154 ELEMENTARY BIOLOGY. [XIII. 



slender digits, which correspond with the five toes in Man, 

 and are united together by thin extensions of the integu- 

 ment constituting the web. The innermost and shortest 

 answers to the hallux, or great toe, in Man. 



At the base of the hallux, the integument of the sole 

 presents a small horny prominence, and sometimes there 

 is a similar but smaller elevation on the outer side of the 

 foot: but there are no nails upon the ends of any of the digits 

 of either the pes or the manus. Thickenings, or callosities, 

 of the integument, however, occur beneath the joints of the 

 digits, both in the pes and the manus. 



During the breeding season, the integument on the 

 palmar surface of the innermost digit of the manus, in the 

 male, becomes converted into a rough and swollen cushion, 

 which, in the Grass Frog, acquires a dark-brown or black 

 colour. 



The Frog, when at rest, habitually assumes a sitting pos- 

 ture much like that of a dog or cat. Under these circum- 

 stances the back appears humped, the posterior half being 

 inclined at a sharp angle with the anterior half. The ver- 

 tebral column, however, will be found to be straight, and the 

 apparent hump-back arises, not from any bend in the 

 vertebral column, but from the manner in which the long 

 iliac bones are set on to the sacrum. 



The walk of the Frog is slow and awkward, but it leaps 

 with great force, by the sudden extension of the hind- limbs, 

 and it is an admirable swimmer. 



In a living Frog, the nostrils will be seen to be alter- 

 nately opened and shut, while the integument covering the 

 under side of the throat is swollen out and flattened. The 

 alternate pumping in and expulsion of the air needed for 

 the Frog's respiration is connected with these movements. 



The upper eyelid of the Frog is large and covered with 

 ordinary pigmented integument, and it has very little mobility. 



