages of life from immaturity to perfection. 



In the young, or tadpoles gf the wa,ter- 

 newt, (Trito7i,) so abundant in clear ditches and 

 ponds, similar transformations take place to 

 those in the tadpole of the frog, with these 



