8 ANIMAL BIOLOGY. [Parti. 



causes them to contract, so that the skin becomes paler, duller, 

 and more uniform in tint. Light causes their relaxation, so 

 that the skin becomes brighter and more diversified with spots. 

 But the darkness and the light do not seem to act directly on the 

 cells, but only through their effect on the eye of the living frog. 



There are four apertures in the frog's body, two median and 

 two lateral or paired. In front is the large mouth with a broad 

 long gape extending back to behind the eyes. On opening 

 widely the mouth, two slit-like apertures are seen at the back, 

 one ventral, clean cut, and longitudinal, the glottis, leading to 

 the lungs : the other dorsal, irregular, and transverse, the open- 

 ing of the oesophagus, or gullet. At the hinder end of the body 

 is the vent or doacal opening, whence issue the faeces, the excre- 

 tion of the kidneys, and the products of the genital organs. 



The lateral apertures are the external nares, one on either 

 side of the snout. They may be seen to open and shut as the 

 frog breathes by the alternate rise and fall of the floor of the 

 throat. A bristle passed into one of the nares passes down- 

 wards and slightly backwards, and emerges in the mouth, by the 

 posterior nares, tolerably far forwards in the roof. The nares are 

 the only lateral apertures. There are no external ear openings. 

 The membrane of the drum of the ear is close to the surface and 

 only covered over with skin. It is readily visible in the midst 

 of the characteristic triangular brown patch above and behind 

 the angle of the mouth. If a hole be pricked in this tympanic 

 membrane, and a bristle passed in, it will emerge at the side of 

 the mouth, through one of the two large Eustachian recesses. 



The tongue is large and white. It is fixed by its anterior end, 

 the free posterior end being bifurcated. It is darted or slung 

 out with great rapidity. The prey (insect, worm, or slug, for 

 the adult frog feeds exclusively on animals) adheres to its sticky 

 surface, and is drawn back into the mouth. Its escape is pre- 

 vented by the teeth, which may be readily felt in the upper jaw 

 and in the roof of the mouth (vomerine teeth). There are no 

 teeth in the lower jaw. 



The eyes are large and prominent, and can be withdrawn into 

 sockets, and thus beneath the upper lids. The delicate and 



