AMPHIBIANS 251 



Nostrils and eyes are present closely resembling those of a 

 Salamander, but behind either eye may be seen a well-marked 

 rounded area corresponding to the tympanic membrane covered 

 over by skin. A small rounded cloacal aperture is present at the 

 hinder end of the trunk. The skin is soft and moist, as in 

 Amphibians generally, and is entirely devoid of hard parts, with 

 the trifling exception of the horny spur on the foot. The upper 

 surface is mottled and the under surface pale, and, as in the 

 Chameleons, though not quite to the same extent, the animal 

 has the power to adjust its colour so as to match the surroundings 

 for the time being. A frog which has been kept for some time 

 in a dark place becomes almost black in colour, but if the same 

 animal is transferred to grass it will gradually assume a greenish 

 tint. 



Endoskeleton (fig. 156). The peculiarities of the skull cannot 

 be entered into here, but as regards the backbone, it may be said 

 that the number of vertebrae is reduced to ten, their centra, 

 too, being concave in front and convex behind, as in Reptiles. 

 The first vertebra is a ring-like atlas, and the last fulfils the 

 function of a sacrum, being connected with the hip-girdles 

 supporting the hind-limbs. The hinder end of the backbone 

 is completed by a bony rod, the urostyle. No separate ribs 

 can be distinguished, but there is reason to believe that the 

 prominent transverse processes which project from the sides 

 of the vertebrae are partly equivalent to these. It is quite a 

 common thing for ribs to fuse with vertebrae, at least in certain 

 regions, e.g. in the human skeleton short neck-ribs have un- 

 doubtedly been soldered, so to speak, with the vertebrae which 

 support the neck. 



The sternum is much better developed here than in the 

 Salamander, and is closely connected with the shoulder-girdles. 

 These present the typical regions, but it may be noted that the 

 precoracoid bar is covered over by a collar-bone or clavicle. The 

 skeleton of the rest of the fore-limb corresponds fairly well with 

 the pattern limb (see pp. 196-198), and a vestige of the thumb can 

 be made out. The forearm, however, presents a very interesting 

 case of fusion, for its two typical bones, the radius and ulna, 

 are here closely united together. In the hind-limb a number 

 of instructive variations on the pattern form are to be seen. 

 The hip-girdles are quite unlike the corresponding parts in a 



