PROGRESSIVE MOTION IN BATRACHIA. 397 



plane it can proceed only by leaps ; an action 

 which the length and great muscularity of the hind 

 legs particularly fit it for performing. The toad, 

 on the other hand, whose hind legs are short and 

 feeble, walks better, but does not jump or swim so 

 well as the frog.* The Hyla, or tree-frog, has the 

 extremities of each of its toes expanded into a fleshy 

 tubercle, approaching in the form of its concave 

 surface to that of a sucker, and by the aid of which 

 it fastens itself readily to the branches of trees, 

 which it chiefly inhabits, and along which it runs 

 with great agility. 



The Salamander is an animal of the same class 

 as the frog, undergoing the same metamorphoses 

 from the tadpole state. It differs much, however, 

 in respect to the dev elopement of particular parts 

 of the skeleton. The anterior extremities of the 

 salamander make their appearance earlier than the 

 hind legs, and the tail remains as a permanent part 

 of the structure. The rudimental ribs are exceed- 

 ingly small, and the sternum continues cartila- 

 ginous. The pelvis has no osseous connexion with 

 the spine, but is merely suspended to it by liga- 



* It is singular that the frog, though so low in the scale of 

 vertebrated animals, should bear a strikingresemblance to the human 

 conformation in its organs of progressive motion. This arises from 

 the exertions which it makes in swimming being similar to those of 

 man in walking, in as far as they both result from the strong action 

 of the extensors of the feet. Hence we find a distinct calf in the 

 legs of both, produced by the swelling of similar muscles. The 

 muscles of the thigh present, also, many analogies with those of man ; 

 particularly in the presence of the long muscle called the sartorius, 

 the use of which is to turn the foot outwards, both in stepping and 

 in swimming. 



