2 ELEMENTARY BIOLOGY. [CHAP. 



becoming dark in a dark place, and light if exposed to the 



light. 



The body of the Frog presents only two median aper- 

 tures, the wide mouth and the small cloacal aperture. The 

 latter is situated at the posterior end of the body, but rather 

 on its upper side than at its actual termination. It is com- 

 monly termed the anus, but it must be recollected that it 

 does not exactly correspond with the aperture so termed in 

 the Mammalia. 



The two nostrils, or external nares, are seen at some dis- 

 tance from one another upon the dorsal aspect of the head, 

 between the eyes and its anterior contour. The eyes are 

 large and projecting, with well-developed lids, which shut 

 over them when they are retracted. Behind the eye, on 

 each side of the head, there is a broad circular area of 

 integument, somewhat different in colour and texture from 

 that which surrounds it; this is the outer layer of the mem- 

 brane of the tympanum, or drum of the ear. 



The fore-legs are very much shorter than the hind-legs. 

 Each fore-limb is divided into a brachium, antebrachium and 

 manus, which correspond with the arm, fore-arm and hand in 

 Man. The manus possesses four visible digits which answer 

 to the second, third, fourth, and fifth fingers in Man. There 

 is no web between the digits of the manus. 



The hind-legs are similarly marked out into three divi- 

 sions, femur, crus, and pes, of which the femur answers to 

 the thigh, the crus to the leg, and the pes to the foot, in 

 Man. The pes is remarkable not only for its great relative 

 size as a whole, but for the elongation of the region which 

 answers to the tarsus in Man. It will be observed, however, 

 that there is no projecting heel. There are five long and 

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

 and are united together by thin extensions of the integu- 



