.ii EXTERNAL CHARACTERS OF THE FROG 65 



connection, are facts of considerable significance in relation 

 to the evolution of this form. 



The two pairs of legs are very different in form as in func- 

 tion. The fore limbs are short, and consist of three divi- 

 sions, the upper arm, \heforearm, and the manus, or hand. 

 The hand has four fingers, and the rudiment of an additional 

 digit on the inner side which can be felt under the skin. 

 This rudiment corresponds to the thumb of our hands. The 

 two inner digits contain three joints each, the two outer 

 ones, four. There are no claws or nails on the digits of the 

 frog in either pair of limbs. The fore limbs, when the ani- 

 mal is in a resting position, are held in a peculiar twist ; the 

 forearm, and to a greater extent the hand, are turned in- 

 ward, so that the large inner finger often points backward. 

 The fore limbs of the male frog differ in a peculiar manner 

 from those of the female, being modified in relation to the 

 clasping instinct of the male which appears at the breeding 

 season. The forearm relatively is thicker than that of the 

 female, owing to the greater muscular development of that 

 portion of the limb. The inner finger of the hand also 

 becomes much larger in the male and swollen at the base. 

 The swelling is due mainly to a thickening of the glandular 

 portion of the skin in this region, and becomes reduced in 

 size when the breeding season is past. 



The hind limbs are long and admirably adapted for jump- 

 ing and swimming, but are of little service in walking, as the 

 frog can scarcely be said to employ this method of locomo- 

 tion. Like the fore limbs, the hind limbs are divided into 

 three parts : an upper portion or thigh, a middle part, cms 

 or 'shank, and the /#<?/, or/^. The latter is very well devel- 

 oped and has the ankle remarkably elongated ; there are 

 five toes and the rudiment of a sixth toe, termed the pre- 

 hallux, which is situated on the inner side of the foot. The 

 F 



